Photodetector With Delayed Release And Bipolar Output

- The Outputs Of The 555 Timer Is HIGH When The Phototransistor Is Dark -

- The output LEDs Can Be ON or OFF When The Phototransistors Are Dark -

  The circuit on this page is for a visible and infrared light detector. The input is sensed by LM324 operational amplifier and the output is from a LM555 timer.

  The output has a selectable delayed release time and can source or sink up to 200 milliamps. The output's release delay is set by changing the values for the resistors and capacitors at the TRIGGER and THRESHOLD terminals of the LM555 timer.

  This circuit is ideal for driving signals without flicker or relays without chatter.

Basic Time Delay Output Detector Circuit

  The following diagram shows the basic circuit.

Selecting A Value For The Input Resistor (R1)

  The value of resistor R1 depends on the type of sensor and the desired sensitivity. See below for more details.

  For phototransistors a value of 470K ohms will work for most room light situations. Selecting a higher value resistor such as 1 Megohm will give better sensitivity in low light. This High Impedance Test Voltmeter circuit can also be used for testing phototransistors installations.

  For CdS photocells it is usually best to install the cell and then measure its resistance under the normal lighting conditions. A resistor with a value that is 3 to 5 times the measured resistance of the cell is then selected for R1.

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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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04 May, 2011