Petrol anti-theft alarm - includes electronic circuit sensitive to movement of float controlled resistor normally driving petrol gauge

摘要

The alarm uses the variable resistor which normally controls the operation of the fuel gauge. A connection is made via a diode to an adjustable biasing bridge and resistor to the base of an NPN transistor. The variable resistor controls the sensitivity of the device. The alarm circuit is such that variation in the level of petrol, and hence the gauge potentiometer position activates the alarm. A time delay circuit allows the petrol in the tank to settle after the vehicle has stopped, before activating the alarm. The resistances bias the base of the transistor for a period of time after switching of the vehicle's engine, the length of the period depending on the resistance and capacitor size.