A presence detection system comprising, among other things, a radio transmitter and receiver is described herein. The transmitter includes a motion detection circuit, a microprocessor, and a radio frequency modulator. The motion detection circuit is configured to direct a motion detected signal to the microprocessor upon the transmitter being moved in a predetermined manner. The microprocessor is configured to generate an encoded message that includes a preamble denoting a beginning of the encoded message, an identification code denoting a type of transmitter, and a check message (such as a checksum) containing information about content of the encoded message. Finally, the radio frequency modulator is configured to modulate the encoded message at a transmitting frequency.