The sensor in this circuit is an LDR and the potentiometer serves to control the sensitivity depending on the intensity of ambient light and the intensity of the pulse that should cause its switching. The circuit is supplied with a voltage of 5 V and its output can be connected directly to the pins of the parallel port. In the example, the signal is entering through pin 10 of the port, being activated at the low level. The circuit can also be used as a shield.
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