"Exhaust Fan" and then turn immediately C1 people discharge process, over time, when the C1 to the positive terminal of the voltage drop Vm / 3, the circuit inverted output terminal 3 has returned high, the relay K2 power release, "Exhaust Fan" stop operation, the circuit into the next cycle period. Although the R1 resistance values greater than the resistance value of R2, but the common element for the RP, when RP boom rotated to an end when R2, R1 + RP resistance value greater than the resistance value of R2. Then C1 charging time is much larger than the discharge time, the maximum duty cycle of the output pulse, the this fan running time is less than the stop time. When RP boom rotation R1 to end the resistance value of R1 and R2 + resistance value of less than RP. Then the capacitor C1 discharge time is slightly less than the charging time, the output pulse having a minimum duty. "Exhaust Fan" running time is greater than the stop time.