Pack 389's Cheap Pinewood Race Timer
Display Schematic Page
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Display Board Specifications
In the schematic below, you will see a CPU (PIC16f874), a display driver (MAX7219), a MAX232 chip, 3 2-digit 7segment LEDs, a couple of switches, and connectors. This unit is ment to replace the laptop computer. It plugs into the track via the same cable as was used in the Phase 1 project. The function of this unit is the same as the laptop (to display race results), with the addition of some extra control lines (SPI) to program the threshold resistance for each photo resistor. Since light to each photo-resistor is different (plus I just bought a mixture of simular but not the same part photo-resistors), this allows the CPU to reduced the resistance until the lane starts triggering (through the LM339) and then back off a set amount for the proper adjustment (this is exactly how I calibrated the timer when it was done by hand with a DVM (digital volt meter). The MAX232 was added to allow the CPU to talk to a laptop in the future, and the laptop could provide race results (never got that far in the project before I stopped working on it).
Bill of materials
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C5/4.5 C5B4.5 capacitor-wima
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Display
Schematic