"Exhaust Fan" 555-flop constituting the duty ratio adjustable circuit, the circuit does not exhaust the exhaust cycle; CD4017 constitute the timing counter do circuit disconnect; "Exhaust Fan" operating circuit. In addition to the program of work on a power supply consists of two parts, part of the "Exhaust Fan" is automatically controlled automatic start and stop cycle control, the other is off the number of stops counting electronic counter. Timed cycled on and off by the timing controller 555 integrated circuits in the circuit, the integrated circuit 555 is connected to a pulse duty cycle adjustable ultra-low frequency multivibrator.