Optocouplers are used on switch-mode power supply boards to send feedback from the cold side to the hot side of the power supply board while keeping both sides isolated from each other. Optocouplers or photocouplers can also be used as a switch to switch something on/off from the hot or cold side of the power supply board to the opposite side without having an electrical connection between the hot and cold side of the power supply.
I will also show you how to test optocouplers in the circuit the REAL way.

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