SCR_GAS_SMOKE_DETECTOR

Published:2009/7/5 22:28:00 Author:May | From:SeekIC

Simple circuit uses Taguchi TGS308 gas sensor with SCR Q1 for half-wave control of 24-VAC alarm horn.Sensor is based on adsorptive and desorptive reaction of gases on tin oxide semiconductor surface encased in noble-metal heater that serves also as electrode. Combustible gases increase conductivity of sensor, thereby increasing load voltage enough to trip comparator and initiate alarm. Output voltage across Rl is normally about 3 VRMS. With gas or smoke, voltage can rise to 20 V. When gas or smoke has cleared sensor, SCR turns off at first zero crossing. Drawbacks are absence of time delay for preventing false alarm when power istumed on and reduced sound level of horn with half-wave operation.-A. Pshaenich, Solid State Gas/Smoke Detector Systems, Motorola, Phoenix, AZ, 1975, AN-735, p 3.



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