I’ve been wanting a better tail light for my bike for a long time, but I haven’t really liked what I see in stores. So, I decided to just build one. The idea is to put a small red light on my bike, aimed backward, with a high-visibility flasher mode. Probably lots of DC-Fix on the lens to make the beam spread as wide as possible. I think a 1x16340 host should provide enough battery life without being terribly large, and it’s a lot easier to build/mod one of those than a 1xAA light.
I’m waiting on parts right now, but here’s my design/spec so far. These are kind of just my notes, and probably are lacking some explanation…
- Host: Ultrafire 602C host: $4.25 from FastTech
- Emitter: Red XP-E2 on Noctigon: $6.50 from RMM
- Battery: Efest 16340 V2 IMR 550mAh: $3.95 from RMM
- Driver: Nanjg 3x7135 “AK-47A” driver: $3.00 from RMM
- Bike attachment: cheapo lockblock clone from DX, $2ish, already have a few
- Clip? (maybe): $1.96 from FastTech
- DC-Fix: already have lots of it
Total: $19.70 to $21.66 (less than most of the non-Walmart tail lights I could buy locally, yet should be a better product)
I’ll have to write my own firmware for it, but that shouldn’t be too hard.
While deciding on an emitter and driver power level, I checked out the red XP-E and XP-E2. It looks like the XP-E2 gives a nice boost in overall output and efficiency, but it also costs twice as much.
Red XP-E2 output: testing a red XP-E, and comparing it to a red XR-C, 22/4/'14: XP-E2 results added in post #32 (and blue and green XP-E2, output tests )
Power/output levels, approximated from graphs:
- 350mA: 56 lm, 2.05V, 78 lm/W (or 44 lm/W at 3.6V)
- 700mA: 142 lm, 2.3V, 88 lm/W (or 56 lm/W at 3.6V)
- 1050mA: 180 lm, 2.42V, 71 lm/W (or 47 lm/W at 3.6V)
- 1400mA: 233 lm, 2.6V, 64 lm/W (or 46 lm/W at 3.6V)
Optimal: 142 lm at 700mA (most efficient power level in lm/W, and bright enough for my needs)
Modes: probably five: (2x7135, 700mA, 550mAh battery)
- low 2%: 2.8 lm, 39 hours
- med 20: 28 lm, 3.9 hours (or 15, 21 lm for 5.2 hours)
- high 100%/700mA: 142 lm, 47 minutes
- high-vis flasher 2%/20%: 2.8/28 lm, 20 hours
(steady on at 2, flash three times at 20 once per second)
(10ms med, 35ms low, 10ms med, 35ms low, 10ms med, 900ms low) - high-vis 2Hz beacon: 75 hours
(flash at max for 5ms, wait 245ms, flash again, wait 745ms)
Mode memory: Yes. If the bike hits a bump and the battery contact is briefly lost, I want it to return to the same mode it was in as soon as power is restored.
I tried this general setup with my S10-Ti MELD, and it seems to work pretty well. But I don’t want to use such a nice shiny light on my bike, so I’m building a similar one (or three) with only the features I need for the road.
The biggest potential issue I can see is that the beam will only be about 170 degrees wide. Some tail lights are actually visible for about 270 degrees, but doing that would require a convex diffraction cap. However, this should still be okay since there would be only a narrow angle to the side where someone wouldn’t get direct light from either my head light or my tail light, and the head light is bright enough to make me visible regardless.
In any case, I’ll post more details after I have parts and can start putting them together.