Finish Line Circuit

This circuit detects cars crossing the finish line.  As cars interrupt the laser beam hitting the light detector, the voltage (Vmeasure) jumps in the circuit.  The voltage change is analog, but the microprocessor needs a digital signal to operate properly.  The LM339 converts the analog signal to a digital (ON/OFF) signal.  As soon as the voltage increases above the set point voltage, the LM339 switches (Vout) from digital ON (5 volts) to digital OFF (0 volts).

By adjusting a variable resistor, the circuit can be calibrated for the amount of light hitting the detector.  There are many variables that can impact the amount of light hitting the detector.  These include the brightness of the laser, how well the laser is aligned, how well the track reflects light, and the amount of ambient light.  Light intensity covers an extremely broad range.  So, it is important to have a flexible circuit.  When setting up for the race, the set point voltage (VTrip) is adjusted to be just below the measured voltage (Vmeasure).  This insures that the microprocessor will get the signal at the instant that the derby car crosses the finish line.