First port of call is this fantastically informative page: http://pilom.com/BicycleElectronics/DynamoCircuits.htm - which explains the basics of building a dynamo light circuit. This set me on the search for a 2x LED light head so I could wire up the LEDs in series (you can go for more than 2 LEDs, but the increased voltage requirements mean you'd have to pedal faster for the light to come on at all).
I bought a Solarstorm X2 like this: http://www.ebay.co.uk/itm/SolarStorm-X2 ... 1192072049 - the advantage of going pre-built are mount, housing and lens are all ready (although not necessarily to one's taste). Plus this one had screw access to the rear.
- The original Solarstorm X2 circuit - there is some space at the back of each LED
I then set about building the front half of the circuit shown two-thirds down this page: http://www.candlepowerforums.com/vb/sho ... or-dummies - without the tuning capacitors (space was the main reason for omitting those components), and I didn't need the taillight side.
Space was still too tight for the remaining components, so the bridge rectifier (four diodes which convert the AC of the dynamo to DC which the lights can use) had to go outside the light. I put those inside an in-line switch a little way along the cable.
- Ordinary lamp in-line switch, filled with fiddly Schottky diodes
The original driver in the Solarstorm X2 is of no use for dynamo usage, so I removed the one main component which was wasting space (an inductor coil). I was tempted to remove the main circuit board too, but unfortunately that gives the back plate necessary shape, so I just taped over the components.
The real problem was shoving all the parts in and closing the back again (I had unnecessarily long wire). I swear I broke something, plus my soldering is pretty awful. I'm not looking back in there until the light fails! Plus I had the wrong sizes of heatshrink for the in-line switch, so now it looks a mess of electrical tape.
- The dynamo circuit - extremely tight cram! I lined most parts which could short out with electrical tape... the rest was luck!
Testing it today, it works! Unfortunately I couldn't fit a smoothing capacitor larger than 470uF, which is too small to do any real smoothing, so the thing still flickers under 15km/h. I think it may have flickered less until I hit a big bump, which might have knocked off the capacitor... I don't know, and I'm not opening it up!
The standlight is the real revelation. With a 1.5F supercap (just about fits in there) it's at least as good as the one on my Pico - bright for about 3 minutes, then fading for the next 10 minutes. That was one of the most expensive components though. The circuit dictates that only one light ('LED 2') will be lit by the standlight, but that's no biggie. That LED also lights up first in the circuit (I presume when there's not enough voltage for both LEDs, current flows down the diode towards the supercap, and is then shared between the supercap and LED2), so that actually helps low speed illumination.
The beam shape is so-so. It's an MTB light, after all. The Pico still has better graduated road illumination. But the DIY light has more spread. There are still a few niggles, so I'll ride it a bit more, and then diagnose problems/think of improvements.
Total cost? I'd estimate £30. The light was £14, and the supercap £8. The other components I'm only partially counting, as I still have spares of those.
Hope this helps someone. If anyone has any questions/suggestions, chime in!
