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Toxic Gas Detector

The major device in the circuit is SR1 (a TGS812 toxic-gas sensor manufactured by Figaro Engineering Inc.) The gas-sensitive semiconductor (acting like a variable resistor in the presence of toxic gas) decreases in electrical resistance when gaseous toxins are absorbed from the sensor surface. A 25,000 ohm potentiometer (R5) connected to the sensor serves as a load, voltage-dividing network, and sensitivity control and has its center tap connected to the gate of SCR1. When toxic fumes come in contact with the sensor, decreasing its electrical resistance, current flows through the load (potentiometer R5). The voltage developed across the wiper of R5, which is connected to the gate of SCR1, triggers the SCR into conduction. With SCR1 now conducting, pin 1-volt supply for the semiconductor elements of the TGS812 in spite of the suggested 10 volts, thus reducing the standby current. A 7805 regulator is used to meet the 5-volt requirement for the heater and semiconductor elements.

Parts List:
Component: Value: Datasheet: Qty:


Not Available 01

25K Pot

Not Available 01


Not Available 02
Resistor 10 Not Available 01
Capacitor 25uF Not Available 01
Capacitor 0.006uF Not Available 01
Capacitor 0.01uF Not Available 01
IC 4071 Available 01
IC 555 Available 01
Regulator 7805 Available 01
SCR C206A Not Available 01
? TGS812 Not Available 01