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.