Circuit Stages and Functioning
By referring to the schematic diagram of a smoke detector alongside, the various stages may be understood with the following points:
Gas sensor GS1 is configured in its usual standard mode.
IC2, which is a voltage regulator, provides the required 5 volt supply to the sensor heating grid and also becomes the power source for the entire circuit.
C1 and C2 are placed for the filtering any residual interference and enhance the stabilization of the regulator IC.
IC1 is wired up as a comparator with its pin #2 clamped to half the supply voltage through a potential divider network comprising R2 and R3. This avoids the need of employing a dual power rail generally recommended for op amps.
Pin #3 of IC1 is also rigged to another potential divider network made up of P1/R1 and the internal resistance of the sensor.
On detection of a gaseous element the potential difference across the relevant pin outs of TGS 813 immediately drops.
Pin #3 of IC1 detects this drop as the voltage here immediately rises above the threshold value set by P1.
Its output goes high, activating T1 and the associated load, either a relay or a powerful car siren as shown in the diagram.
This concludes the stages incorporated with the circuit, the next section explains the testing and setting up procedure of the unit.