Recently finding the need to bike home in the dark, and not wanting to regularly charge a battery-powered light, I decided to buy a used bicycle dynamo (for $40) to power a front and rear bike light.

The dynamo is a bottom-bracket dynamo, which means it’s meant to be mounted on the bottom bracket (where the pedals are mounted) and is driven by the tire tread. This contrasts with bottle dynamos which are driven by the sidewall of the tire, potentially wearing out the sidewall. There was no space to mount the dynamo on my bottom bracket, but thankfully it fit just as well on the seat stay (the part connecting the seat with the rear wheel axle). To stably clamp it in place, I had to make a custom bracket out of a piece of galvanized pipe pounded flat and bent into the right shape to conform with the seat stay tubes.

For the rear light, I bought a 1445 automotive lamp, rated at 14.4 V, 1.9 W. Since the dynamo behaves like a constant-current source, it can actually produce much higher voltages than its rated 6 V, so the rear light gets plenty bright at medium speeds. The rear light fixture that came with the dynamo required a threaded bulb, whereas the bulb I bought had a bayonet base, so I had to use a file to cut down the bayonet nubs and form a very crude thread on the base.

The headlamp accepts PR bulbs, the type used for older flashlights. I’m using a 4.5 – 6 V, 40 lumen LED bulb powered by a half-bridge rectifier and a 5 V linear voltage regulator soldered onto a PCB, and housed in a 3D printed case that is mounted on the water-bottle-cage holes on the bike. Since the LED barely consumes any current, its presence does not really affect the voltage outputted by the dynamo, which can still reach the higher voltage needed to power the rear light.

Everything is connected with DuPont connectors, using the bicycle frame as a ground.