"Exhaust Fan" in this circuit, at the beginning of power, since the beginning of charging C1, the voltage is less than the upper end of the VP / 3, IC1 3 feet high output. Then VL2 light indication, K2 relay coil is not energized, the switch remain static. However, this high level in the early days of the formation of a count pulse to the CP end of IC2 input through C3, the output of IC2 move up by one. Then the capacitor C1 into the state of charge, and the output terminal 3 is high. C1 is charged until it reaches the positive terminal of the voltage 2 VDD / 3, the circuit inverted output terminal 3 goes low. Then pull power relay K2, the fans of this kind of power on the work, this type of fans start to run. VL1 working status indicator light indication.