Detection circuit with hysteresis

Abstract

A battery-powered fire alarm including a smoke detector circuit, a controllable horn circuit and a battery monitoring circuit. The smoke detector employs a pair of complementary field-effect transistor switches with gates respectively connected to an ionization chamber and a potentiometer of a Wheatstone bridge circuit connected across the battery. 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 a preselected alarm voltage at the potentiometer, both field-effect transistors turn on to energize an alarm circuit to sound an alarm. Hysteresis circuitry is provided to ensure that the complementary switches, once turned on, will not turn off and thereby terminate the alarm until after the alarm condition has terminated. In one embodiment, a feedback signal causes the preselected alarm voltage to increase in response to alarm activation. In other embodiment, hysteresis is achieved by means of a feedback signal proportionately lowering the sensing voltage from the ionization chamber in response to alarm activation. A test switch and associated circuitry is also provided to manually lower the sensing voltage below the preselected alarm voltage to test the detection circuitry in addition to the battery and alarm horn.