Posted on Nov 2, 2012

In the presence of smoke or gas, the ac output voltage increases and becomes rectified, filtered and zener-diode coupled (D2 for thresholding) to sensitivity control R3. Under no gas condition, the output equals approximately 0 V (high). When gas is present, the output will be a negative value (low) sufficient to overcome the threshold of MeMOS gate 2 and D2. The circuit shown uses a TGS 308 sensor, a generalpurpose gas detector that is not sensitive to smoke or carbon monoxide.

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If smoke is the primary element to be detected, use the TGS 202 sensor. The two sensors are basically identical; the main differences lie in the heater voltage and the required warm up time delay. The TGS requires a 1.2 V heater and a 2 minute delay, whereas the TGS 202 requires 1.5 V and 5 minutes, respectively. The system uses a MeMOS gated oscillator directly interfacing with a triac-controlled ac hom. Using the MC14572 HEX functional gate, four inverters, one two-input NAND gate and one two-input NOR gate, the circuit provides the complete gas/smoke detector logic functions time delay, gated astable multivibrator control and buffers operation. The 24-Vac hom produces an 85/90-dB sound level output at a distance oflO ft. Controlled by the astable multivibrator, the hom generates a pulsating alarm-a signal that may be advantageous over a continuous one in some noise environments.

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