Smoke sensing fire alarm

Abstract From the Canadian patent

A battery-powered fire alarm including a smoke detector, a controllable horn circuit and a battery monitoring circuit. The smoke detector employs a pair of complementary field-effect transistor switches respectively connected to an ionization chamber and a potentiometer of a Wheatstone bridge circuit connected across the battery. Unlike known circuits of this type, the field-effect transistors are biased off to minimize standby power consumption and are connected such that the threshold voltages thereof are offsetting to minimize supply voltage sensitivity of the detector. When the voltage from the ionization chamber assumes a value approximately equal to the potentiometer voltage, both …