An alarm condition sensor for triggering a signal incorporated in an alarm system and adapted to be mounted on a motor vehicle or other object to protect it against burglary, theft, tampering, or the like. The sensor comprises a sensing mass mounted atop a deformable substrate equipped with piezoelectric material operable to generate a signal voltage upon stressing or deformation of the substrate in response to forces effecting relative movement between the mass and the substrate. The sensor may be mounted on a cantilever spring support which enables the mass to move and thereby stress the substrate and generate a signal voltage. Voltage sensitive means responsive to voltages generated by the piezoelectric material is operable to trigger the alarm system. The sensitivity of the sensor is adjustable to vary the threshold value of the voltage operable to activate the alarm.