Phototransistor / Comparator / 555 Timer

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.

  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.

Detector Circuit Schematic

Using The 555 Timer With Photocells

555 Photo-Detector Design Guide Sheet

  An optional resistor, R6, can be added to the circuit to lower and compress the detection voltage range but this only partially alleviates the problem of the wide detection band.

  The graphs at the bottom of the diagram show the input voltages at which the OUTPUT of the LM555 will change states. The effect that resistor R6 has on the circuit can be seen in the right hand graph.

LM555 Photocell Detector 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.

LM 555 Photo-Detector With Delayed Release

LM555 Photo-Detector Circuit Notes

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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

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