This short collection of projects - some useful, some instructive, and some amusing - can be made for around 10 UK pounds (16 US dollars). The estimated cost does not include an enclosure (box). All of the projects are battery powered, so are safe to build.

An intercom has many uses around the home and office, including acting as a baby alarm. This simple low-cost circuit operates on the Master/Slave principle. This is a halfduplex system, in which there is communication in both directions, but in only one direction at any one time. This helps to make the circuit simple and cheap, and means that only a single pair of wires is needed to connect the stations.

One station is the Master, and controls the direction in which messages pass. The other station is the Slave and consists of no more than a loudspeaker, which functions as a microphone or as a speaker, depending on how it is switched into the circuit by the Master station.