An electronic component
that consists of a light-emitting diode
fused to a photodiode
. 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.