Posted on Feb 6, 2014

The circuit on this page is for a visible and infrared light detector circuitboard that has 8 detectors. LM339 voltage comparators are the active element. These detectors can be used as part of other light detector circuits shown on other pages at this site such as these Light Activated Detector Circuits at this site. For phototransistors a valu

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e of 470K ohms will work for most room light situations. If the light is dim, selecting a higher value resistor such as 1Megohm will give better sensitivity. 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. A 1K ohm resistor will allow about 10 milliamps to flow through a typical LED if the supply voltage is 12 volts. The value of the resistors at the outputs of the comparators can changed depending on the desired current through the LEDs. The following diagram shows the circuit that is on the printed circuit board. There are 8 independent photo-detectors with open collector outputs that can sink up to 15 milliamps each. All of the comparators on the PCB are wired so that when the photosensors are exposed to light, the output of the comparators will be LOW and the LEDs will be ON. The detection voltage level for the circuit as shown is set at 1/2 of the supply voltage. If a lower or higher detection level voltage is needed, the values of resistors R9 / R10 and R19 / R20 can be adjusted. This circuit does not need a regulated power supply and can operate on supply voltages of up to 32 volts. For more general information on Voltage...

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