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Gas smoke alarm circuit

2015-03-02 12:17  
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Gas smoke alarm circuit
TGS308 gas sensor conductance ( Figure 5.18 ) increases in the event of combustible gas, and the voltage is got by the potentiometer RP1 slide point, and its value is increased from normal 3V rms to 20V, this elevated voltage is added to transistor VT1 by diode and 4.7kΩ resistor, so that it gets conduction, VT1 conduction makes Triacs 2N6070A get conduction. Thus, full-wave AC voltage can drive E to produce up to 90dB alarm sound. H is Deltal6003168 type 24V AC alarm. When gas disappears from the sensor, the circuit is restored to the original state, so the alarm automatically stops.