How to test an optocoupler

How to Check an optocoupler. A common type of Optocoupler consists of an LED and a photo-transistor in the same opaque package.
In electronics, an optocoupler, also called an Photocoupler or optical isolator is a component that transfers electrical signals between two isolated circuits by using light. Optocoupler prevent high voltages from affecting the system receiving the signal.

Correction by Mr. Mario
Hitting the internal IR LED (Vf ~1.5v) inside an optocoupler and a red indicator LED (Vf ~ 2.0v) with straight 3.7v from a lithium cell without any current limiting resistors is a great way to destroy what was once a perfectly good chip.

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