An optocoupler (aka photocoupler or opto-isolator) is an electronic component used to transfer a signal between different parts of a circuit which need to be electrically isolated from one another - for example, where a high voltage is to be switched with a low voltage signal. Often this is because the source and destination are (or may be at times) at very different voltage levels, like a microprocessor which is operating from 5V DC but being used to control a triac which is switching 240V AC. In such situations the link between the two must be an isolated one, to protect the microprocessor from overvoltage damage. Typically in such situations a relay is used, however these can be quite power hungry, prone to failure since they contain mechanical moving parts, and a relay is physically very large relative to the other components on a typical circuit board. Optocouplers are essentially digital or switching devices, sothey're best for transferring either on-off control signals or digital data. Analog signals can be transferred by means offrequency or pulse-width modulation.