A communication device comprising an internal power source, first and second receiver elements for receiving signals transmitted from the base station, and a processor selectively responsive to a modulation of the received signal to generate a data signal for transmission to the base station. The first receiver element has lower power consumption than the second receiver element does when energised. At least during a standby state of the device, the first receiver element is activated to detect the presence of a received signal, the second receiver element and the processing means being de-energised. The first receiver element is responsive to detection of a received signal to trigger an active state in which the second receiver element is activated to respond selectively to the frequency of the signal received to actuate the processor. The invention is applicable to vehicle access control systems, and tire pressure monitoring systems.