The main principle of "Exhaust Fan" in the program in use, 555 IC enables oscillation circuit is connected between the power source through the circuit 7,6,2 feet resistors R1, R2 butt to the ground terminal of the capacitor C1 the charge and discharge process, with three pins each cooperating function to complete. In front of the already described oscillation circuit, the charging circuit of the capacitor C1 is formed by R1, R2 and C1 the ground; and the discharge of the capacitor C1 is positive to 7 feet by R2 completion. As long as the circuit R1, R2 and C1 of the value set, the oscillation frequency of the oscillation circuit and a pulse duty ratio is fixed and can not be changed.