Optoisolator

 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.

Optoisolator have become much more sophisticated in recent years. Though standard optoisolators are still available today, the many improvments have  increased the already widespread application of optoisolators. Input control-logic circuitry has been added to determine under what conditions the IRED should be activated. Such output devices as photo-darlington transistor configurations, ligt-activated SCRs and triacs, and even constant-current ouput amplifiers have replaced the single phototransistors.