smoke and gas detector

This circuit is intended for use as a preventive device. We all know about accidents that occur through the accumulation of gas or of people over- come by smoke. The preventive character manifests itself by timely warnings in case of high gas concen- trations in a manner that does not cause the gas to explode. The circuit is based on sensor type TGS109 which is sensitive to gases enumerated in the accompanying table. Power is provided by an 8-volt bell transformer which is tapped at 5 V. The voltage developed across the 5 V winding is rectified by D3, smoothed by C1, and regulated by R2, D4, and C2. The resulting direct voltage of about 5.6 V is used to supply IC1. The 3 V alternating voltage is used to operate the sensor, which needs 1 V at about 0.5 A. Resistor R1provides the necessary voltage drop. The mutual inductance between the two windings of the sensor increases with rising gas concentrations. Note that there is no difference in the two windings: the sensor may therefore ...