bicycle lighting

Free CAD software: 3D, 2D, and PCB layout

2012-02-10T22:53:00.000-05:00

I rely heavily on CAD freeware for all my weird obsessive bike light projects. Never having been a professional engineer or designer, I haven't had the luxury of being able to snag a copy of any commercial CAD software from my workplace. Licenses for such things run into the thousands of dollars (I emailed Solidworks a few months ago to get a quote for an individual license for hobby use and they asked for a cool $2000). No thank you!

It took me a long time to discover the tools that I use to design parts in 3D, draft them in 2D, capture schematics and layout PCBs. Here's a list of the free CAD software I find most useful:

3D CAD

eMachineShop

Man, I really like this software. If you're a CAD veteran you might hate it but for the beginner I don't think it could get any easier than this. The software is specifically for designing single machine parts and the design restrictions do a good a job of preventing you from designing something that can't be machined. Living within the software's rules can be a frustrating limitation at times and I'm often forced to think outside the box (so to speak) to design a specific feature on a part (cutting a feature out of a solid, for example, uses a kind of confusing 'air inside/outside' paradigm). It kind of forces you to think like a CNC machine, which is no bad thing if you want your part design to actually be machinable. Making parts by revolving a profile is very straightforward. The autosnapping is fantastic - no key combinations are required to snap to midpoints, endpoints, centers, tangents, etc. Best snapping interface I've encountered, allowing me to work fast. However, complicated parts with many surface features on different axes are not this application's forte. If I need to do more complicated Boolean shaping then I modify my part in FreeCAD.

eMachineShop wants you to use their machining service and there's a handy online quote generator that allows you to get pricing for different materials and tolerances. For pieces out of steel and aluminium, I find their prices competitive with other big online CNC services like QuickCut and Firstcut. All three online machine services quote about 1.5-10 times higher for the same material than most of the prototype makers I've contacted in China. I've used eMachineShop's fabrication service once and was disappointed with the quality of the material and the use of an inferior machining method (plus they initially over charged me for shipping - $128 UPS to Canada!). The part was within tolerances and performed its function, but eMachineShop's off my list of prototype makers for now. Most folks report a good experience, so I'm aware that my dissatisfaction was an exception.

To their credit, eMachineShop's CAD software allows you to export 3D IGES files as well as 2D DXF files. The 3D export has a disclaimer that it's still a work in progress, but for the most part the original design survives the export. For some reason, adding holes to curved surfaces starts to screw things up, a problem that can be solved by modifying the part in FreeCAD.

Although I haven't used it, eMachineShop does support CNC bending, so you can design parts bent out of sheet metal. Unfortunately, the bends are not exported in the 3D IGES file, so you wind up with a flat version of your part on export. I'm not sure if the .igs file plus a 2D description of the bend locations, their radii and angles would be enough to get a quote from a third party fabricator.

FreeCAD

I love it and hate it. It's deep but challenging. For de novo part design of anything but the most basic fundamental shapes, I stick with eMachineShop and then modify the part in FreeCAD. For the most part, I use FreeCAD to make part assemblies so I can see how everything fits together. The Boolean operations are good for cutting one complicated shape from another, something that I can't do in eMachineShop. FreeCAD is still under heavy development, which means it can be a bit buggy and frustrating to use sometimes. It has a small community of enthusiasts where you can get help. The forum is particularly valuable for finding scripts to make complicated features that would be difficult or impossible to do with just the GUI. A helical thread, for instance. FreeCAD looks and feels very promising, but the frequently buggy Boolean operations and a lack of complete GUI implementation of all features is holding it back from a wider audience of prospective users. For my purposes, though, it is perfectly adequate.

One feature I've discovered recently is the Drawing Module, which lets you generate 2D drawings of your part, including isometric views. The GUI implementation is almost nil, so you spend a lot of time manipulating objects by command line, but the results are pretty cool:

Taillight concept 3D

Taillight 2D isometric view

The results of the isometric projection can be cleaned up with DraftSight.

IGSViewer

Prototyping services seem to like IGES format for quotations, so I use IGSViewer to check the outputs of both eMachineShop and FreeCAD to make sure nothing wonky has happened during export.

2D CAD

DraftSight

A new piece of software for 2D drafting, it is quite deep with a reasonably good interface. Like eMachineShop, the implementation of snapping to endpoints, midpoints, tangents, perpendicular, etc. is very handy, although you need to turn it off if you don't want to snap to the nearest feature, no matter how far away it seems. I usually import the DXF output from eMachineShop into DraftSight where I can build up my 2D drafts. The isometric views that FreeCAD spits out in .svg format can be imported (via InkScape) and cleaned up to produce pretty nice looking technical drawings. Dimensioning is pretty straightforward, although I often wind up fighting with the global attribute system. For freeware this is very well-developed. Not sure the story behind it. Perhaps it's just intended as a halo application for its super expensive big brother, Solidworks.

InkScape

Big freeware competitor to Illustrator. I've only used it to convert .svg to .dxf for FreeCAD drawing import into DraftSight, but it looks pretty cool. I've never been an Illustrator user, so I don't imagine using his much, but it sure is handy for the very specific file conversion I need to do.

PCB CAD

Eagle CAD

This software is an electronic hobbyist's dream come true. I found the learning curve a bit steep, but once you get it Eagle CAD becomes a great all-in-one schematic capture and PCB layout application. There is a good tutorial from Sparkfun to get you started. The large libraries of components never seem to have the specific parts I want to use, so I've had to get used to designing my own schematic symbols, laying out the pads according to the datasheet's description of the device's footprint and adding the part to my own custom library. This is kind of a pain in the butt, but I guess you can't expect the libraries to be completely exhaustive. The freeware version has a limited board size (plenty of space for what I want to do) and only supports 2 layer designs (the PCB house I use only offers 2 layer fabrication). The autorouter seems to do a good job of routing the connections after placing the parts. I typically don't route manually, instead shifting parts around until the autorouter hits 100%. In the world of PCB design, that makes me a bit of a loser. So be it.

I'm sure I'm missing out on other great CAD tools. Let me know what your essential pieces of free CAD software are.

Classic bicycle taillights

2012-02-06T21:19:00.000-05:00

Luxor Le Martelé taillight. Photo by J Ferguson

I love classically styled bicycle taillights, especially those of the French persuasion. I have a handful of Luxor tail lamps waiting for the right project to come along (yes, I'm the type that would build a bike around a taillight). I just discovered this gallery on Flickr dedicated to curating photos of both vintage and handmade taillights. As well, there's this great Japanese site that has examples of classic lights from Radios, Luxor, Soubitez and JOS.

Unfortunately, modern taillights are invariably plastic and utilitarian in nature. Compass Bicycles had a recent post bemoaning the indifference to elegance in contemporary taillight offerings. If you want a nicely designed light suitable for your modern classical build, you're looking at acquiring something vintage (these options from Kimura notwithstanding). If you're into classic randonneuring bikes, then vintage options can get very expensive.

Although I'm partial to the fender-mounted teardrop-shaped Luxor lights, I also really like the JOS tail lamps, especially the Fu model. Here is a beautiful example from Flickr:

JOS Fu taillight. Photo by spoke sniffer.

It occurred to me that the symmetrical design would lend itself well to CNC machining. For fun I drafted up a concept:

This would obviously be equipped with an LED, so the protruding bulb holder with it's knurled binding post isn't required, although it might be nice to incorporate it. The bracket might be a bit of a challenge to have made, but the body could be turned on a lathe without much trouble. The lens, on the other hand, would have to be injection molded, which is associated with high setup costs for unrealistically large volumes. Perhaps a 3D printer could produce something that might work. For now, my classically inspired taillight design will have to remain in silico. What do you think? Does the world need a new taillight?

Ledil optic improves Sturmey Archer Cree XM-L upgrade

2012-02-03T00:37:00.001-05:00

I recently posted about a Cree XM-L upgrade I designed for the vintage Sturmey Archer head lamp (at least the one seen most commonly on North American Raleighs from the 1970s). The light output is high, but the beam shape has some significant shortcomings, rather embarrassing ones considering the centre of the beam from the original bulb was still brighter than the LED upgrade. Yikes!

The wise and wonderful people at CPF suggested I try a small modern optic instead of relying solely on the vintage reflector. The choice of sub 20mm optics for the XM-L is quite limited. The best I could find was this 16.1mm 37º optic from Ledil (This 23º optic would probably be better but wasn't stocked by either Mouser or Digikey). It arrived from Mouser today. I had to snip off the mounting posts, but it otherwise fit perfectly over the XM-L's PCB, attached firmly with its own adhesive tape:

Ledil 16.1mm 37º optic mounted on XM-L

As before, I measured the beam profile at 2.5m from my lux meter's sensor. Here's the data I would show if I was a savvy marketer:

Beam profile of Cree XM-L with Ledil optic compared to original incandescent bulb

Well, that looks great. Big fat wide beam compared to the puny and narrow beam of the original bulb. Of course, that doesn't tell the whole story. Here's the same beam profile compared to not only the original but also to a 0.35A incandescent. The profile of the untreated XM-L's beam is also plotted:

Beam profiles of Sturmey Archer lamp with different light sources

While the Ledil optic does a nice job of evening out the profile of the XM-L's beam and doubles its brightness in the center, the peak output is still a lot lower than a simple incandescent upgrade. Granted, an original Dynohub barely puts out 0.35A at high speeds and the front bulb is usually run in parallel with a 0.1A taillight bulb. What you get from the Cree XM-L is a bright wide beam with lots of spill. This results in a nice amount of off axis visibility, which is important to me in the city traffic where I do most of my riding.

With the Ledil optic I'm willing to declare my Sturmey Archer LED upgrade a success. There's even the possibility of upgrading to the 23º optic if it ever makes its way over to North American shores.

Incandescent optics versus LED optics: implications for upgrading vintage lamps

2012-02-01T00:51:00.000-05:00

Sometimes facts get in the way of a great idea. I've devoted a lot of time to the problem of upgrading vintage lamps to power LEDs. Power LEDs produce a ton of light, but they get hot and require adequate heat sinking to maintain their output and limit the chance of failure. This led me to develop a series of rather elaborate copper heat sinks. I've made a screw base LED bulb, a wedge base LED bulb, and custom LED heat sinks to fit the classic Sturmey Archer headlight as well as the Luxor 65 series of vintage French lamps, all using Cree XP-G or XM-L power LEDs. I fired them up from both a DC bench top supply and a hub dynamo and I was impressed with the results. For the Sturmey Archer and Luxor headlamps, the outputs were bright, producing huge floody beams. Before I could make claims about the superiority of these upgrades, though, I thought I should take some lux readings and compare the output of the LED upgrades to that of the original incandescent bulbs.

Cree XM-L mounted in Sturmey Archer headlight

Here is some lux data comparing the output of an original 6V bulb at 350 mA (in actual fact, the original bulb was 6V 0.2A with a parallel 0.1A taillight bulb), a 3W halogen bulb (Reflectalite GH106) at 500 mA and the Cree XM-L upgrade at 500 mA (low end modern hub dynamo output) and 700 mA (high end hub dynamo output as well as Dynohub magnet upgrade). Lastly, I measured the output of a Cree XP-G mounted in a 41.5mm reflector designed specifically for Cree LEDs. The lux meter's sensor was placed 1M away from the light source and the centre hot spot of the beam was measured. The lux reading recorded represents the brightest spot of the beam.

Lux output of Sturmey Archer headlight with different light sources compared to Cree XP-G with specialized reflector

Well, what the heck? Is ancient technology really winning? Well........... yes, in a nutshell, it is. :(

Although I haven't photographed the beams, allow me to emphasize how different the incandescent/halogen beam patterns were compared to the LED patterns. The hot spot of the incandescent/halogen beams that yielded the highest lux reading was very concentrated. The slightest change in position of the beam could see the lux reading tumble by half or more; the focused hot spot was very small and the overall beam was quite narrow, certainly less than 45º. In contrast, the LED upgrades mounted in the same reflectors produced a very diffuse hot spot and a very evenly illuminated flood of probably greater than 120º. The LED upgrade is clearly putting out more light, but it isn't focused like the incandescent bulbs.

To document this quantitively, I placed the sensor of my lux meter on a tripod and focused the center of the beam on it (highest lux reading). I then took readings at finite distances from the centre of the beam while maintaining the same distance from the light source, essentially creating a brightness profile across the beam's diameter. I rather arbitrarily set the distance at 2.5m. Some bike light makers measure performance by lux at 5m. Lux at 1m also seems to be a common standard, however I was limited by the distance between my work bench and my wall, so 2.5m it is, which is fine for a relative measurement. Here I compare the XM-L's beam to the beams of a 6V 0.35A incandescent as well as the 6V 0.2A incandescent that originally equipped the Sturmey Archer lamp.

Brightness profile of Sturmey Archer headlight beam with three different light sources

As you can see, both incandescent bulbs produce a narrow symmetrical beam that drops off to zero between 40-50 cm from the center. The 0.35A bulb has a surprisingly bright peak, whereas the original 0.2A bulb is considerably dimmer. The XM-L beam doesn't have much in the way of a center peak, but provides a wide profile of even illumination. Unusually, it seems to have some asymmetrical artifacts, which I confirmed by measuring the beam in a different axis (not shown). My room wasn't big enough to measure the full width of the XM-L's beam. If was able to, I expect the area under the XM-L's profile curve would be a lot larger than the area under both incandescents' curve (ie. greater total light output).

The 0.35A bulb isn't an especially fair comparison as the original configuration was a 0.2A bulb, however it does indicate that a simple incandescent upgrade can dramatically improve the light output. Although I didn't measure the beam profile, you can bet that the 0.5A halogen bulb would be even brighter!

The obvious problem with the XM-L's performance is the optics. The vintage optics are optimized for incandescent bulbs. The limited viewing angle of the LED means that, although a lot more light is coming out of the lamp, the optics aren't focusing it into a tight beam. I had originally fretted that the LEDs weren't in the right position relative to the vertex of the reflector's parabola, but changing that position doesn't have much of an effect on beam pattern. The glass globe of an incandescent lamp allows the filament's light to enjoy a greater than 180º viewing angle. It is my suspicion that it is this backward directed light that is collected and focused by the reflector.

This LED/incandescent performance difference is less pronounced with the Luxor 65 reflector. Interestingly, the results indicate that the Sturmey Archer lamp does a considerably better job of focusing incandescent light into a tight beam.

Cree XM-L mounted in Luxor 65 head lamp

Lux output of Luxor 65 head lamp with different light sources

So, I think the lesson to be learned here is that there are rather strict limitations to what you can accomplish with a LED retrofit of a vintage lamp. You really are at the mercy of the reflector, which does a surprisingly good job of focusing a small amount of incandescent light into a tight beam, but isn't very compatible with the narrower viewing angle of power LEDs.

It also really emphasizes how deceiving lumen ratings of LEDs can be. I suppose this is why Maglite uses 'Beam Distance' to measure the performance of their flashlights rather than total light output or lux at a finite distance (1m, 5m, etc). This is the maximum distance at which a light source will produce 0.25 lux, thus taking into account both the total light output and the throw provided by the optics.

Modifying or replacing the reflectors of vintage lamps isn't especially practical, so I think I've hit a performance wall here. While the LED upgrades don't produce a tight focused beam, their output still produces a bright flood that is certainly adequate for riding around town at night and maybe along a dark bike path at normal speeds. I haven't done a side-by-side comparison yet, but I'm pretty sure the greater total light output from the LED upgrade will result in greater noticeability in traffic. Careening down a long hill on a moonless night on your vintage French randonneuring bike is probably not advisable though...

The bright side (forgive me) to all of this is that the rear red LEDs are much much brighter than their original incandescent counterparts. I fret constantly about being visible from behind when I'm riding, so I'm more concerned about having highly optimized lighting for the taillight. The LED upgrade offers a bright standlight and a super visible rear light (with the potential to flash!), which, for my purposes, makes up for the fact that the head lamp beam pattern is less than optimal.

Still, hope for a better beam shape is not lost. The opening into the reflectors of both the Sturmey Archer and Luxor lamps is less than 20 mm, but there are modern plastic optics with a diameter less than that. It's possible some modern optic made for the XM-L might improve the beam shape. This one from Ledil looks promising:

Ledil Cree XM-L 16.1mm optic

Need to order a couple of these and see if they can improve the beam shape.

Update: optics arrived and the results are pretty good.

Power LED upgrade for Luxor 65 bicycle lamps

2012-01-31T00:33:00.000-05:00

Luxor 'Le Martelé' lamp set. Photo from jp weigle.

I'm a huge fan of the Luxor 65 series of bicycle lamps, especially the hammered aluminium 'Le Martelé'. It took me a while, but after a few months on French Ebay I managed to acquire a set. Naturally, I want to upgrade them with modern power LEDs, as I've done with the classic Sturmey Archer lamp set. Both front and rear lamps use a nicely designed lamp holder that snaps into the reflector/lens base with a pair of spring clips:

For the rear lamp I can use a screw base bulb with a super bright red Cree XP-E LED. For the front lamp, I wasn't satisfied with the way my LED bulb sat in the reflector, so designed a heat sink that could be clamped into the opening of the reflector with set screws:

Here it is machined in copper:

Heat sink for Luxor 65 LED upgrade

and with a Cree XM-L LED mounted directly to the nub that pokes through a slot milled in the PCB:

The holes for the set screws weren't quite in the right place to grab the base of the reflector so I had to drill and tap new ones for a 4-40 set screw. Here's how the LED/heat sink assembly clamps onto the base of the reflector:

Original set screw hole was in the wrong spot!

Here's how the original bulb holder fits:

Notice the double dip in the middle of the edge of the reflector base. I failed to notice this when placing the holes for the set screws!

The final result:

The light is bright and very floody, much like my LED upgrade to the Sturmey Archer headlight. The beam does not produce a concentrated hot spot, which makes me wonder if this is a limitation of using LEDs with 100-140º viewing angles in reflectors designed for incandescent bulbs with >180º spreads. Some lux data will be required to sort this out.

Here's the matching Le Martelé rear lamp with a LED bulb replacement:

Sturmey Archer headlight Cree XM-L power LED upgrade

2012-01-29T22:22:00.000-05:00

Raleigh Superbe's Sturmey Archer headlight. Photo by lowercase design

I really like the Sturmey Archer lamps that equipped later model Raleigh Superbes. The older Raleighs (and other English 3 speeds) seem to have come with a variety of different lamp styles over the years. My best guess is that when the Superbe hit North American shores in full force in the late 1960s and throughout the 1970s, Raleigh/Sturmey Archer had standardized their lamp offerings, which resulted in the now classic chromed bullet head lamp and tail lamp. This version of the lamp set is what I see most commonly on the streets of Toronto, as well as in Flickr feeds. They aren't especially rare and certainly don't show the craftsmanship and quality of the earlier French lamps, but a Sturmey Archer light set can still command prices of well over $100 on Ebay. They can even occasionally be found as a NOS.

The output from the incandescent bulb is rather pathetic. This is mostly due to the low output from the Sturmey Archer Dynohub that typically powers the lamp, possibly exacerbated by the seemingly low quality of the reflector. Halogen replacements are available from Reflectalite, but even these are rated at a rather paltry 18 lumens. I tried upgrading the bulb with an screw base LED replacement, but found the output rather disappointing. I even went to the trouble of making a custom screw base power LED mounted on a copper heat sink. This produced a big floody beam, but the flimsy bulb holder that mounts the bulb in the reflector probably wasn't up to the task of supporting the extra weight of the power LED's heat sink.

So, back to the drawing board. To start, I thought I'd replace the flimsy stamped bulb holder with a copper heat sink. The reflector is made of coated metal, so it shouldn't have any problem with carrying the weight of the heat sink, which can be wedged into the opening in the reflector in the same fashion as the original bulb holder. This time I decided to use the super high output Cree XM-L, which is rated at 290 lumens @ 700 mA. As in my previous designs for the Cree XPG, a nub pokes through the PCB so the LED's thermal pad can be soldered directly to the heat sink. Something like this:

Design for Sturmey Archer headlight heat sink

Looks a bit like a top hat. Here it is machined in copper:

Copper heat sink for Sturmey Archer headlight power LED upgrade

It flares out a tiny bit where the bottom of the pillar meets the rim. Here's the heat sink with a Cree XM-L soldered on to it:

This is how the original bulb holder fits in the reflector:

and here is the copper heat sink mounted in the same opening:

Copper heat sink mounted in reflector of Sturmey Archer headlight

It fits very snugly in the reflector and doesn't feel like it's going anywhere. A dab of Loctite wouldn't hurt though. The LED sits low in the reflector, about where the original bulb's filament would be.

Sturmey Archer headlight with Cree XM-L power LED upgrade on copper heat sink

Coupled with an upgraded LED bulb for the rear lamp and a Dynohub magnet upgrade, a modern and powerful dynamo system emerges. It will eventually even be possible to add a standlight and flasher. The lamp is still a victim of its original optics, which obviously aren't nearly as good as the highly optimized optics of modern bike lights, but the high output of the Cree XM-L does result in a bright, floody beam. Certainly enough for cruising around city streets in relative safety and quite possibly enough to illuminate a poorly lit country bike path.

I'm thinking of making this LED upgrade available, along with an LED upgrade for the matching taillight. Let me know if you're interested either by commenting or by emailing me directly (bottom of page).

The XM-L can be run at up to 3A to produce a whopping 943 lumens so a battery powered retrofit could produce an impressive amount of light, although it remains to be determined if the heat sink would be adequate. That would require some performance testing.

Programming an Attiny10 with AVRISP mkII and AVR Studio 5

2012-01-27T23:33:00.000-05:00

Jump to Attiny10 programming tutorial if you want to skip my long winded preamble.

I've been tinkering with my dynamo powered LED flasher circuit recently. While the standlight works well, the blinking circuit requires that a 5W Zener diode shunt all 500-600 mA of dynamo current during the off cycle of the flash. At a frequency of about 2 Hz and a duty cycle of about 50%, I didn't think this would be a problem, but the damn Zener hits 100°C within a minute and showed no signs of slowing down, suggesting that I was taking it to within an inch of its life. I got some advice about using a power transistor to shunt the current, but using a 20W transistor still resulted in the device getting too hot too fast. I don't have room to put a big heat sink on it so I finally gave up on trying to shunt the current and getting rid of the off cycle energy as heat. I'm sure there is an elegant solution out there that allows the energy to be recaptured rather than wasted, but I'm already running the rear red LED close to its maximum current. Instead, I decided that I could do away with a flashing front light and just flash the rear light. This way, the dynamo is never disconnected from the load and the Zener does not need to shunt any current except in the case of a connection failure. A low side N-FET seems to do the trick, shorting out the red LED when the transistor is on:

N-MOSFET shorts red LED when ON

Anyway, in the process I also decided that I wanted a flash that was below 50% duty cycle. Ideally around 30% or so to better mimic the strobe pattern of commercial LED flashers. You can achieve this with a 555 by adding a diode, but I thought I'd try using a microcontroller to generate the flasher's pulse instead. Why, you might ask, would I want to do something so complicated? Adding a microcontroller to a bike light seems kind of overkill. Well, it turns out I can do it in a smaller package with fewer parts for less money (and learn something along the way). Win!

In my previous design I was using a 555 astable oscillator to generate the pulse. The smallest package size is SOIC8, which is pretty bulky as far as surface mount devices go. The smallest microcontroller I could find was the Atmel AVR Attiny10 that comes in a breathtakingly small SOT23-6 package.

TI CMOS SOIC8 555 timer ($1.72) next to SOT23-6 Attiny10 ($0.89)

Surface mount devices are pretty annoying for prototyping, but the advantage of their tiny footprints becomes readily apparent when you need to cram a lot of parts into a small pace. Once I realized how useful SMDs are for creating physically small circuits, I accepted that the prototyping complications were worth it. Getting the Attiny10 onto a breadboard requires a little adapter from the nice people at Proto-Advantage. I soldered it using a soldering iron. Not as tricky as it seems.

Attiny10 on SOT23-6 to SIP board

Attiny10 programming tutorial

The Attiny10 part of the Atmel AVR family. The Attinies are the baby siblings of the microcontrollers that are used in the Arduino development platform. You can even program some of the Attinies with the Arduino, such as the Attiny85.

Unfortunately, the Attiny10 (and variants 4/5/9) won't work with the Arduino IDE and it isn't entirely clear if you can compile C for it, leaving assembler (ASM) as the most straightforward option for programming the whopping 1024 bytes of Flash ROM. It took me a few days of fiddling around to get an Attiny10 to put out a pulse to blink my LEDs. I've put together a little tutorial about programming the Attiny10 to fill in the blanks I found in online documentation, both official and in forums and the blogosphere. Perhaps this will make things a little easier on the very rare occasion that another layperson might want to program an Attiny10 using the available Atmel tools.

[rant]
Engineers seem to, rather annoyingly, assume that everyone using their products or protocols is also an engineer and their technical documentation reflects this assumption. They seem to communicate by some Borg-like collective knowledge (otherwise known as an Engineering Degree) that is mostly impenetrable to the rest of us. This is often a barrier to the minority of wannabe engineers who want to occasionally parachute into the engineering world to get something specific done without actually becoming, you know, an engineer.
[/rant]

This is a tutorial aimed at programming the Attiny10 in assembler using the Atmel AVRISP mkII programmer and AVR Studio 5. By extension, it should also work for all members of the Attiny4/5/9/10 family. It really is just about connecting the Attiny10 to the AVRISP and being able to load hex code onto it because even this can be a frustrating challenge for the uninitiated. The Attiny10 datasheet, the AVR Assembler User Guide and Instruction Set are wonderfully indecipherable documents that you need to refer to for actual programming in assembler. There is also this huge tutorial.

Adding to the confusion is the fact that there are numerous accounts that AVR Studio 5 is either incompatible with the AVRISP mkII, the Attiny10 or both. As of January 2012, I can say that AVRISP mkII, AVR Studio 5 and Attiny10 all play nicely together on Windows 7.

So, let's start:

Step One: Get what you need

You need an AVRISP mkII programmer, AVR Studio 5 and some Attiny10s to get programming. A breadboard and a SOT23-6 to SIP/DIP adapter will also come in handy.

There are a bunch of AVR programmers out there. I chose to use the AVR branded one, the AVRISP mkII, which is a little pricer than the USBTinyISP but requires no assembly. One gentleman was so averse to using the available Atmel tools that he made his own programmer and his own IDE.

AVR Studio 5 is a rather huge and cumbersome piece of software that I'm sure is great for managing complicated multi-thousand line projects written in C. You may or may not be able to compile C for the Attiny10, but you can definitely use AVR Studio to write assembler and use the very handy simulator to debug your code before flashing your Attiny.

I'm pretty sure you could also use AVR Studio 4, which is now mature. In fact, many people seem to have delayed upgrading to AVRS5 because it is considered buggy.

I'm sure you could also use avrdude instead of AVR Studio. It has been documented to work with the Attiny10. I have a strong aversion to command line interfaces, so avrdude was out (dead give away that I'm not an engineer...).

Step Two: Connect the programmer

After downloading and installing AVR Studio 5 you should be able to connect the AVRISP mkII to your PC and see a green light glow inside, next to the USB connector. An exterior LED is red to indicate that there is no external power (weirdly, the AVRISP, despite having Vcc and GND connections, cannot be used to power the Attiny during programming).

Confusingly, the Attiny10 and its ilk are not programmed using the same 3 wire MISO/MOSI/SCK ISP protocol used by other AVR microcontrollers. Instead, they use the 2 wire Tiny Programming Interface, or TPI. What wasn't at all clear to me from the various documentation available was how you connect the AVRISP mkII to an Attiny10. The AVRISP mkII User Guide makes no mention of TPI. The mkII is documented as being compatible with the Attiny10, but I had to really hunt around to confirm the pin connections from the programmer to the Attiny10.

Pin connections from here

AVRISP mkII	Target	ATTINY10
connector	pin name	pin number

1 MISO	TPIDATA	1
2 VCC	VCC	5
3 SCK	TPICLK	3
4 MOSI	–	–
5 RESET	RESET	6
6 GND	GND	2

The programmer's Pin 1 is opposite to the red mark on the cable and is marked with a hard-to-see arrow/triangle. I hooked it up wrong the first time, so double check that you've identified pin 1 correctly!

AVRISP mkII connected to Attiny10

From left to right: (1)MOSI/TPIDATA, (2)GND, (3)TIPCLK, (5)VCC, (6)RESET

You need to connect an external 1.8-5.5V power source to VCC and GND before you can program the Attiny10. The programmer's exterior LED will go from red to green when it senses the external power. I use a 3.7V lithium ion battery for power.

Step Three: Program the Attiny10

Launch AVR Studio 5 and make a new assembler project: File:New Project... In the dialog select the AVR Assembler Project template. Then select Attiny10 from the Device Selection dialog that pops up. Write your code (I wanted to blink an LED, so I grabbed and modified some ASM I found here - if you follow that link note that the Attiny10 has PORTB instead of PORTD). This performs a nested decrement of registers. Two registers for the ON delay and three registers for the OFF delay (ie. longer OFF time than ON time). This is what my code looks like:

rjmp RESET  ;go and set up PORTB as an output

;name registers (selected >r15 arbitrarily)
.def counter1 = r16
.def counter2 = r17
.def counter3 = r18

;set some variables
;time1 and time2 set the value for the final loop in each delay
.equ time1   = 170 ;between 0 and 255
.equ time2   = 1
.equ led     = 2 ;LED at PB2

RESET: ;set PB2 as an output in the Data Direction Register for PORTB
sbi   DDRB, led  ;connect LED to PB2 (Attiny10 pin 4)

flash: ;main loop

cbi   PORTB, led ;LED off - cbi/sbi swapped for N-FET switching (ie.LED is OFF when FET is ON)
ldi   counter2, time1 ;load counter1 delay
rcall onDelay
sbi   PORTB, led ;LED on
ldi   counter3, time2 ;load counter3 delay
rcall offDelay
rjmp flash  ;return to beginning of loop

onDelay:
clr   counter1  ;clear counter1

loop1: ;nested loop that decrements counter 1 (255) x counter2 (time1) times (ie. 255*time1)
dec   counter1 ;decrement counter1
brne loop1   ;branch if not 0
dec   counter2 ;decrement counter2
brne loop1   ;branch if not 0
ret

offDelay: ;same as onDelay but with a third loop
clr   counter1
clr   counter2

loop2: ;decrement counter 1(255) x counter2(255) x counter3(time2) (ie. 255*255*time2)
dec   counter1
brne loop2
dec   counter2
brne loop2
dec   counter3
brne loop2
ret

This is pretty rudimentary. It basically just kills time by pushing bits around registers. A more elegant solution would be to set up an interrupt triggered by the Attiny10's 16 bit timer (more on that another time).

Build your code by selecting Build:Build Solution. This converts the ASM to machine code. If it builds successfully it will generate a .hex file in the project's folder. This is the file you need to load onto the Attiny10. You can simulate your code by going to Debug:Start Debugging and Break and and then hitting F10 to step through the instructions and looking at how the contents of registers and ports change, etc.

When satisfied, you program the chip using Tools:AVR Programming. Select AVRISP mkII in the Tool drop down list and Attiny10 in the Device list. TPI will come up in the Interface list by default. Click 'Apply'. If there is a connection problem, you will discover it here. This is the time to make sure that the Attiny has its own power and is connected to the AVRISP correctly.

Go to Memories and click '...' to select your project's .hex file if the contents of the Flash box aren't pointing to it already. Then click 'Program'. The programmer will load the code and then you can test it out.

I found that the programmer needed to be reset by unplugging it from USB and then reconnecting it before I could get each upload to run properly on the Attiny10. I chose PB2 (pin 4) to blink my LED because it isn't connected to any programmer lines, but the other lines should work even if connected to the programmer, so long as it has USB power. If the programmer is off then the Attiny10 didn't like being connected to it. Once satisfied with your program you can disconnect the programer lines and operate it stand alone:

Attiny10 blinking an LED

These are the two sites I found most useful for learning the ins and outs of Attiny10 programming:

http://irq5.wordpress.com/2010/07/15/programming-the-attiny10/
https://sites.google.com/site/wayneholder/attiny-4-5-9-10-assembly-ide-and-programmer

There's also information scattered all over the avrfreaks forums.

Beautiful retro styled lights by Kimura from Jitensha Studio

2012-01-17T17:57:00.000-05:00

Browsing around today I came across these for the first time:

Kimura flashlight

Kimura tail lamp

They are Kimura lights from Jitenshi Studio, a bicycle shop in Japan that specializes in high quality touring and randonneuring bikes. The lights are handmade by one Mr. Kimura. They're not inexpensive ($195 USD for the flashlight, $125 for the taillight), but the quality looks amazing, easily rivalling similarly priced offerings by Schmidt or Supernova, but with infinitely more retro charm. Too bad Mr. Kimura isn't in the business of making dynamo lights.

Mounting a vintage flashlight on a front rack seems to be a popular way to dress up your vintage (or modern classical) touring/randonneuring bike. Historically I think these flashlights served as auxiliary lighting for bottle dynamo systems when stopped. Here is a particularly gorgeous example from the Velo-Orange site:

Vintage flashlight on a bike. Image from Velo-Orange.

The tail lamp is especially intriguing. I wonder if the lens is pulled from a mass-produced light or if it was made specially for this application. They also have a nice selection of reflectors.

Dynamo digital buck converter: progress report

2012-01-02T17:31:00.000-05:00

Inspired by this thread at CPF, I built a prototype of a digital buck converter using the Arduino development platform. The point of it is to extract more power out of a hub dynamo without adding extra LEDs in series. To rehash: modern hub dynamos typically saturate at about 500-600 mA and their voltage is primarily determined by the load, so the easiest way to passively get more power of them is to add more LEDs in series. Each LED gets the same current in the series circuit, while Vf of the whole series circuit goes up by the forward voltage of each additional LED. White LEDs typically have forward voltages of about 3V, and most hub dynamos easily put out 6V at low speeds, making this a viable arrangement. Even 3 series LEDs with a Vf of 9V can produce a lot of light at low speeds, although the minimum speed that reaches Vf will, of course, be higher than with fewer LEDs. However, I'm primarily interested in single LED designs for retrofitting into vintage lamp housings. With proper heat sinking some white power LEDs can take well over 1A. In order to extract more current out of the hub we need a buck converter that converts the higher voltage available from the dynamo at higher speeds into more current for the LEDs. This is done by switching the buck converter with pulse widths below 100% duty cycle. Here's the schematic:

Schematic for Arduino-controlled digital buck converter for hub dynamo

This is a synchronous buck converter that uses two switches rather than a single switch and diode. This increases efficiency by reducing switching losses (ie. the voltage drop across the diode). MCP14628 is a MOSFET driver specifically designed for digitally controlled SMPSs; when UGATE is high, LGATE is low and there's a buil charge pump to drive the high side FET. The PWM signal comes from the Arduino. This circuit worked fine with a bench top power supply, taking a higher voltage input and converting it to a higher current at Vf of the series LEDs. However, when I connected it to a dynamo Q1 kept getting fried. This was probably due to Vdyn (rectified DC dynamo input) spiking when the load is disconnected while Q1 is off, exceeding Q1's maximum Vgs. I added D3 to take care of this (Vgs(max) of these FETs is 30V). Despite this, I still would occasionally fry Q1. C3 and D2 were added to protect the high side FET from getting zapped by exceeding its Vgs, this time perhaps due to parasitic inductance of the long wires in my messy breadboarded prototype. C3 and D2 seem to work and during normal operation Q1 doesn't get fried anymore. However, when the load is disconnected or I mistakingly set the duty cycle to 0% in the software, Q1 gets toasted. I asked for help here, and am starting to get some answers. Making C3 larger might be a good start. It is situations like this one where my complete lack of electrical engineering training becomes a bit of a barrier to progress...

The ZXCT1009 is a high side current monitor that is used as feedback for the PWM signal.

Here's what it looks like on the breadboard:

Buck converter spaghetti!

Pretty messy, which is certainly reducing efficiency. Once it's tidily committed to a PCB I hope the efficiency will go up a bit and there won't be as many issues with parasitic inductance. I'd also like to double the switching frequency from 32 KHz to 64 KHz so I can use a smaller inductor.

Here's how my buck converter performs with 2 series LEDs with a Vf of about 5V powered by a Sanyo H27 dynamo:

Current versus speed of a dynamo powered buck converter at different duty cycles. Switching frequency is 32 KHz

The advantage of dropping the duty cycle appears just over 20 km/h, although it is quite modest. Above 30 km/h I think the converter is getting a worthwhile amount of extra power, roughly 200 mA (1W or about 40% more). Efficiency is roughly 80%. I need to test other duty cycles and perhaps go below 50%. Thus far it seems like there's not much peak power tracking to be done. A simple implementation would be to leave the duty cycle at 100% below 20 km/h and then drop it to 50% above that. The H27 saturates at around 500 mA, whereas other hub dynamos saturate at slightly higher currents. Perhaps they also have better low speed performance and if they can generate a higher output voltage at lower speed then there might be more of an advantage to using a buck converter.

There is another microcontroller-based dynamo LED driver out there. Its performance seems to be considerably better than mine; it produces surprising amounts of power at low speeds, whereas mine can only begin to harvest more power out of the hub just before it saturates. To be fair, because of my specific application, I am measuring current into two series LEDs rather than total power output, so this is a bit of an apples to oranges comparison. However, there is no denying that this converter seems to perform better at speeds below 30 km/h. I naively speculate that its switching topology is something other than buck, but really I have no idea how it's done or how complicated or expensive the implementation is.

I'm not entirely sure if the complication of a buck converter is worth the effort. Human perception of brightness is commonly described as logarithmic, meaning that a linear increase in actual luminous flux does not result in a correspondingly linear increase in the perceived brightness. I've not really found this put very succinctly on the Internet, so this might be an oversimplification (and hopefully not entirely incorrect).

Whether or not this whole exercise is mostly academic remains to be determined...

Sturmey Archer Dynohub magnet upgrade: progress report

2011-12-30T17:35:00.000-05:00

I designed a magnet adapter to hold 20 neodymium magnets in the same positions as the poles of the original Sturmey Archer GH6 Dynohub ring magnet. The first adapter I made was for 0.125" block magnets. This adapter worked better than I expected, more than doubling the power output available from the original magnet. Well, it turns out that, after calibrating my current monitor, the upgraded magnets work even better than I first thought, nearly tripling the dynamo's power output. In a short-circuit current reading on my multimeter, the upgraded hub saturates at around 1.0A! Running two power LEDs in series, it gets to around 0.9A at about 45 km/h.

Great. However, this is actually probably more power than I need. While white power LEDs can take well over 1A, the brightest red LEDs I want to use in tail lamps are rated at a maximum of 700 mA. I think I need to tone it down a bit and try to get that saturation current below 0.7A. The other incentive to do this is to reduce the drag. Spinning the wheel by hand in the testing jig doesn't give the impression there is much more resistance from the new magnets, but spinning the armature itself on the upgraded hub is much harder than with the original magnet. So, I designed a new adapter that places 20 0.0625 (1/16)" magnets with the same spacing from the armature (about 0.04"). The sixteenth inch magnets are N40s instead of the N42 eighth inch magnets and they are a lot cheaper. This time I got them from these fine folks, just north of Toronto. As before, they are epoxied on with JB-Weld.

Dynohub magnet adapter with 1/16" Nd magnets in place

Here's how it fits over the armature

I fitted this into the Dynohub wheel on my motorized testing jig and measured the current at different speeds, logging the data with an Arduino:

Current versus speed of GH6 Dynohub

The 1/16" magnets seem to be just about right. At 45 km/h the hub hits just over 700 mA, although it doesn't look like it's quite plateaued. I might be wise to incorporate some back up current limiting circuitry to protect my red LED at very high speeds. The resistance from the 1/16" magnets is about as much as you'd expect from a contemporary hub dynamo. Based on these results, I think I'm going to stick with 1/16" magnets. They're cheaper, easier to install, offer less resistance and produce a peak current that is mostly suitable for the red LEDs I want to use.

The current design leaves the magnets a little exposed. I doubt they'd ever come loose, even in the exceptional case of contact with the armature. Still, I've designed a new version of the magnet holder that leaves them less exposed:

Proposed protective magnet holder

It's more complicated than my current design, so will be more expensive. The exterior machining is to reduce the weight, but it doubles the price. This should weigh about 190 grams. Without the outside machining, it would weigh about 270 grams. Not sure if the extra cost is worth the reduced weight...

I could also get it made out of aluminium, which would make it very light and a little less expensive. The advantage of using steel is that the magnets stick to it and the attraction acts as a clamp while the epoxy sets. An aluminium adapter would require that each magnet be individually clamped to prevent them from jumping out of their slots and sticking to one another.

Upgrading the wedge base bulb in a Sturmey Archer tail lamp

2011-12-27T14:07:00.000-05:00

I'm quite partial to the bullet-shaped tail lamps that were part of the Sturmey Archer Dynohub lighting set (and I'm not the only one). While the head lamp takes a standard E10/MES screw base bulb, the taillight takes a hard-to-find wedge base bulb.

Original capless wedge base bulb. Get 'em while you can at sjscycles.

When I converted my first set of Sturmey Archer lamps to LEDs, I resorted to soldering a jumbo leaded LED onto a piece of PCB that I could wedge into the the bulb holder. Now, in order to take advantage of the extremely bright red Cree XP-E, I used a slightly modified version of my E10/MES LED heat sink to make a wedge base LED bulb.

Cree XP-E red. 131 lumens @ 0.7A!

Copper heat sink for red Cree XP-E

LED board and wedge base board.

The copper heat sink has a #4-40 tapped hole to mount a rectangular piece of PCB perpendicular to the LED platform with a machine screw. Wires come through the heat sink platform and are soldered to the wedge contacts:

Completed wedge base bulb with copper heat sink and red Cree XP-E

To get a good fit in the lamp's spring contacts I added a blob of solder at the base of each contact, on both sides. A tight fit is essential as the heat sink assembly is much heavier than the original glass bulb.

Original bulb and Cree XP-E replacement bulb mounted in lamp

The beautiful bullet!

The Cree XP-E has a maximum current rating of 0.7A. In a passive dynamo system, it'll get up to 0.6A, so I'll need to ensure that the heat sinking is sufficient. Unlike my screw base LED bulb, there isn't really any thermal connection between the heat sink and the bulb base, preventing much heat transfer to the lamp body. I'm hoping that it will run up to 0.7A in flashing mode, the off time lowering the steady state heat sink temperature.

Flashing dynamo light prototype

2011-12-22T16:03:00.000-05:00

As I've discussed previously, I'd like to build a dynamo powered lighting system with a powerful standlight and the option of having the lights flash. This is a feature that doesn't seem to be available on commercially available dynamo lighting systems, which don't flash and typically have rather weak standlights. It took me a long time to develop the circuit and I got a lot of help from the fine people on CPF, particularly in this thread. After a couple of months of tinkering I had a working circuit breadboarded. It charges a 20F supercapacitor using a current limiting load switch and uses a Zetex boost driver to power the LEDs while stopped. A high side P-channel MOSFET switches the LEDs between dynamo power and capacitor power and a low-side N-channel MOSFET is used to flash the LEDs with the output of a (special low-voltage CMOS) 555 timer.

Now the challenge was to turn the breadboarded circuit into something that could fit into the housing of a vintage lamp. This is what I started with:

Breadboarded dynamo circuit with standlight.

First, I needed to capture the schematic:

Schematic of flashing dynamo standlight circuit (click to enlarge)

I used EagleCAD to do this, which is a very capable and free piece of schematic capture and PCB layout software. Most of the components I had to make in my own custom library, which I'll eventually post someday. Most standard packages (SOT23, 0805, SOIC, etc) are already available. I placed the parts as logically as I could and used the autorouter, which did a pretty darn good job of routing everything in a circle about 1.6" in diameter. My design rules included larger minimum trace widths and more generous spacing around components than the default rules. After much revision, I wound up with a two-sided PCB design:

PCB layout in EagleCAD

Sparkfun has an excellent PCB layout tutorial using EagleCAD. I sent the Gerbers off to BatchPCB and a few weeks later received the boards:

BatchPCB boards. Not bad for $3 each!

Now came the hard part. In order to keep things small I chose all surface mount parts. I don't have much experience soldering SMT components, but there are several tutorials on Youtube plus some from Sparkfun. After a couple of hours I felt I had the hang of it and wound up with a reasonably good looking board. For most components I used a soldering iron and occasionally would resort to using a hot air rework station.

Populated PCB. Huge 20F supercapacitor dominates!

It really wasn't as bad as I thought it would be. Fine tweezers are essential! Having a stereomicroscope didn't hurt either (all the time I spent dissecting fruit fly larvae brains and injecting zebrafish embryos is finally paying off...).

To my amazement, the whole thing worked on the first try. The only problem was that I didn't include an input capacitor for the 555 timer which I had on the breadboard. Without it, the 555's output is very erratic while the supercapacitor is charging. I soldered a 10µF leaded capacitor across pin 1 (GND) and pin 8 (Vcc) of the 555 and it worked exactly as it did on the breadboard. Revision 1.1 of the PCB will have to correct that!

The 6.8V Zener, which regulates the input voltage, gets darned hot as it shunts all the dynamo current during the off cycle in flashing mode. This could cause a potential failure of the Zener. I need to test it extensively to see if it fails. If it does, then I'll have to figure out what to do with the dynamo voltage when the LEDs are disconnected during the off period of the flash. No ideas yet.

Another potential pitfall is that the LEDs are connected in series, so if the rear light becomes disconnected, the series circuit will be broken and the front light won't work. The advantage of being in series is that both LEDs get the same current, simplifying the standlight circuit. For now, I think it's a good compromise.

Sturmey Archer GH6 Dynohub magnetic keeper ring

2011-12-21T18:03:00.000-05:00

The Sturmey Archer Dynohub 'Keeper Ring' has become the stuff of legends. Frequently mentioned in discussions about servicing the Dynohub but never described, it is as rare as hens teeth. It is, apparently, essential to maintain the GH6 ring magnet's field strength during circumstances in which the armature needs to removed. A google search did turn up one on auction back in February, which even included a photograph:

The only photograph of a Dynohub keeper ring I could find on the internet

Update: just after posting this I found a wealth of information on the keeper ring in this thread on bike forums, including dimensions and part number!

Other than infrequently turning up on eBay, they are unavailable. Gentleman Cyclist does, however, mention on their parts page that they will soon have keeper rings for the Dynohub (you should take a look at the other interesting bits and bobs they carry for the 3-speed enthusiast).

Well, I had an old Dynohub from '75 that was rusty as all heck. It came from a trashed ladies Raleigh Superbe that was languishing in the basement of Polly's Recycle in the east end. The armature could barely spin within the ring magnet and the whole assembly was covered in a coat of rusty grit. Usually a Dynohub can be serviced without the need for removing the armature from the ring magnet, but in this particular hub's case, the armature need a good going over with a wire brush. So, I had my Internet Material Synthesizer make one out of mild steel #45. Now, I think my Dynohub magnet upgrade makes the original magnet kind of obsolete, but I was getting a bunch of other parts made so I thought I'd get a keeper ring made as well. It arrived today:

Machined Dynohub Keeper Ring

It has a 2.700" outer diameter, the same as the armature, and is 0.75" thick.

Dynohub Alnico ring magnet with armature. Don't remove that magnet without a keeper!

Dynohub ring magnet with keeper

Does it work? I don't know. I pushed the armature out with the keeper, so the field wasn't disrupted. The magnet feels about as strong as it did before the armature was removed. The design is very similar to the original, so I expect it's doing its job.

Do you want one? I can easily get more made. They would cost between $15-$30 each, depending on number I get made in one go. I'm also thinking of offering a magnet upgrade for the Dynohub (this is still a work in progress though...)

If you're interested send me an email (find the address in my profile).

Copper-based LED bulb fitting and performance testing

2011-12-18T11:50:00.000-05:00

I must admit that I was rather enamoured with the elegant design of the finned copper heat sink I had made. Unfortunately, it didn't actually perform as well as I'd hoped, so I revised the design with the aim of getting the LED lower in the parabola of the reflector and giving up on improving the thermal properties (which were just OK). This is basically just a chopped down version of the previous design with a larger hole through the centre and a couple of countersunk holes on the top to ensure complete electrical isolation from the PCB.

Revised LED bulb pillar heat sink design

The Internet Material Synthesizer took about 10 days to make this:

I soldered the Cree XP-G LED (still tricky even with a hot air rework station), ran the wires and used a thermal epoxy to stick it into an E10/MES Edison threaded bulb base:

E10/MES Edison LED bulb with copper heat sink

This can now be threaded into any flashlight or vintage bike lamp that takes a miniature screw bulb (albeit in need of a separate driver circuit). I first tried it out in a Sturmey Archer head lamp:

Flimsy bulb holder of SA headlight

Unfortunately, the SA lamp has a rather low quality and flimsy way of mounting the bulb in the reflector. The actual bulb holder is a thin piece of stamped metal that doesn't hold the bulb straight. This is then pressed in and held by friction in the base of the reflector, where the positive bulb end makes contact with a springy metal tab:

The switch quality is terrible and even a NOS lamp had enough oxidation on the contacts to make switching finicky and provided enough resistance that getting the full current available current from the power supply wasn't possible. So, the original switch and contacts are useless. I'll have to retrofit a higher quality switch on the bottom of the lamp and leave the original switch purely for aesthetics.

Ok, so I've given up on fitting a screw bulb into the original holder of the SA head lamp. I think I can design something that can be pressed (and maybe epoxied) into the reflector. It will be simpler, won't rely on the flaky original contacts and will probably have better thermal properties.

So, how does the bulb perform in a decent quality lamp holder? For this, I turned to the trusty and well-designed bulb holder of a Luxor lamp. Conveniently (and unlike SA lamps), both head and tail lights use the same well-made bulb holder:

Luxor makes a better bulb holder than Sturmey Archer

It uses side springs to snap securely into a the base of the lens, which is then fitted into the lamp body with clips. I drove the LED at different currents from a bench top supply and measured the temperature with my thermocouple-equipped multimeter.

LED bulb ready for testing! Brown wire is thermocouple.

Now that I've automated data logging of wheel speed, LED current and light output, I should probably just go ahead and make an Arduino-based thermocouple to log temperature as well. However, that hasn't happened yet, so all I did was log lux data over 20 minutes, and make note of the steady state end temperature.

One requirement of my LED bulb is that it outperform commercially available E10 LED bulbs, so I pitted my bulb against the (I think) now discontinued TerraLUX MiniStar1 TLE-1S (the flanged version still seems to be available). It is rated at 50 lumens and is the same bulb that I used in my original (and doomed) Sturmey Archer LED retrofit. I also tried out the 35 lumen E10-WHP, but it put out a paltry 6 lux at 1 meter, so I didn't bother logging the data. The TLE-1S is rated 1W with its own built in driver. Given 3.4V, it sucked up 0.35A, so about 1.2W.

The defending champion: TLE-1S

Here are the results:

Lux data (no secondary optics) collected every 30 seconds over twenty minutes.

Great, the TLE-1S gets blown out of the water by my LED bulb. It also got surprisingly hot (74.5°C!), although its output didn't drop much as a consequence. Now, to be fair, I didn't collect data for my bulb at 0.35A, but that's because I have no intention of running it at 0.35A. Ultimately, it will get any one of the three currents tested: 0.5-0.6A from direct dynamo output or 0.75-1A from a dynohub magnet upgrade or buck driver. Still, speaking as a former scientist, this experiment was not properly controlled...

Surprisingly, the revised LED bulb performance is better than my original design; although the initial output is about 3-5 lux lower across the board (this could be due to LED-to-LED variability as well as the fact that I burned off the LED dome of my first prototype!), the thermal performance is better, resulting in lower steady state temperatures and, consequently, less light drop. This is probably because the bulb base is in contact with the fairly bulky Luxor bulb holder, which is increasing the surface area by at least two fold. Although the contact area between the bulb base and holder is small it seems to be enough to improve thermal performance.

Overall, I'm pleased with the result. This bulb will work at high currents in a variety of vintage tail lamps and, at the very least, in the Luxor head lamp. Unfortunately, the bulb holders of both the Sturmey Archer head lamp and the Radios No. 18 are too flimsy and will require individualized solutions. I think I can design a simple replacement LED/holder combination that won't require any modification to the lamp.

One problem is that I still think the LED is a little high in the parabola of the Luxor reflector. Not sure how much more length I can shave off my copper pillar while leaving enough room for mounting and wire routing holes...

Logging lux data with a hacked DX light meter

2011-12-15T00:53:00.000-05:00

I've been using an Arduino Uno to log data for my buck converter project. I've been logging the LED current using a high side current monitor and the wheel speed on my testing jig using the pulsed output of the dynamo, which I can count using a clever zero cross detector that is inherent to the inputs of all AVR microcontrollers. The data is sent to a computer via serial interface. Measuring LED current is all well and good, but the real performance measurement is the LED light output, which is not only current dependent, but also temperature dependent. So, I got a cheap-o digital lux meter from DealExtreme branded as 'Ceto', which allowed me to take a few readings to see how the light output dropped with increased LED temperature.

DealExtreme SKU 5100 digital light meter

It would be nice, though, if I could log this data digitally in the same way I can log current and velocity data. I thought there might be a way to decode the signal going to the LCD to get a digital output, but it turns out that this is way complicated, requiring, at the very least, a datasheet for the LCD (and a touch of genius doesn't hurt either). Disappointed, I resorted to looking up the identifiable ICs on the printed circuit board. There are some quad gates, bilateral switches and a dual flip flop, which I speculate are involved in analog to digital conversion and range switching. Anyway, there was one lonely 27M2BC dual opamp right by the sensor input that looked promising. The datasheet even has a little "Photo-Diode Amplifier" application circuit on page 32.

27M2B opamp on lux meter circuit board

Pin 7 has an output whose voltage matches the lux reading on the LCD

I probed around a bit to see if the designers had used the same circuit in the datasheet, but quickly found out that this chip didn't have the same pinouts as the label on the package suggested. Maybe it's a counterfeit? The Texas Instruments logo sure does look a bit fuzzy. In any case, I found one pin (pin 7) whose voltage changes with light input, from 0 to about 2.2V. Turns out the lux value displayed on the LCD is a base 10 multiple of the voltage on pin 7, the order of magnitude being determined by the range switch. For example, when in the 2000 lux range, 0.178V = 178 lux:

Opamp output voltage is scaled by some factor of 10 to the actual lux reading

So, all that needs to be done is to read the voltage on the opamp's pin into the Arduino, multiply according to the range setting and be done with it. The only issue is that my current monitor is read relative to the Arduino's 1.1V internal reference, so any value over 1.1 volts will be clipped by the ADC. This essentially halves the lux value that each range can read, which isn't too big a deal. The default reference of 5V would allow the full range to be measured.

I put a 3.5 mm mono jack in the front panel to get the signal out to the Arduino:

Pin 7 is brought out to the front panel via 3.5 mm mono jack

Some tweaking in code was required to get an accurate voltage reading. I'm using a fast ADC conversion on the Arduino, which makes things a little noisier.

Arduino pulling data from lux meter

So, now I'll be able to gather lux data at different speeds!

Vintage Radios bicycle lamps

2011-12-13T23:24:00.000-05:00

I've only been able to find a little bit about these lamps. I have a Radios No. 18 head lamp. Its reflector is slightly splotchy and tarnished and there is a little dent at the point. They are a pretty and classic example of the streamlined bullet style of the era with a little Art Deco flare on the spring loaded lens release. A recent auction on Ebay France turned up a couple of really nice specimens. Unfortunately, somebody else wanted them more badly than I did. Photos are from the seller.

They seem to be going up in price with some NOS examples going for over $100. Someday I'd like to get a nicer No. 18 like the one pictured here, with what looks like a spotless and well-preserved reflector. The fender-mounted tail lamp is also very beautiful:

Hot LED: lux readings of a Cree XP-G on a copper heatsink

2011-12-09T12:25:00.000-05:00

After musing about it for a while, I got some advice from Kevin, of Lambda Lights, on designing a copper heat sink for my LED E10 bulb project. Kevin's elegant solution to heat sinking is to solder the LED's thermal pad directly to a copper nub poking through a slot milled in the PCB. However, he ultimately concluded that there would be little point to machining pillars out of copper for an LED light bulb application as the heat has nowhere to go beyond the surface of the copper (ie. it is not attached to a larger heat sink or in contact with a housing or chassis of any kind). My enthusiasm and greenness beat out Kevin's sage wisdom and I went ahead and had a prototype copper heat sink made for a Cree XLamp XP-G. I had them machined by a company in Shenzhen that did a great job for a great price (less than one tenth what a couple of USA prototyping services quoted me). I reasoned that adding a few fins would aid in shedding heat to the surrounding air, so the design became fairly baroque:

Copper LED heat sink for Cree XP-G. The fins add about 2 square inches of surface area.

In a nifty kind of internet 'digital to analogue' conversion, my design went from virtuality to reality in 10 days:

Copper nub pokes through hole milled in PCB

For the first time I used Seeed Studio's (yes, there are 3 Es there!) Fusion PCB service, which turned out to be cheaper and faster than than BatchPCB, although the board quality doesn't seem to be quite as nice. The main impetus for using Seeed was that they can mill internal slots in their boards, whereas BatchPCB cannot.

I soldered the LED on to the board and heat sink with a heat gun, which proved to be a challenge; I quite literally blackened one side of the PCB and melted off the dome of the LED! The LED survived, although I don't know if its performance is at all affected by its rather rough (and out of spec) treatment. This experience prompted me to finally invest in a hot air rework station, which is currently in the mail.

Cree XP-G soldered directly to copper heat sink. Overzealous heat gun use

After a few days of admiring my creation, a couple of things arrived that actually allowed me to test its performance. After wanting one for years, I finally caved and got a Fluke multimeter. It's the 'Not For Sale Outside of China' 17B model available through DealExtreme. Genuine Fluke for a about one third of a similarly equipped US market model, although the warranty is automatically voided by export. Most relevant for this application though is that it has a thermocouple thermometer, so I can measure the temperature of the heat sink by direct contact. With a bit of thermal grease, I was able to wedge it into the space between fins and make good contact with the heat sink.

Thermocouple on LED heat sink with thermal grease

New Fluke 17B multimeter!

In the same package was a cheap lux meter to let me measure the relative output of the LED being driven at different currents at different temperatures.

Rudimentary lux meter set up. Response time is impressive: meter is showing lux from the camera flash!

My excitement was tempered almost immediately by two realizations: First, my design puts the LED way too far above the vertex of the parabola of the reflector, negating any advantage of using the original vintage optics. That was kind of a dumb oversight! Second, as Kevin predicted, the heat sink just gets hot; there isn't enough air flow to cool the heat sink sufficiently, even with the addition of fins. Running at 1A it takes about 20 minutes or so for the heat sink to plateau at almost 100°C. That is considerably hotter than my goal of keeping the heat sink at or below 80°C. Luminous flux decreases as the junction temperature of the LED increases, reducing efficiency and LED life.

I measured the lux at different currents, reading the initial lux as well as the lux once the heat sink had reached a steady state temperature. Readings were taken at 1 meter in an otherwise dark room (lux = 0) without any secondary LED optics (and a missing LED dome!). Ambient temperature was about 22°C.

As you can see, a higher steady state temperature results in less light output. About 20% reduction at 1A, 13% at 0.75A and 9% at 0.5A. So, I think there will be little point to driving these LED bulbs beyond 0.75A, which is about what an upgraded GH6 Dynohub can put out. Of course, I have to change the design to drop the LED down to the approximate level of a bulb filament so that it's in the optimal position within the parabola of the reflector. This necessarily will be a smaller heat sink (lower mass and surface area), so it will heat up faster but I expect the steady state temperature shouldn't be too much higher. Waiting for the new design in the mail to find out...

Hetchins dream bicycle with Dynohub

2011-12-01T03:03:00.000-05:00

1947 Hetchins Super Special

Currently on auction. This is my dream bicycle configuration. Essentially a Hetchins version of the Raleigh Lenton Sports (a kind of club bike), it combines the reliability and elegance of a Sturmey Archer 3-speed hub (in this case the very rare ASC fixed gear version) and the utilitarianism of fenders with a lightweight and sporty racing frame. With a GH6 Dynohub and light set, this represents a perfect union of style and function. If only I was shorter and richer...

GH6 Dynohub

Full album is here.

Peter C. Kohler has written a great deal about the Lenton Sports, which is worth looking at if only for the great collection of vintage ads.

Raleigh Lenton Sports from Peter Kohler's article.

Sturmey Archer Dynohub magnet upgrade: early results

2011-11-28T18:02:00.001-05:00

My idea for a GH6 magnet upgrade seems to be working. I had an adapter machined out of mild steel to space out 20 0.25" x 0.75" x 0.125" neodymium magnets, which I attached with J-B Weld. It took a while to fit it into the hub shell and I ultimately had to steal a spacer clip from another GH6 to make it fit without the armature rubbing. Unfortunately, I didn't document any of this with pictures, so you'll just have to take my word for it for now! The cogging effect is quite strong, but once mounted in the fork the wheel spins fairly easily. Not sure how this will translate into actual drag while riding.

I connected it to my test circuit, which consists of an Arduino controlled buck converter driving two LEDs in series at 100% duty cycle. The series forward voltage of the two LEDs is about 5V so you can infer power from the current but I didn't actually measure Vf, which goes up modestly as current rises. The data was acquired on my motorized testing jig. I'm using the Arduino to log the current at different speeds. Speed is calculated using zero-cross detection to measure the frequency of the pulses coming from the hub. My jig has crappy speed control so all I can do is ramp it up and slow it down and catch the current at each speed. At each speed I collect the average current of two complete wheel rotations (again, as measured using hub pulse frequency) four times and plot the average of that. Error bars are standard deviation of the mean.

The hub seems to saturate around 750 mA, although my multimeter is reading 900~~-1000~~ mA at higher speeds (update: confirmed with another multimeter that the hub is indeed saturating at 900 mA, so my high side current monitor is not linear :( grrrr!). Thus far, it looks like the magnet upgrade is a success. I was worried, based on the accounts of others, that the armature would saturate at a much lower current, but that doesn't seem to be the case here. I'm going to try to use 1/16" block magnets to see if I can get a similar power output with less drag. There is a chance that the 1/8" magnets are overkill, causing unnecessary drag and reducing efficiency.

Decreasing the gap between the magnets and the claw poles of the armature might also help make the hub more efficient. At the moment they are about 0.050-0.060", whereas the original magnet is probably closer to 0.010-0.020".

Will post photos of the upgrade as soon as I have them!

Update: I found a photo of the adapter sticking to my fridge.

GH6 magnet adapter upgrade with Neodymium magnets in place

Update 2: here it is mounted in the hub shell with the armature (the magnet cover plate has been removed)

Specific aim 4: a Dynohub magnet upgrade

2011-11-18T18:34:00.000-05:00

I think I have established that I have an unhealthy fascination with the Sturmey Archer GH6 dynohub. The hub has an output of about 1.8W, although doing a little experimenting with my digital buck converter on my dynamo testing jig, it appears that the hub reaches its saturation current at considerably higher speeds than contemporary hub dynamos (at least in my test setup with a 1966 GH6). If the point of the buck converter is to extract more power out of the hub, there's not much practical use if that power isn't available until you're going 40 km/h!

Speed versus current of a GH6 powering two series LEDs through a digital buck converter at 100% and 50% pulse width duty cycle. Error bars are standard deviation from the mean of 4 readings taken at each speed.

The GH6 is old technology and seems primarily limited by the fact that it has a rather weak 20 pole Alnico magnet, which is reported to lose a significant amount of flux over time. The few GH6 experts out there maintain that if a Dynohub has been serviced there is a good chance that the mechanic damaged the magnet by removing the armature. Amazingly, Jobst Brandt, of The Bicycle Wheel fame, built a re-magnetizing device (very bottom of page) to refurbish the magnet in the event of the hub being disassembled improperly.

GH6 20 pole manget (GL343) and armature (GL603)

I wondered if it would be possible to upgrade the magnet with modern Neodymium magnets. After some more searching I discovered that I wasn't the only one that had thought of this. Ben over at gotwind.org tried a couple of configurations and got something that worked, although not well enough for his application. His results are promising for my application though as he reports getting higher voltage due to the increased flux of the new magnets and perhaps even a higher saturation current.

Ben@gotwind.org replaced the 20 pole Alnico ring magnet with 20 neodymium magnets in a GH6 hub

Based on Ben's description, a magnet upgrade to the GH6 might be able to bring it up to spec with modern hub dynamos (rated voltage at low speeds, saturation current around 0.5A), albeit with more drag and probably lower efficiency. After some pondering and a bit more research, I think I'm going to have a go at making an adapter to fit the GH6 with 20 new Neodymium magnets. One planned improvement over Ben's efforts will be to keep the spacing between the stator and the magnets as close to the original specs as possible. The actually spacing is probably in the order of 0.010 to 0.020". I'm aiming for around 0.050", whereas Ben's design is reported to have 3mm (0.118") spacing. The trick will be to pick magnets of appropriate strength that are strong enough to saturate the stator at low speeds, but not so strong that they add undesirable cogging effects and drag. Thus far, I have a design I'm shopping around to machinists:

Proposed magnet adapter with armature

3D view of proposed GH6 magnet adapter

Not sure yet what material it should be made of. At first, I thought of using ferromagnetic steel so the magnets would stick to it. This is more expensive and heavier than using 6061 aluminum, which might be a better solution as the longer I consider it the more I realize that whatever the material, I'll need to epoxy the magnets in place.

So, fingers crossed that these efforts will result in a GH6 with improved lower speed performance!

Update: magnets arrived today and, boy, are they strong! It's been said that Nd magnets are 10 times stronger than Alnico magnets. Now that I have some in hand I believe it. As stated previously, I think the challenge will be to use a magnet that is just strong enough to saturate the stator but not so strong that it adds additional drag due to cogging torque.

Specific aim 3: a dynamo wheel testing jig

2011-11-17T18:27:00.000-05:00

In order to test my buck converter with a dynamo I need to be able to spin the wheel at different constant speeds. This is not done practically by spinning the wheel by hand, so some kind of motorized system is required. Fortunately, (DIY dynamo lighting legend) Martin over at pilom has already built one and he gave me some helpful advice on how to build my own. An abandoned bicycle fork recovered serendipitously from under a bridge and a washing machine motor salvaged from my in-laws' basement are the main ingredients. Rudimentary speed control comes from a router speed controller, which I believe is TRIAC-based. It took about 3 hours in my Dad's wood shop to put it all together:

hub dynamo testing jig with 700C wheel

1/3 hp washing machine motor. Routed slots allow motor adjustment to accommodate different sized wheels

The 6 inch drive wheel is based on achieving a maximum speed of 50 kph, or close to 400 RPM, from the motor's full speed of 1750 RPM. Speed control is not great; the sweet spot cruising speed (25-35 kph) is very difficult to maintain. In this region the motor's relay cycles on and off, resulting in a sinusoidal speed with a period of a few seconds. Constant speeds in the 5 kph - 20 kph are easily achieved, then it cycles and finally takes off to a maximum speed.

So far so good. The contraption remains stable with minimum vibration at high speeds. I ordered a super cheap bicycle computer from DealExtreme to measure the wheel speed, but while I waited for it to arrive I figured how to calculate the speed based on the pulses coming out of the hub using a zero cross detector. The cycle computer has arrived, so I'll be able to see how close my zero cross dynamo speedometer works (more on that later...).

Specific aim 2: a dynamo digital buck converter

2011-11-16T18:25:00.000-05:00

I am developing hardware and software for a dynamo peak power tracking system. Maximum Power Point Tracking (MPPT) is something I learned about over on a thread at CPF and have subsequently become fascinated about. Widely applied in the solar industry to extract the maximum available power from a photovoltaic array under all conditions of illumination, MPPT is, in essence, a way to tweak the load resistance to reach a peak power point on a current/voltage curve under dynamic conditions. Bicycle dynamos operate primarily as constant current sources. They reach a saturation current at reasonably low speeds and the only extra power to be gained from a 'passive' system is through the increase in voltage that comes at higher speeds. Passively extracting more power is done typically by adding more LEDs in series. If you have 1 LED with a Vf of 3.0V you can get a maximum of 3V x 0.5A = 1.5W. If you add a second 3.0V LED in series you can get 6V x 0.5A = 3.0W, a third gives you 9V x 0.5A = 4.5W, etc. The problem here is that for each LED added there is an increase in the minium speed at which the dynamo reaches Vf of the series chain. Below that you get no light, or, at best, very blinky lights. However, there are some clever passive low speed boost circuits out there to get more power at lower speeds.

Today's power LEDs have maximum currents in the range of 1-3A, so a dynamo that saturates at 0.5 or 0.6A is not able to run these even close to their maximum theoretical outputs. I want to a employ a single LED design for front and rear lamps, so running the LEDs at more than the dynamo saturation current would be useful. In order to pull maximum power out at all times, a switched mode buck converter can be employed. I'm working on a digitally controlled synchronous buck converter that uses LED current and bicycle speed as feedback. Thus far I've developed a peak power tracking system that works with a bench top supply. Essentially, under fixed current conditions (like a hub dynamo), it modifies the pulse width of the switching signal to maximize the power from the available voltage.

200 mA in, 370 mA out!

LED Current vs pulse width duty cycle at different input voltages where Vf of the series LEDs is 4.1V

Putting dynamo LEDs under microprocessor control may seem excessive, but I'm not the first to do it. In theory, it should be possible to drive LEDs up to and beyond 1A at cruising speeds, with the obvious caveat that the extra power has to come from the cyclist's legs!

Buck converter hardware

Hardware is under Arduino µC control

The software still needs some significant tweaking to work with the fluctuating DC of the rectified dynamo output. I'll post a full description of this buck converter soon, hopefully once I have the peak power tracking working with a dynamo. I'm using the Arduino platform for development, which is great and easy to use, but I did need to employ a few under the hood tricks to get it working. I will ultimately have to switch to another, smaller AVR microcontroller for the final design.

Specific aim 1: making an LED light bulb

2011-11-15T18:22:00.000-05:00

I need to mount a single LED with sufficient heat sinking into the base of an E10 Edison screw thread bulb.

I've collected some lovely French lamps. Historically, these would be powered by a bottle dynamo. My intention is to power them using one of the... um... 10 or so hub dynamos that I own. Old incandescent bulbs don't light up much so a LED upgrade is in order. Experience has taught me that commercially available LED E10 Edison bulb upgrades are OK, but I think I can do better, especially with the selection of high power LEDs now available. Power LEDs get hot, so whatever retrofit I come up with will need to be able to deal sufficiently with the heat.

Late night scrawlings

Thus far, I think I have an idea involving a turned copper pillar of some sort that can be pressed/epoxied into an E10 bulb base. Something like this:

Copper pillar LED heat sink

Pressed into something like this:

MES/E10 lamp base

This way, any cycle lamp that takes an E10 threaded bulb can be retrofitted without modification. Getting the heat out of such a small package will be a challenge, though; something that my previously proposed solution might not be up to. The best solution would be to mount the LED directly to the copper heat sink, which eliminates any of the thermal resistance encountered in an FR-4 board perforated with plated vias or in a metal core PCB that is then attached to a heat sink with thermal epoxy.

There are several elaborate methods described for doing this. Lambda Lights seem to have mounting LEDs directly to a large copper heat sink down to a science, so I might contact them before attempting to reinvent the wheel. From what I can tell, they turn a copper pillar and then machine a square nub on the top where a PCB is mounted with a slot milled in it so the nub can poke through and be soldered directly to the thermal pad of the LED. Something like this:

While ideal, this solution may be cost prohibitive...

Sturmey Archer lamps: second attempt

2011-11-14T22:56:00.000-05:00

Back in the summer of 2008 I was quite proud of myself when I managed to get a hold of a set of NOS Sturmey Archer lamps for my wife's Raleigh Superbe from a Canadian seller for about half of the going rate at the time ($67, to be exact). Two years later my faith (what little was left) in humanity was shattered when a very bad man (I've called him worse) very craftily stole the rear light. This faith was somewhat restored when a very nice reader of this blog offered to send me a replacement rear lamp at no charge (thanks Jeff!). A last minute discovery of an auction with less than an hour to go has turned up another NOS Sturmey light set, also going for well below market price (and also, coincidentally, sold by a Canadian seller). I bid, finished my dinner and was delighted (but not surprised) to find that I had won!

Sturmey Archer NOS light set

Just a week ago I won an auction for a NOS GH-6 Dynohub, a 1978, from the same seller:

1978 NOS Sturmey Archer GH-6 Dynohub. Not quite as elegant as earlier models.

Setting goals and things in the works

2011-11-10T18:58:00.000-05:00

Being housebound taking care of two small children leaves the mind with enough excess capacity to come up with an increasingly complex and ambitious list of aims. To try to keep track of all the stuff bumping around up there, I'm going to commit some of these goals to this here blog page.

Specific aims:

1. Mount a single LED with sufficient heatsinking into the base of an E10 Edison screw thread bulb.

2. Develop hardware and software for a dynamo peak power tracking system.

3. Build a speed-controlled dynamo wheel testing jig.

4. Upgrade the Sturmey Archer GH6 magnet to modern Neodymium magnets.

A flurry of posts will address each of these aims.

Luxor dynamo lamps

2011-10-21T21:44:00.000-04:00

These really are my favorite dynamo lights of all time. I have one set of the hammered aluminum Luxor Martele lamps. I find them to be achingly beautiful. Just stumbled across a NOS set on offer through Classic Rendezvous back in February of this year. Have a look.

getting the heat out of LEDs (as cheaply as possible)

2011-10-14T17:03:00.000-04:00

Well, it turns out designing a PCB for power LEDs isn't as simple as I'd previously thought. Now, to be fair, with the intention I had in mind, I was only going to run my power LEDs at about half of their rated maximum current. With that in mind, I'd put a single via through the thermal pad to connect it to a large plane on the bottom side. It was also connected to a plane on the component side. Like so:

Cree XP-G SMT pad with via through thermal pad and connection to component side thermal plane

I figured that was a lot of copper to dissipate the heat. After some more reading, however, I realized that while my design was probably adequate for my current requirements, there was no way I was going to get away with running the LEDs at their maximum current. At about 300-400 mA the thermal planes are quite warm, but not alarming. At 500mA the LEDs on this design do get pretty darn hot to the touch and that isn't good as it reduces their efficiency which ultimately results in more heat, less light and reliability issues. I do have a planned application where I want to run these guys up to 1A or more, so my current PCB design needs revision.

Many power LEDs come mounted on the familiar metal-core PCB star:

Cree LED on MCPCB star

MCPCBs have a metal core that conducts heat much more efficiently than the standard FR-4 PCB laminates from the top layer to the bottom layer (where a heat sink can be attached). In other words, the metal core has a much lower thermal resistance than FR-4; 5-10 times lower. MCPCBs seem like a great solution for mounting LEDs, but they are very expensive to have custom made. Fortunately, there are several application notes out there that describe how to design an FR-4-based PCB using thermal vias to get a top to bottom layer thermal resistance closer to metal core. I used this one from Cree to guide my design. The application note is summarized thusly: instead of using a metal core to get heat away from the LED's thermal pad, use as many thermal vias as you can fit in the thermal pad to transfer the heat away from it through to the bottom layer, where a heat sink can be attached.

First, I had to rejig the XP-G solder pad, as the orignal layout created a real bottleneck for heat transfer:

The big 'I' can now be filled with vias to conduct heat away from the thermal pad. Riddling the board with thermal vias isn't necessary; just getting the heat way from the immediate vicinity of the thermal pad has the greatest effect. Here's what the PCB design looks like:

Lots of thermal vias. On the back there is a 14x14mm square solder pad to attach a copper heat sink:

These are little RAM sinks. You can get 8 for about $15. They come with their own thermal adhesive already attached but the general consensus among user reviews to to scrape that off and use something like Arctic Silver thermal adhesive instead.

Hopefully the vias will provide sufficient transfer to keep the LED cool enough to remain bright and not overheat at full current (1A+). They are the bottleneck between the LED's thermal pad and the heat sink. The application note recommends more vias in the actual thermal pad itself but my cheap PCB fabricator has a minimum drill size that doesn't allow this. Following their recommended design they claim an FR-4 PCB can have a thermal resistance close to that of a metal core PCB. In my case, I doubt I can achieve that with only 3 vias in the pad proper (they recommend something like 14!). If these aren't sufficient I might have to find another fabricator that can do smaller drill hits, although this could ultimately defeat the purpose of doing this on the cheap!

Another potential pitfall here is that BatchPCB, who I'm using for fabrication, only fabricate 1 ounce copper PCBs. Thicker copper would provide a larger conduit to take the heat away, but for the sake of not spending a fortune on prototyping, I'm kind of stuck with their 1 oz limit. I placed the order last night, so I'll know if a few weeks if this design will work.

An ode to the Raleigh Superbe and its Dynohub

2011-09-29T00:24:00.000-04:00

On my walk to work today (I didn't ride as rain was expected and I had the child seat on my Orange

bike) I encountered no fewer than five Raleigh Superbes. A Superbe is a fairly common sight on the streets of Toronto, both the gentlemen's version as well as the lady's step through frame. I usually happen across one or two a week, but five in one day is exceptional.

Superbe from an eBay auction

Another beautiful example from this blog.

Another Superbe from mytenspeeds.

The Superbe came in a variety of colours since its inception, but it is the 'Bronze Green' model of the sixties and seventies that is by far the most common Superbe you see here in Canada. One day, I hope to own one! The Superbe undeniably has classic retro appeal, but what fascinates me most about this bike is its Dynohub lighting system.

Dynohub advert from SA heritage site.

The Sturmey Archer GH-6 Dynohub was the centerpiece of this system and it preceded the recent glut of contemporary hub dynamos by more than 60 years. Introduced in 1945, it was in production until 1984 according to this Sturmey Archer heritage site. It is often mistakingly rated the same as contemporary hub dynamos ( 3W @ 6V) but it actually puts out around 1.8W @ 6V. I've confirmed that my GH-6 saturates at about 330-330 mA, which is consistent with the later power rating.

The Sturmey Archer GH-6 Dynohub. My favorite hub in the whole world.
Photo from Sheldon Brown's Dynohub article.

In the world of LEDs this is still a very respectable amount of current, so the GH-6, despite its antique status, ~~is quite capable of powering a modern lighting system~~ (Update: after more testing I've determined that the Dynohub saturates at speeds above 35 km/h, so is, in fact, not very useful for powering LEDs without a magnet upgrade). They are reasonably easy to disassemble and service, provided the bearing races aren't shot and the armature coils are intact. I pulled one apart that was in very rough shape but, despite a lot of internal corrosion, it seems like the armature windings are intact (at least by testing continuity with a multimeter, although I guess this could also mean they are prematurely shorted!).

Product photo from Sturmey Archer. Oddly, the front hub has been placed in a rear drop out.

I find the GH-6's lopsidedness appealing, although some might call it an odd looking duck with its one gigantic spoke flange on the dynamo side and a regular sized flange on the other. I discovered that this unusual configuration can make getting a wheel built around the GH-6 take a little longer as the build calls for odd spoke lengths that most bike shops need to special order; I had the GH-6 from a Superbe rebuilt into a modern aluminum rim. Before the build I stripped the hub apart, cleaned it and repacked the bearings. Despite its 42 years (at the time of servicing), it cleaned up beautifully and there was hardly any wear on the races. I expect it will last a lifetime.

Choosing LEDs and batchPCB success!

2011-09-27T15:09:00.000-04:00

My main aim is to design a dynamo powered LED lighting system that can be retrofitted into vintage bicycle housings. My earliest approach was to use T10 Edison screw replacement LED bulbs. These are LEDs with a driver circuit retrofitted into a standard 10 mm threaded incandescent bulb housing, which were very common in flashlights until the flange base bulb came into vogue. Almost all of the vintage lamps I've come across use the Edison T10 thread. In theory (and mostly in practice), this is an easy and reasonable way to upgrade vintage lamps to use modern LEDs. All you need is a rectified DC output from the dynamo and the LED's driver circuit handles the rest. However, these LEDs aren't the brightest ones available and you are, for better or worse, at the mercy of the built-in optics of your lamp housing.

After experimenting with these LED flashlight bulbs for a while I decided I wanted to use my own LEDs and drivers. When being driven by a dynamo LEDs don't really require a regulated current source, defeating the purpose of the driver circuit built into these bulbs. The dynamo itself acts mostly like a constant current source nicely limited to 500-600 mA (depending on the hub). Many high power LEDs are available with maximum current limits above what the dynamo can generate. You're really spoiled for choice in this category, so picking one is really just a matter of price. I went for the premium XP-G series for my front light (490lm @ 1.5A), which is step below the XM-L series that can produce a whopping 943lm @ 3A. I am debating between a Cree XP-E (98lm @ 0.7A) or an OSRAM Golden Dragon Plus (118lm @ 1A) in red for the taillight.

Cree XP-G. 490lm if you can get 1.5A into it (and get rid of the heat!)

Keep in mind that, without some extra driver circuitry, you can't drive these LEDs at their maximum current rating with the hub dynamo, nor would you want to; getting out the heat they produce at high currents requires careful thermal management design. So, by picking LEDs with maximum current ratings considerably higher than what the dynamo can generate there should be a fairly wide safety margin for overcurrent and overheating conditions.

Cree XP LEDs are really, really small (3.45 mm square to be exact) and come in a leadless surface mount package. While small is good, it's also a pain in the butt for prototyping. So, I designed a PCB that I should be able to mount in a variety of lamp housings. I'm not exactly sure of the mounting details yet, but this one fits nicely into the Luxor lamp housing. I used EagleCAD to design the PCB and got this:

EagleCAD LED disc board

There's a pad to mount the LED and a spot for a through-hole backplane connector. A via is placed in the thermal pad to connect the top and bottom planes to dissipate heat. There's probably a total of 1.5 square inches or so of PCB that acts as a heatsink. Hopefully this will be enough! I also have a couple of holes for mounting a reflector. The holes are spaced for this particular reflector from DealExtreme, although I've yet to drill and tap the pilot holes in the reflector body. I used BatchPCB to get a few of these made. For $30 I got 8 boards shipped, but they wound up sending me 16! It takes about a month, so not great if you're in a hurry!

LED disc bottom

LED disc top

Ok, now what? Well, with leadless packages you need to reflow; a soldering iron isn't much use. I use the stove top 'frying-pan' method, which is a variation of the hotplate/skillet method (scroll down...). The frying pan method does not employ any temperature control. I just pop the PCB in the hot pan, press it down with tweezers, wait for the solder paste to reflow and then pull it out as fast as I can. This is potentially risky, as most parts have a maximum reflow temperature/time above which you can cause damage. Here is the finished product:

White XP-G on LED disc board with reflector

I put a plastic adhesive reflector on it. Not sure if a reflector will be necessary or if the optics of the original lights will suffice. Here's the red XP-E with 400 mA running through it:

Not much to see here except that it is extremely bright. For the rear light, I doubt I'll need optics. The lamp's lens should be enough to diffuse the light. At this point, I think decisions about which LED to use become a bit philosophical. The Gold Dragon Plus can put out 118 lumens of red light at 1A. The XP-E in the above photograph is probably putting out about 50lm and it is blinding. When flashing it will be even more noticeable. It also gets quite warm, so it might not be a good idea to put more current through it. I'm going to have some PCBs made for the Golden Dragon just to try it out, but I doubt the existing design will be sufficient to handle the heat it will produce at full current.

charging supercapacitors

2011-09-25T12:27:00.000-04:00

Supercapacitors are super because they can have much higher energy densities than capacitors that aren't so super. A capacitor becomes super in the tens to thousands of Farads range, compared to the measly pico to microfarad range of not-super capacitors. The eventual development of cheap supercapacitors seems to be what has led to the 'standlight revolution' in commercial dynamo lighting systems. Back in the old days, when you stopped your lights would go out, although I've seen references to a few obscure vintage lights with battery powered standlights. Now, when you stop, a charge stored in your light's supercapacitor can keep the LED on for a few minutes. Supercapacitors are a good choice for standlight power as they can be charged up very quickly and can go through many many more charge/discharge cycles than any kind of battery.

3V 20F supercapacitor. Yours for only $8.38 CAD!

While these commercial standlights work well, my main complaint about them is that the lights dim significantly when they switch to capacitor power at a stop. Commercial standlights typically use a 5.5V 1F capacitor. This doesn't store enough energy to keep the LEDs on at full brightness, so the discharge of the capacitor is slowed by dimming the LEDs (this applies mostly to the standlight for headlights). Personally, I'd rather have a brighter standlight, which should be possible with a larger supercapacitor. Higher value supercaps are, of course, bigger and more expensive than the little Gold Caps that are used most frequently.

b&m taillight guts

The biggest issue with supercapacitors is that they can only handle small voltages, typically between 2.5 and 3.0V for supercaps over 10F. If you charge them beyond that voltage they can be damaged, or worse. So, strict voltage regulation is required. Another big issue is that you need to limit the current going into a supercapacitor. They'll take what current they can get, as they appear to your dynamo as a much lower resistance load than your LEDs. This means your lights won't come on until your capacitor is charged up. Finally, the low voltage of of the capacitor creates a problem for lighting LEDs. LEDs typically have forward voltages (Vf) in the 2 to 3V range, but don't operate at any voltage below their Vf. So, a 2.7V supercapacitor can only drive a 2V LED until it reaches 2V. With 2V across it the capacitor still has lots of juice left in it, but the LED can't get it out on its own.

This is a functioning DFN-DIP adapter, but it took a while...

My first solution was to double them up in series. Two 2.7V 20F caps can become one 5.4V 10F cap, giving plenty of voltage overhead for powering LEDs. Turns out, voltage balancing is crucial when charging a supercap stack, as any unbalanced voltage can cause one cap in the stack to go overvoltage. Linear Technologies makes some nice looking chips that take care of that balancing and limit the current too. I started with the LTC3225, which, on paper, looked like a perfect solution. I went to the trouble of soldering the tiny 8 pin leadless DFN package onto a homemade DIP board before I wizened up and found an adapter board from Ancaster, Ontario's very own Proto-Advantage. The LTC3225 worked as advertised except for one crucial aspect: despite programming it to charge the caps at 200 mA, I could only get it to charge at 30 mA. Either I was doing something wrong or the one they sent me was way out of spec. Either way, I opted to try another Linear supercap charger, the LTC4425.

LTC4425 application schematic

The LTC4425 worked and I was able to get it charging up a supercap stack at 200 mA (or whatever current was programmed with the Iset resistor). In practice, however, the LTC4425 couldn't limit the inrush current of a discharged stack, which meant that for the first few seconds, all of the available current went into the supercap stack and none went to the LEDs! As soon as the stack had some minimum voltage across it, the current regulation would start working properly.

LTC4425 MSOP-12 package. Pain in the butt!

While this was kind of a bummer, it pushed me towards a cheaper and probably better solution. While working with the LTC4425 I was introduced to a wonderful little chip called the ZXSC310 by the wise folks at candlepowerforums. This is a LED boost driver than can drive LEDs with a Vin as low as 0.8V. With a supercap charged up to 2.7 or 3.0V, there's plenty of voltage overhead to drive power LEDs at reasonable currents. In practice, the 310 worked great and eliminated the need for a stack of supercapacitors, which meant I only needed to accomplish the relatively simple task of charging a single supercapacitor. This is easily achieved with a voltage regulator and a current limiter.

In my case, I chose a 3.0V linear regulator (potentially a bad idea as it has to drop a lot of voltage from the dynamo under certain circumstances), the MIC5209, and a current limiting load switch, the FPF2125. This circuit very effectively limits the current going into the supercapacitor under all circumstances, so the LEDs get their share, eventually getting all the current from the dynamo once the supercap is charged.

Supercapacitor charging circuit

tail lamps: to flash or not to flash

2011-09-22T13:10:00.000-04:00

After the Sturmey Archer bullet lamp was stolen from my wife's 1966 Raleigh Superbe and my rage (which I documented on Craigslist) had subsided, I decided it was time to get working on a new lighting project. I had also been eyeing my Mom's 80s Raleigh Sprite mixte that was sitting neglected and dusty in the garage for years. My wife complained her Superbe was heavy and clunky (it was), and the elegant and lighter mixte was a better size and the frame was in much better shape than the beat up old Superbe. I planned to fix it up, and a new bike was a good excuse for a new lighting system.

One thing that impressed me about the first system I built was how incredibly bright the rear lamp was. It was easily as bright as a the tail light of a moped, if not brighter. My night riding is primarily done in the city where I feel especially vulnerable in traffic from behind, where, of course, I can't see what's coming. Most bicycle lighting (and especially the premium kind), both battery and dynamo-powered, is focussed on the head light. People take painstakingly staged beam shots of their commercial and homemade head lights using the latest and greatest Cree emitters and optics. There seems to be a great amount of interest in the design, output and illumination patterns of head lamps. As I don't often find myself careening down steep hills in the woods or mountains on moonless nights, I must admit that I don't share this fascination with my fellow bike light aficionados. I do, on the other hand, fret quite often about getting mowed down by a car coming at me from behind as I wait to turn left at a traffic light in the city. So, in my lighting system I plan to devote as much or more attention to the design and output of the tail lamp.

First and foremost, I want the tail lamp to be bright. Really, really, really bright! Second, I want it to be diffuse, so it can be visible from a wide angle. Most commercial tail lights fail rather miserably at being both bright and visible at a wide angle. Typically, they use multiple low power LEDs with optics to produce a bright but narrowly focussed beam. I also want to be able to fit the LED and its associated circuitry and heat sinking into a variety of vintage tail lamp housings, which are considerably more elegant than contemporary offerings. I'm especially fond of fender-mounted tail lights.

Finally, I want my tail light to be able to flash. Interestingly, Germany, the mecca of dynamo bicycle lights, has laws governing bicycle illumination and has outlawed flashing bicycle lights. Several other European countries have also made blinking lights illegal. Since these are the markets that dynamo light makers primarily sell to, there are no (as far as I can tell) commercial offerings for flashing dynamo lights. The wise lawmakers of these nations must have well-considered reasons for outlawing blinking lights (although apparently Great Britain has revised their lighting regulations to allow flashers). They mesmerize tired or drunk drivers, interfere with night vision, make distance hard to estimate, annoy other cyclists riding behind you, etc, etc. There seems to be a litany of reasons for their banishment from a nation's pedal cycles. However, over here in North America I can think of two very good reasons why one would want a flashing tail light:

Almost every cyclist uses a flashing tail light for urban night riding. That's just what's been marketed to us by light makers and it is now the unofficial standard of North American bicycle lighting (blame the half-decade ubiquity of the knog or the superlfash!). As a consequence, it's what both cyclists and drivers are accustomed to.

In an after dark urban environment, solid, low power, narrow beamed tail lights have a lot of other light to compete with. In these parts there's been a proliferation of European-style commuter bikes with dynamo lighting. As such, they come equipped with solid tail lights. My experience riding behind such bikes is that their tail lights kind of get lost in the optical cacophony of car lights, street lights, commercial signage, utility and emergency vehicle lights, etc. So, in the same way an emergency vehicle has strobed lights to grab your attention in an already visually busy environment, a bright strobing tail light on a bicycle screams out 'DON'T RUN ME DOWN!'

That said, in cases where flashing lights are not appropriate (avoiding arrest on cycling trips to Europe or long night rides with other cyclists, for instance), there's a requirement for the flasher to be disabled.

During my design process, I discovered one reason why light makers haven't bothered to make a flashing dynamo light. Turns out, they are kind of difficult to implement. The big issue is what to do with the power of the dynamo during the off cycle of the blink. Dynamos are essentially constant current devices and their voltage is primarily determined by their speed and the resistance of the load. So, disconnecting the load (during the off-cycle of a flash) causes the dynamo voltage to spike, which has the potential to make things smoke!

After much effort, though, I now have a working prototype of a dynamo-powered flasher, the details of which I'll post soon.

back to the workbench!

2011-09-22T01:07:00.000-04:00

Well, it was three summers ago that I worked on the lithium ion dynamo charging circuit. The lighting system itself worked very well as a 'be-seen' light for riding around the city, especially the rear light, which was really impressive. The LED was very bright, without optics and the lens on the tail light diffused it nicely to make it visible from a fairly wide angle.

Unfortunately, there were a couple of mechanical and design issues that ultimately led to its retirement. First, the old Sturmey Archer lamp switch was pretty flaky and getting the knob in just the right position to make contact and turn the lamp on could be tedious. The first failure came when one of the internal wires popped loose, most likely due to a combination of vibration and my bad soldering!

The real problem, though, was the lack of a proper load switch in the design. The battery was in parallel with the charging circuit and the LEDs and I think those LEDs could draw more current from the battery than the dynamo could replace while riding. This meant that proper battery charging could only happen during day riding with the lights off. Riding at night for long enough could deplete the battery completely. The charging circuit could provide enough current to drive the LEDs from the dynamo, but they went out, of course, as soon as you stopped.

An improved design would have incorporated a load switch that would disconnect the battery from the LEDs while the bike was moving and connect them only when stopped. However, even lithium ion batteries have a limited lifetime, so in the long run, this design wasn't viable for the obsessive technical idealist in me (in practical terms, with a proper load switch I expect a lithium ion powered standlight would've lasted years). Commercial dynamo lights use supercapacitors to store energy to keep the lights on while stopped. Supercapacitors can be charged up very quickly to hold just enough energy to keep your lights on for a few minutes. Better yet, they can be charged and discharged hundreds of thousands of times (or more) before their performance declines. Adding a supercapacitor requires careful design, requiring both the voltage to be strictly regulated as well as limiting the charge current. At the time, I felt such a design was beyond my technical expertise.

Anyway, the lithium ion standlight on my wife's bike didn't get much use; she was very pregnant by the time I finished the project and didn't really start riding again until autumn of 2009. At this point, some of the aforementioned issues arose, but the impetus to start working again on dynamo lighting circuits in earnest came when some absolute fucker decided that he needed the Sturmey Archer taillight more than my wife's bike needed it. The thief, who knew what he was after, snipped the wire and, with tools he must have brought specifically for the job, removed the taillight and courteously replaced it with a crappy chromed plastic Cateye taillight. Most of the Raleigh Superbes I see on the street have dangling, unused Sturmey bullet tail lamps, so it was just my luck that probably the only other person in a city of 2.5 million people who had a thing for vintage Sturmey Archer tail lamps happened across my wife's bike and decided to steal the rear light.

Around the same time, I was starting to get obsessed with vintage French bicycle lights from the 50s and 60s. Unlike their British and American counterparts, who used chromed steel, many French lamp makers used polished aluminum, resulting in many elegant and well-perserved specimens. After several months hanging around Ebay France, I had secured my first set of hammered Luxor lamps.

So, with a nice set of lamps and the demise of my wife's bike's lighting system, I started to look again into designing a modern dynamo-powered LED lighting system with a standlight that could be retrofitted into vintage lamp housings.

Thus far, much progress has been made, which I will chronicle here!

dynamo lithium ion battery charging

2008-07-10T15:14:00.000-04:00

Introduction

For a while now I've been obsessed with bicycle hub dynamos. Two of my three bikes are equipped with them. I ride quite a lot at night around the streets of Toronto, and for almost half of my year-round commute I ride home from work in the dark. A well designed dynamo-powered bicycle lighting system can provide a surprising amount of light with very little load on the rider and almost no extra drag while riding during the day with the lights off.

I bought my wife a very dilapidated 1966 Raleigh Superbe from a friendly street bike peddler with an enthusiasm for old English 3 speeds. The Superbe was the high-end model of the Raleigh 3 speed family and came equipped with a Sturmey-Archer GH6 6V 2W (some sources say 3W) Dynohub and a shiny chrome light set (unfortunately missing from this example).

I overhauled the Dynohub and was excited to find out that it still worked. After some hunting around on ebay I got lucky and managed to track down a NOS Sturmey Archer chrome light set for a reasonable price (some folks are willing to pay over $100 for a used set!).

Brand new out of the box (hand courtesy of the gentlemen who got far less than 100 bux for his shiny wares).

The original configuration did not throw out much light at low speeds and, of course, the lights went out completely when the bicycle came to a stop. A few modern dynamo lights have a supercapacitor that can power a small auxiliary LED while the bike is stationary (called a standlight). Busch & Muller even make a nice chrome retro-styled headlight with this feature but offer no matching rear light. I have one of these for my bike equipped with a modern 3W Shimano hub, but the old GH6 doesn't quite put out enough juice to satisfactorily light up the 2.4W halogen bulb. Being an obsessive weirdo this just wouldn't do, so I thought I'd try to build a modern dynamo lighting system with a standlight into the original Sturmey Archer lamps for my wife's Raleigh.

Step One: Finding Suitable LEDs

The GH6 has a lower output than modern dynamos, so using LEDs as the main lamps instead of the original incandescent bulbs makes good sense to get high brightness at low speeds. With a total power output of about 2W, I figured 1W front and rear should do. Bicycle dynamos put out AC, so an AC to DC conversion needs to be worked out as well - more on that later...

Scrounging around google, I found that there are several manufacturers that make drop-in LED replacements for flashlight bulbs. The TLE-1S is a 1W white LED with a screw base that fits the Sturmey Archer head lamp. $15.99 for a flashlight bulb? Only a crazy person would want this. They do brag about a 'power push' circuit which is supposed to suck every last drop of juice out of the batteries that are meant to power it without dimming. Most importantly, it accepts a fairly wide input voltage (3-9V) and presumably has some way of limiting the current going into the LED.

Finding an appropriate taillight proved more difficult. The Sturmey Archer tail lamp uses a bulb with an old fashioned automotive wedge base. There are LED replacement bulbs available in a rainbow of colours, but they all seem to be 12V only. I experimented with a mini maglight replacement bulb that used pin contacts and soldered them to the filament terminals of a broken wedge bulb base and then encased the whole thing in silicone, but unlike the TLE-1S the TLE-20 didn't seem to have suitable current limiting circuitry. Unfortunately it took blowing two $12 bulbs to learn this. bummer. :(

I finally settled on a 2V 350mA 1W jumbo red LED from digikey, but it's a bit steep at $5.17. This is just a LED so it will need a current limiting resistor.

Step Two: Designing a standlight

Many modern dynamo bicycle lights use a supercapacitor to power auxiliary low power LEDs, which provide some light while the bicycle is stationary. I only wanted to work with the main bulbs in the front and rear lamps and keeping 2W worth of LEDs lit for more than a few seconds would require some hefty supercapacitors. I originally tried to work out a design using a 120F supercapacitor, which is slightly shorter and fatter than an AA battery. The supercapacitor solution seems the most elegant, but with a typical supercapacitor rating of only 2.3V and the dynamo's output ranging from 3-20 VDC (after running it through a bridge rectifier) the design requirements quickly exceeded my ability.

Using a battery to power the LEDs and using the dynohub to recharge the battery seemed like a less elegant but more practical solution to keeping the lights on when stopped. There are a few resources for dynamo-powered battery chargers out there, but they were too complicated and bulky for my requirements. I needed a small charging circuit and a small battery that would fit together into the housing of the headlamp. High capacity 3.6V lithium ion CR123A camera batteries are a good fit and are available for only a few bucks complete with a built-in circuit to prevent over-charging, over-discharging and over-heating. I went with the Ultrafire Li-Ion from DealExtreme ($5.08 for two!)

Here's the 880mAh Ultrafire with its cover peeled off to reveal the protection circuitry.

Step 3: Design a regulated battery charging circuit

Single cell lithium ion batteries have very specific charging requirements. Just connecting the rectified output of the hub wasn't going to cut it. For starters, 3.6V Li-Ion cells need exactly 4.1 or 4.2V and carefully controlled current regulation during charging, but the GH6 throws out anywhere from 3V at low speeds to over 20V at higher speeds (based on my measurements of the rectified output).

Fortunately, there are several integrated circuits available that are designed specifically to monitor the charging of Li-Ion cells. I went with the simplest one I could fine: the National Semiconductor LM3622, available from Digikey for $3 (or you can ask National for samples). The LM3622AM-4.1-ND takes anything from 4.5V up to 24V and puts out exactly 4.1V. It controls the current going into the cell by monitoring the cell's charge status and in its most basic configuration requires only 5 external components. National even has a handy application note about it, which suggests exactly what components and values to use for the highest efficiency.

Perfect! There's only one problem:

It only comes in a tiny 4.9mm x 3.9mm SOIC8 surface mount package. Having only ever soldered through hole components, I thought I'd hit a wall. But, it turns out that the SOIC8 is one of the larger SMD packages out there and that soldering with conventional techniques is not that bad (although I remained skeptical until I actually did it).

So, I designed the charging circuit, including a bridge rectifier, based on the design in National's application note. To make the schematic and layout the circuit board I used EagleCAD, a difficult to learn but ultimately useful FREE (for my needs at least) software package that can run on Mac OS 10. To make things even easier, Lady Ada has EagleCAD files on her site that include an LM3622 device used in her Wave Bubble, saving me the trouble of having to figure out how to configure custom devices in already hard-to-learn software.

Step 4: Test a prototype

I bought all the parts from Digikey, including an SOIC-8 to DIP-8 adapter (virtual robbery at $6.43 a piece) to facilitate the use of a prototyping breadboard.

Soldering the SOIC8 package to the DIP adapter proved to be easier than I thought. I was even drunk and shaky from too much coffee.

Using a regulated DC power source, I was able to get 4.09V over a range of input voltages and when connected to a discharged Li-Ion cell, the charging circuit drew about 180mA and took about 2 hours to drop the current to a couple hundred µA, indicating that the charge was complete. I tested a rectified AC source from the motor-crank assembly of a wind-up flashlight to roughly mimic the output of the dynohub (albeit with a much higher frequency) and the circuit worked fine. Success!

Now to see if the GH6 Dynohub could power the charger:

Even at low to moderate speeds, the GH6's output was good enough to power the circuit. Spinning the wheel as fast as I could made the voltage across the rectifier peak close to 20V, a little over 4V shy of the LM3622's maximum rated input. I doubt my wife will ever go that fast, even downhill in relatively flat Toronto.

Here is the complete parts list for the charger:

1 LM3622 IC
1 D45H8 transistor
1 MBRS130L diode
2 10 µF capacitor
1 0.22ohm resistor
1 1000 µF electrolytic capacitor
1 W02M 1.5A 200V bridge rectifier
1 CR123A 880mAh 3.6V Li-Ion battery

Step 5: Make a PCB

The charging circuit needs to be assembled into as small a package as possible to fit into the housing of the headlamp. Here's what I came up with:

With no mad PCB-routing skills, this is the best I could do for a single-sided design with large traces and pads suitable for DIY etching. Q1 is a large 10A power transistor and probably could have been swapped for a smaller SMD component such as the STN479 used in the design of the Wave Bubble. I decided to stick with the D45H8 suggested in National's application note, even if it was overkill. The final footprint of the PCB is a little less than 1.3" square. Download the EagleCAD project here.

Using a PDF (derived from the EPS output of EagleCAD's CAM processor), I made the etch-resist mask using the toner transfer method. This was considerably harder to master than advertised. After several attempts, I had a reasonable looking etch-resist pattern that still required a bit of touching up with a black Sharpie pen. I etched the board using ammonium persulphate (220g/L) which we have in the biochemistry lab where I work. Using a magnetic stirring hot plate to keep the the temperature at 60ºC and the solution in constant motion the board was nicely etched in less than 10 minutes.

A few minutes of soldering later, the circuit was complete!

I got a little too excited after finishing it and soldered in test pins, making the connection of the battery and dynohub leads a little sloppy.

Here's the bottom of the board with the surface mount LM3622 and diode. This time I was sober and coffee-free.

Step 6: Connect to the dynamo

Now the whole thing needs to be connected to the dynamo and stuffed into the headlamp.

All connected. On the upper right you can see a 6.8ohm 2W current limiting resistor connected in series with the rear light.

Wrap it all up in electrical tape and stuff it in there. Plenty of room left over.

Now for the rear light. The LED won't fit into the wedge socket by itself so I cut out a small piece of PCB and soldered it to the leads to approximate a wedge base bulb.

Fits snuggly in the socket. The bulb looks like it's a bit too long.

But the lens fits!

How's it look in the dark?

The rear light is super bright. Blinding, actually. Even brighter than the front, but using about half the current.

Final thoughts

Everything seems to work. The only problem I've had so far is that the battery wasn't fully charged when I installed it. The two lights discharge the battery faster than the dynamo can charge it, so on the first night out I wound up draining the battery completely, preventing the lights from staying on when stopped (the charging circuit seems to provide enough current to power both lights while the bike is in motion when the battery is dead). I expect a long ride during the day with the lights off should get it charged up sufficiently. That said, for long night rides this design is somewhat lacking. Under the conditions my wife is likely to ride, I suspect it will rarely be a problem.

Another potential design flaw is excessive battery charging time. Other charging circuits incorporate a timer so that after the battery is held at full-charge for a determined amount of time the circuit shuts off completely (I think). The LM3622 doesn't incorporate at timer; as long as the wheel is spinning the charging circuit is on. I think this is probably OK for the long term health of the battery because the LM3622 monitors the charge state and drops the current to a few µA when the battery is fully charged . If the bike was ridden for hours and hours with the lights off then there could be an issue with battery lifespan, but my wife's bike will never be taken out for more than an hour or so once or twice a week so I don't think it will be problem.

Finally, there is the issue of the dynamo's output exceeding the 24V maximum rating of the LM3622. This is an unlikely but not impossible scenario. I don't doubt that flying down a steep hill at top speed would probably blow the chip. A better design would incorporate voltage regulation.

Back to the drawing board...