A sensing system includes an acoustic wave device comprising a body of piezoelectric material, and a first electrode assembly mounted on the piezoelectric body whereby an appropriate input applied to the electrode assembly produces an acoustic wave in the piezoelectric body and a first frequency response. The acoustic wave device also has a second electrode assembly to produce a second frequency response, the second electrode assembly being in contact with an area sensitive to a supplied stimulus to cause a characteristic of the sensitive area to change when a supplied stimulus is sensed to thereby change the second frequency response. The acoustic wave device has an output connected to the input of an amplifier, the amplifier having an output connected to the input of a coupling device. The coupling device is connected to the input of the acoustic wave device and has an output connected to a detector. The acoustic wave device, the amplifier and the coupling device are configured to be in an "off" condition when no supplied stimulus is sensed by the sensitive area of the acoustic wave device and to be in an "on" condition when a supplied stimulus is sensed by the sensitive area. The coupling device is configured to send a change signal to the detector when a change from the "off" condition to the "on" condition occurs.