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PS2501 Optocoupler

In the 21st century you don't need wires to transmit a signal, and in fact it might be dangerous to have two devices connected if there is a sudden surge either way. Also, maybe the voltage supply of your signalling device needs to be different or isolated from the voltage supply of your receiving device. Perhaps you hate signal noise and are looking to clean it up optically.

For whatever these reasons, you can use an optocoupler such as the PS2501 to bridge the gap between signal and receive as follows. This little device has an LED transmitter (1,2) and an NPN silicon phototransistor (3,4). The IC allows a signal transfer across an optical gap, with no physical connection between the circuits.

Hook up power on both sides and watch the...er...lack of connection?