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My first rear standlight

Circuit description

This is a first attempt at a bicycle standlight (ie so that I'm still visible when I stop). C1 is a 1 Farad goldcap memory backup capacitor, which is charged from the dynamo input through a diode, and resistor to avoid dimming the main lights too much. Zener diode D2 makes sure that the capacitor's max voltage isn't exceeded. D3 is a directional high-brightness red LED. D4 and D5 are diffuse wide-angle versions. Circuit diagram

Performance

The circuit itself works as well as I'd hoped, and the light is bright enough. The main problems are mechanical: the wires from the dynamo kept shaking loose from the terminal blocks, and eventually the LEDs broke off from the circuit board from the vibrations. The next version will need more thought to keeping it in one piece.

Future improvements

  • Design for harsh vibration so it actually survives!
  • Switch off when dynamo power is available, so it can charge more quickly and draw very little current once the capacitor is charged.
  • Possibly drive LEDs with current source so that brightness is more constant. However I'm not sure that the sudden turn-off is any better than slowly fading as it does now.