Photo Inverting

Posted on Feb 6, 2014

For phototransistors a value of 470K ohms will work for most room light situations. If the light is dim, selecting a higher value resistor such as 1 Megohm 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

Photo Inverting
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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 photodetectors with open collector outputs that can sink up to 15 milliamps each. 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 Comparators please see the Voltage Comparator Information page at this site. The 1K output resistors can be replaced by jumper wires if they are not needed such as for inputs to control or signals circuits that have their own current limiting resistors. WARNING - If the polarity of the power supply for this circuit is reversed or the circuit is connected to an AC or DCC source this circuit will be damaged. The maximum supply voltage for this circuit is 15 Volts. The diagram above can be...

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