"Exhaust Fan" IC2 for the CD4017 decade counter circuit with its output Q0 Q7 to control a pull off the relay with the KI. When Q0 a Q7 as long as there is a high output, relay K1 will not be disconnected. When the circuit is powered, Q0 outputs a high level so that VT conduction, the relay K1 pull. In subsequent work, although the output will end by the Q0, Q1, Q2. ..., Q7 changes, but VT conduction state does not change until the Q8 output high resets the counter. IC2 counting pulses output from the ICI, ICI output whenever a pulse period, IC2 will count once, the output terminal that is a forward. If you do not add a preset, then when the "Exhaust Fan" operation seven cycles will be cut off the power supply to stop working. SB2 is preset buttons, each press of a button, then to IC2 input count pulse, the output terminal that is a forward, the remaining work will reduce cycle time. Preset is through so-called pre-reduction "Exhaust Fan" work cycles.