Pack 389's Cheap Pinewood Race Timer

Photo Trigger Specifications

The Photo Trigger board takes light input, and creates pulses when the cars cross over the light sensors (a floresent lamp sits over the track, and the sensors are mounted under each lane). This is the same architecture as the Phase 1 system, with the exception of some extra lines to supply power (to the CPU board), and SPI data lines to the digital potentiometers to calibrate the photo-resistors to the reference voltage.

In the diagram below, you can see that each lane has a photoresistor hooked up a digital pot (two per package), in the same way as the PHASE 1 model.

This schematic is only marginally more complex than the original, but required me to start from scratch on this board.  Also,  at the last minute, I decided that I would put the CPU on this board, but did not change the schematic.  This was not a huge deal, but I thought it might reduce the chance of noise in the system if the CPU was powered from the track (the track is where I put the power supply), not the cable.

Photo Sensor Board

Bill of materials (most items purchased from Digi-Key):