Semiconductor gas sensor - heated to alternate temps. with resistance drop by gas absorption triggering fire alarm

摘要

A gas detector for use as fire alarm contains a metal-oxide semiconductor gas sensor which is repeatedly heated through an astable multivibrator with transistors to a low temp. of 30-40 degrees C at which it absorbs a gas component (CO), and to a higher temp. of 100-200 degrees C at which it releases the gas. When heated to the lower temp. a resistor is connected in a series with the sensor to indicate by its voltage rise that a reduced sensor resistance has been caused by a gas concn. above a certain threshold. This gas detector requires little electric power; it is simple yet it has a high efficiency. It can readily be set selectively for the detection of gases other than CO and is free from malfunction.