THE GAS SENSOR is mainly composed of tin dioxide on a ceramic base; the resistance of the sensor varies depending on the concentration of reducing gases in the air. The circuit shown is useful for the detection of dangerous levels of combustible fumes or gases. It uses a comparator circuit to trigger an alarm buzzer. The sensor`s resistant element is connected in series with resistor R1 to form a voltage- divider circuit;
R1 is specifically matched to each gas sensor by the manufacturer.