Circuit arrangement for the evaluation of a signal produced by a semiconductor gas sensor

A gas-sensing system for controlling an air-recirculation valve in a motor vehicle having an interior to which air from outside can be supplied or in which air can be recirculated in accordance with a setting of the air-recirculation valve, said sensing system comprising:

• a semiconductor sensor providing an output signal corresponding to a concentration of external noxious gases having a critical high level of the concentration;
• an adjustable electricity source connected in series with said sensor and forming therewith a voltage divider;
• control means connected with said divider and receiving the output signal for realizing a time delay of said output signal with a time constant higher than a time needed for forming said critical level of said noxious gases and for producing a control signal and feeding the control signal to the source to modify the output signal of the sensor when the time constant is exceeded so as to compensate for a gradual change in concentration exceeding the time delay;
• and
• a comparator receiving said control signal and operatively connected with said voltage divider and with said control means, said comparator processing said control signal and a reference signal corresponding to said critical high level, said comparator switching the air-recirculation valve from an exterior air-supply position to an interior air-recirculation position upon said control signal exceeding said reference signal during a time at least equal to said time constant.