Friday, April 20, 2012

Sturmey Archer light set LED upgrade with standlight

It's been a long time in the making, but I've finally finished retrofitting the Sturmey Archer head/taillight set for my wife's bike with power LEDs and a standlight. It all started back in 2008 with my efforts to upgrade a GH6 Dynohub powered lighting system with LEDs and lithium ion powered  standlight on a dilapidated old Superbe.  Sadly, some jerk stole the taillight. The system didn't perform well anyway, which led me to explore ways of charging supercapacitors to power standlights.  After acquiring another NOS set of Sturmey Archer lamps, I set to work on an updated retrofit.

I had originally designed the circuit so a switch could select between flashing and solid modes, but it turned out that the Zener diode that needs to shunt the dynamo current during the off-cycle of the flash wasn't up to the task.  I'm quite convinced of the value of flashing lights in urban settings, but in this case I've settled for solid lights while moving and a flashing standlight.  The circuit has had a reasonable amount of bench testing on my dynamo testing jig, but it's now time to bundle it all into the lamp housing, put it on the bike and start riding with it.  Here are a few pictures of the board with switch and copper heat sink mounted LED (described elsewhere):

The orignal selector switch on the top of the housing proved unreliable, so I enlarged an existing hole in the bottom of the housing to hold the switch, which has a nice rubber boot.   Mounting the PCB was a little fiddly. A tiny L-bracket with one #4-40 tapped hole is used to mount the circuit board to the lamp housing:

The board looks like an odd fit, but that's because I made it as small as I possibly could, with the hope of fitting into smaller vintage lamp housings, such as the Luxor 65.

It all fits together nicely, with the retrofitted LED/heat sink/optics and switch looking fairly inconspicuous:

The taillight LED is ready to go, although I need to get the brown plastic taillight chromed (a gift from a generous reader!).  I intend to save my really nice NOS SA lampset for some other project.  Once the taillight is chromed (technically, vacuum metallized), I just need to hook up the taillight and dynamo wires and mount the lamps on the bike.

Future improvements could include a taillight flashing system, where the headlight is never disconnected from the dynamo, alleviating the problem of surging dynamo voltage when flashing both LEDs.  I implemented this on a breadboard using a low-side N-FET to short the taillight LED, under the control of an Attiny10.  This also has the advantage of smaller and fewer components than a 555-based flasher.

Another improvement I'd like to eventually make is to upgrade the optics.  The headlight uses a Cree XM-L, which can take up to 3A of current.  Unfortunately, small format narrow beam optics aren't yet available for the XM-L, but they are for its cousin the XP-G, whose 1A current maximum can easily handle the dynamo's output.   The XM-L optic certainly improves the beam shape, but it still doesn't have as much throw as I would like.  The best solution would be to fabricate a new heat sink for the XP-G and uses a narrower optic.


Tom Reingold said...

Excellent work! I have a friend who does similar work. I'll make sure he knows about you if he doesn't already.

How is the light output? The LED looks like it casts light forward, so does it make use of the reflector?

minisystem said...

Thanks! Please spread the word; it's always nice to find folks with similar interests. The LED does not make effective use of the reflector. It took me ages to figure this out, even though it's fairly intuitive: incandescent bulbs have an effective 'viewing angle' that extends beyond 180º and their reflectors were very well designed to capture and focus this 'backwards facing' light. It makes the old 1.8W incandescent dynamo systems concentrate a surprising amount of light into a tightly focussed beam. However, it does not work very well all for LED upgrades due to their typical 120-140º viewing angles. You can see a detail comparison here.

minisystem said...

This gentleman explains the problem of putting LEDs in reflectors meant for incandescent bulbs better than I do:

Anonymous said...

Good job, I have a Raleigh DL-1 with a GH6 Dynohub and lights. Good info.

scott said...

That's a fantastic implementation. Just a couple of questions. Are you planning on selling boards (either empty or full) when the attiny version comes along? Why did you originally choose the XM-L over the XP-G?

minisystem said...

Thanks. :) No plans to sell anything yet. The thought crossed my mind, but I'm not convinced demand would be high enough to make it worth the effort; even a small volume run would still be too expensive for most sensible people. This version is now installed and working on my wife's bike and I have a plan to do an Attiny version for a set of Luxor lamps. If I remember correctly, the XM-L is modestly brighter than the XP-G at rated 500 mA dynamo output, so that's why I chose it, despite using the XP-G in previous designs.

Unknown said...

Are you still tinkering around with this stuff? I have a Sturmey Archer XL-FDD that I've built into a replacement wheel for a 1932 Burgers ENR Model 58 Step-through that I'm restoring for my wife. She is keen to get some lighting for it, but I'm loathe to put some new-fangled system on it. I actually purchased a cool old battery powered headlamp that I was hoping to retrofit with a bulb that could be powered by the dyno hub. Anyway, I'd love to have you build an LED unit for it. I'd love to discuss price, etc.
james [at] worden [dot] org