This low cost FM transmitter is connected in series with your telephone line using the included modular telephone cables. It transmits the telephone conversation over the FM band when you pick up the telephone handset and the transmitted signal can be tuned by any FM receiver. The circuit includes an On Air LED indicator and also provides a switch which can be used to turn off the transmitter. A unique feature of the circuit is that no battery is needed to operate the circuit since power is taken from the telephone line. The transmitter uses only a short aerial (about 4 x 10 cm long) since some of the RF signal is also radiated through the telephone line itself.