Secure wireless switching can be useful in a variety of applications, such as activating or deactivating an alarm system, operating a garage door opener, or operating an anti-start device in your car. The circuit described here is compact and quite secure. It consists of a transmitter and a receiver operating on a frequency in the 433-MHz ISM band. Frequencies in this band can be used without a licence if the transmitter power does not exceed 10 mW (10 dBm). The transmitter is small enough to be fitted in a key fob, and it is powered by a 3-V lithium cell.