An optoisolator, in its simplest from , is an LED and a phototransistor, set in opposition, in a permanently sealed package. Often a dua-in-line package (DIP) is used so that devices connected to the tranmitter and those connected to the receviver. Information is transmitted through an optical link. If a computer, operating at 5V DC, were directly connected to circuitry controlling a 240 V AC load, for example, the possibiliy would exit that a circuit failure could result in the high voltage being applied directly to the computer. Isolating the computer from the high-voltage circuit by using an optoisolator prevents the permanent damage that such a circuit failure colud cause , since there is no physial connection betwen the transmitter and the receiver inside the optoisolator.
