In SX Microcontrollers, SX/B Compiler and SX-Key Tool, JonnyMac wrote:

If you can get a copy of this month's (January 2007) Nuts & Volts magazine you'll find that the project for my column is... (drum roll, please) a pinewood derby lane timer.  It uses an SX28 to time the car, detect the finish, and display the running time (in milliseconds) on five, 7-segment displays.  I've attached the schematic and code (get the magazine for an explanation of everything).

My circuit uses the comparator with a calibration resistor to set the sensitivity of the photo-detector (part of an IR LED/transistor pair from RadioShack).

In the SX/B help file, under the INTERRUPT keyword, there is a demo that uses port B edge triggered interrupts to determine which button (of four) was pressed first.  If you can adapt your detection circuit this may be the best way to go.  The project discussed above is really designed to aid in fine-tuning the performance of a pinewood derby racer by measuring its "lap" time.

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