new and improved (sort of) rear bike light, rejigged for 12 volts

Quite a few years ago I decided that my bike was far too light, so I experimented by adding lead to it—in the form of various sealed lead acid batteries (SLAs). Conveniently, and by sheer coincidence, the extra weight could also power a head light. Who knew? Then I got to thinking, why not power the rear light with the same battery? At the time I built this thing, commercially available rear lights were adequate, but none too bright—a bit like me. Really bright LEDs were not available then, so I decided to go for volume instead.

I’ve been running this light for quite a few years. It’s reasonably bright and has never let me down. Ten LEDs, slightly brighter than stock, point rearward replacing the original, anemic five. Two banks of four LEDs, one facing left, the other facing right, give added visibility from the sides. I’ve also added a switch to the headlight pod, so I can turn it on and off with the greatest of ease. No more NiCad AAs to charge, yeah!

don’t try this at home, kids

I don’t know what I was thinking, perhaps I wasn’t, but the diminutive circuit board is crammed with chips, LEDs, resistors, capacitors, and wires. I used three 14051 CMOS hex buffers to drive the LEDs, probably because of my irrational fear of biasing transistors. For much the same reason, where any sane person would have used a 555 timer for the clock signal, I used two of the four gates from a 4011 quad NAND gate. Hey kids, don’t try this at home: there are much better, simpler ways.

completed light Velcroed to rack pack

And there you have it. The four LEDs on each side add extra visibility, and despite the quirky design, it’s been ultra-reliable, even in torrential downpours. One annoying habit (feature?) is that sometimes I have to turn it off and on twice before it will blink. I think this is because the NAND gates don’t like being operated at such a low frequency.

Update: 2013-02-16

The above design is overly complex and not very efficient—it actually generates quite a bit of heat. Since engaging in this bit of silliness, a pile of parts that had been magically accumulating on my workbench (no more “hexed” buffers, thankfully), has been transformed into a much better design: Do-It-Yourself Flashing Rear Bike Light: Part 2. The adventure continues…

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