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Light based Proximity Detection

Light Activated Train Detector Circuits

Using the popular 555 timer IC as a photocell controlled train detector.

Circuit 1:


Shown on the schematic is a secondary output that uses the open collector at the DISCHARGE terminal (Pin 7) of the timer. This output can sink a fairly large current and would be ideal for driving relays.

Circuit 2:    Using Photocells



Circuit 3:   Photocell with delayed release

The next circuit uses two LM555 timers to provide train detection with a delay in the signal returning to the unoccupied condition


xPhoto555TDLM 555 Photo-Detector With Delayed Release

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.
  •   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

Circuit Credit: Rob Paisley

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