An electronic component that consists of a light-emitting diode fused to a photodiode or phototransistor. When it works properly, any current applied to the LED should cause the phototransistor to source a bit of current through its leads.

Since they completely block any current, opto-isolators are useful when you are controlling something noisy and high-voltage, such as an electric motor, with something more sensitive to noise and at a lower voltage, such as digital electronics.

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