- The light shall not affect the "stock" look of the bike.
- The light shall operate only when it's dark enough to be effective.
- The light shall operate automatically when the bike is in use and for around 30-60 seconds afterward.
- The light shall blink to enhance visibility at night.
- The battery on the light shall last "indefinitely" when not in use.
- The light shall efficiently utilize the battery's power when in use.
- The electronics of the light shall be reasonably weatherproof -- at least enough that road spray will not affect operation.
Based on that, I made the following initial design decisions:
- The light will use an existing bike reflector to maintain the stock look.
- The battery type will be a 9V rectangular battery.
- The light will use 3 2-volt red LED's for a 6 volt voltage drop. This will additionally allow a 8.4V rechargeable battery to be used in place of the 9V.
- A wire-through-a-spring will be used to detect vibration indicating the bike is in use. The spring will wiggle when the bike is jostled, making electrical contact between the wire and spring.
- A capacitor discharging through a resistor will be used to time power usage on the light for 30-60 seconds.
- A 555 timer will be used to provide make the lights blink.
P.S. In case you're wondering, I used EAGLE (the circuit design package from CadSoft) to lay out the circuits.