![]() ![]() In some cases the values of some resistors may have to be adjusted to compensate. The supply voltage for the circuits is specified as regulated 12 volts DC but this can be changed if needed. This will be explained in the NOTES sections of this page. For information on Voltage Comparators please see the Voltage Comparator Information page at this site.Īll of the circuits on this page are configured to have the LED's turn on when the sensor element is dark (covered by a train.) The LED's can also be made to turn off when a train is detected. The Photo-detectors on this page use LM339 (Quad) or LM393 (Dual) voltage comparator, integrated circuits to detect the change in voltage across the sensor. The phototransistor or photocell would normally be placed between the rails in the circuits on this page. (Phototransistors change their 'conductance' while photocells change their resistance depending on the intensity of the light falling on them.) ![]() Both of these sensors allow less current to flow when they are dark. The sensors used for these circuits are silicon phototransistors or Cadmium Sulfide (CdS) photocells. This page features basic, visible light photo-detector circuits that can be used to detect trains or other light blocking objects. Photo Detector Circuits Basic Visible and Infrared Light Detectors ![]()
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