Been a while between builds but heres my latest.
Smaller, easier, brighter, smarterer.
Theres been a few ideas from here and over on MTBR that I wanted to try out, and I wanted to try a reflector setup.
Heres what I came up with
- Brightness adjusts smoothly as speed increases
- Bright standlight with reasonable runtime.
- Single mode, no high low.
- Small all in one design.
Firstly, I've been chasing lumens for a few years now, going from 2 to 3 to 4 to five and then back to three LEDs. Last year while in between builds I resorted to using an old dual cree p3 build, using those nice tight cree optics. Had a good ride with it. Occurred to me two LEDs with the right optics should be enough.
Settled on XP-E R2s in Khatod
KCLP20 optics. On smooth, one stippled. Around 6 and 11 deg FWHM.
I've been a fan of Martins
circuit 10 for a while as it gives good low speed and high speed performance. But you need to flick a switch (yes, flicking a switch is actually quite a chore after 10hrs on the bike), and you only get 500mA.
With only two LEDs the low mode shouldn't be needed as you get light down to around 7kph in high mode. So 2 LEDs and a single mode circuit.
Ktronik showed that you can tune the circuit to give boost at certain speeds. A little investigation showed I could design in whatever peak I wanted. 40kph seemed like a good speed as there aren't many places where I spend a long length of time going faster than that. Yes, Im overdriving the XPs a wee bit.
Heres the curve for a dual led tuned for 40kph. Nice.
My previous standlight was a simple supercap across one LED, very simple, worked ok, but Frontranger in this
thread shows a simple way to power the standlight for longer, and brighter. The cap charges off both LEDs with the voltage limited by a string of diodes, and powers one LED through a resistor. This means more power for longer.
Heres the complete circuit.
I used 4 caps on the input instead of two as I found they got quite warm.
Heres the bits, theres a few diodes and resistors hiding between the caps on the right.
All together. 8cm long.
After squeezing everything in I used carbon fiber to seal the ends as the plastic end caps from my previous build use up too much space.
Mylar seals the front holes.
Very efficient use of space in there. Bit annoyed the super cap isn't straight.
On the bike
Only had one city ride(damn this cold) in the bright moonlight but it seems to work well. Nice spot and a good spill.
The standlight is very bright, rivals my cateye, and lasts for a useful amount of time.
Righto, that should do it until the XP-Gs arrive.
