Wireless Alarm Transmitter and Receiver

(Summer Circuits 2010 article)

Here are two circuits that make it possible to add up to eight detectors to an existing alarm system without running a single cable. Each transmitter has a unique number that is reported to the central unit in the event of an alarm, and the transmitter battery condition is monitored. Transmissions between the transmitters and the central unit are coded and are in one of the two ISM frequency bands: 433.92 MHz (US: 315 MHz) or 868 MHz (US: 915 MHz).