G - Physics – 08 – B
Patent
G - Physics
08
B
358/21
G08B 17/00 (2006.01)
Patent
CA 1071774
ABSTRACT OF THE DISCLOSURE 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 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. The battery monitoring circuit employs a pair of complementary field-effect transistors connected with each other to establish a reference voltage to which the battery voltage is compared. An advantage over prior battery monitoring circuits of this type is that the reference voltage is a function of both the threshold characteristics of the field-effect transistors and the magnitude of a preselected control voltage. Both field-effect transistors remain off until a low battery voltage condition is sensed to minimize standby power consumption.
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