Great sensitivity for the circuit can be obtained with a Darlington amplification step for the sensor current, as shown in the figure. This circuit can be adjusted to respond to small variations in the temperature of the sensor, with greater readiness, being suitable for more critical applications. The operating principle is the same as the first circuit. When the temperature rises, the resistance of the sensor decreases and the voltage at the 4093 port input increases. At the switching point, the inverter output goes to low level bringing the transistor Q3 to saturation. When transistor Q3 starts to drive, the motor starts up, providing the desired ventilation.
