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Light Activated Detector Circuit

Light Activated Detector Circuit

This page shows three circuits for using the 555 timer IC as a photocell controlled train detector. The circuit is shown driving light emitting diodes but any load of less than 200 milliamps could be used.
LM555 Photo-Detector Circuit Notes

Inductive loads such as relay solenoids should have anti-ringing diodes across their coils.

The use of R6 in the circuit would make this type of detector more suitable for use with photo transistors.



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Resistor R1 is used to prevent excessive current flow to the THRESHOLD terminal of the timer ar through the photocell if R2 is set to a zero position.

Cadmium Sulfide photo-resistors come in a wide variety of characteristics and ratings. Some testing and adjustment may be required to get reliable detection with this circuit.

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