Sound activated switches are useful in many circumstances, especially when "hands free" operation of a piece of equipment is required. They are often used, for example, to automatically switch on a tape recorder (or a digital solid state equivalent) to record a sound or conversation without "wasting" tape during quiet periods.

Many inexpensive cassette recorders have a remote switch input to enable the recorder to be switched on from the microphone and a circuit of this type can easily be connected to it to start the recording automatically. As well as this, sound activated switches can be useful in applications such as intercoms, baby alarms, security alarms, or photographic work, and there are no doubt many others.

This project shows how a logic gate can be used as an audio amplifier and a voltage switch. This circuit arose from a requirement for a basic microphone interface to logic circuits without having to build one from scratch each time. Since it was not built for any specific application, a number of outputs were provided, including an amplified version of the sound waveform. With a few additional components, however, the circuit can easily be used as a sound-operated switch with other equipment.