US Patent 7761212 Wireless communication apparatus

A wireless communication apparatus including a master unit and a slave unit. The master unit includes a transmitter configured to transmit a beacon periodically and a receiver. A detector is provided that is configured to output a detected signal, and the slave unit is configured to receive the detected signal from the detector. The slave unit includes a receiver configured to receive the beacon periodically at a receiving timing determined based on the beacon, and a transmitter configured to transmit the detected signal to the master unit receiver at a transmitting timing determined based on the beacon if a value of the detected signal changes by a threshold amount.