The circuit on this page is for a simple light detector circuit board that has 8 detectors that can be used with visible or infrared light systems. The detectors use LM339 voltage comparators as the active element.
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. 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.
Selecting A Value For The Output Resistors (R4)
The value of resistor R4 is chosen to give a desired current flow though the LEDs See below for more details.
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.
For information on calculating the value of current limiting resistors please see the Current Limiting Resistor Calculator page at this site.
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