| 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. |
