(57) An alarm system uses piezo-electric crystals (2) which when vibrated produce an electrical
signal that is used to generate an alarm. Each crystal is in a sensor unit (1) with
a latch circuit (13) and an indicator (15), and the threshold at which the alarm triggers
latch-on and activates the indicator is adjustable. The latched-on alarm signal in
normal use is passed by cable to a remote control module (21), where the actual alarm
can be raised. For setting up, the sensor is provided with a periodic power supply
from the control module and the alarms (24, 29) are disabled, so that during power-on
periods an operator can adjust the sensor threshold without disturbance at the control
module. If latching occurs at the wrong level, the subsequent momentary power-off
restores the sensor to its primed state, enabling re-adjustment. As well as being
responsive to sensor vibration above a set level, an alarm is raised if the cable
circuit is broken or short circuited.
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