A circuit and method for providing an auto-off capability for a wireless transmitter, of a type having an audio plug which mates with the output jack of an audio source. The portable transmitter modulates signals from a baseband signal source onto a carrier and transmits the RF carrier to a receiver. The auto-off capability is provided to prolong battery life and eliminate the transmission of unmodulated RF carriers. The auto-off capability comprises turning power off to the transmitter circuit when baseband input signal is absent for more than a predetermined amount of time, and immediately turning on the transmitter circuit when baseband signal is present.