The Pinewood Derby electronics consists of a "which came first" circuit and an optional timer. An IR transmitter and receiver at the finish line detects the car crossing the line. There is no schematic for the receiver because I used a unit from Radio Shack, Part Number 276-137. You will drill a 1/4 inch hole in the track and mount the receiver under the track. A frame will be needed to hold the IR transmitter and the lamps which indicate which car was first. Most tracks have three slots but I have only seen two used. The "which came first" schematic includes the power supply, but the timer works from 5VDC and any good supply will work.

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"Which came first circuit

Timer schematic

Wire wrap layout for timer

Pinewood Derby system layout (interconnects)