After the "Exhaust Fan" This circuit is in use, first press the start button SB1 connected to control circuit power supply, then IC2 pseudo output high so that VT conduction, the relay K1 pull power from the power switch by K1 lock, the circuit into working condition and has been maintained. The circuit IC1 output pulse period of 1h, the pulse duty cycle can be varied by adjusting the RP, thus changing the fan opening and stop time. Element analysis: the circuit is a counter circuit counts by 4017, by eight LEDs to display the count, the relay K1 to control the entire circuit, complete a round count by K1 is closed to close the circuit.