PURPOSE: To prevent deterioration of a gas detection element of self-heating type for detecting as concn. from its electric conductivity, by lowering the temp. of the element after a prescribed delay time when a gas not above a prescribed concn. is present.
CONSTITUTION: Power supply is turned on, a prescribed voltage is applied to the element 3 and its heater. When a gas concn. in the atmosphere exceeds an alarm concn., a comparator C1 actuates a buzzer 4. When the gas concn. in the atmosphere is in the range defined by comparators C2 and C3, FET 10 is made nonconductive, and a CR timer T1 begins to act. When it is charged, a transistor Tr1 is made nonconductive, and voltage applied to the element 3 is dropped. Thereafter, when a comparator C4 outputs H level, a CR timer T2 is actuated, after charging, the transistor Tr1 is again made conductive, and the voltage applied to the element 3 is again raised. When a comparator C5 outputs H level, the FET 10 is made conductive, and the timers 1, 2 are discharged.
KIRA MITSUJI
SUNAHARA SHIYOUZABUROU
