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The four 3mm Phototransistors are mounted 55mm bellow in the track, for grater sensitivity you can wire two in parallel for each lane (x8). They act like switches that are normally closed when lined up with the working emitting diodes, pulling the signal lines to ground, when a car passes over them the circuit is broken and parallel port TTL lines go high to 5v.
NB: We also have independent rally tracks that all run on incorporated 5Vdc regulators, with 2 LEDís in series(at 2.8V needs a resistor to limit current <100ma) and 2 phototransistors in parallel giving better sensitivity over grater distances, but still using the same software.
Maplin YH70M Infrared emitting diode IED x 4(8)
Maplin YY66W Phototransistor PT x 4(8)
1.2v (1.4) NiMH AA Battery x 1 *
AA battery holder Maplin YR59P *
* Replace with 5Vdc 1A regulated system as this can be used to drive track Relays and indicator LEDís(each of the LED's will need a resistor to limit current to <100ma)
HOW TO USE THE PARALLEL PRINTER PORT FOR LIGHTS AND TRACK CONTROL
Components: (Maplin part No.)
1 x ULN2803A Darlington Driver (QY79L)
1 x DIL 18 pin socket (HQ76H)
4 x 2A DPDT relay (N17AW) *1
4 x DIL 16 pin socket (BL19V)
4 x 10mm 2.5V Red LED (UK25C)
4 x 430 ohm resister (M430R) *1
*1: To run at 5volts, replace 12Vdc with 5Vdc using a 5V relay, and for the LED replace the 430 ohm resistor with 180 ohm.