Abstract: A security system for monitoring the presence of one or more objects in a monitored zone. The system includes an antenna and at least one tag having a tuned resonant circuit, the tag being associated with one or more objects to be monitored. A transmitter, coupled to the antenna, is operable to transmit at one of a plurality of frequencies in response to a frequency selection signal. A receiver, coupled to the antenna producing a tag detection signal for stopping the cycling of the transmitter frequencies in response to a signal received at the receiver from the tag thereby causing the transmitter at a single frequency when a tag is detected. Also provided is a device for decreasing the output power of the transmitter when the receiver signal is above a predetermined level.