The simple exhaust fan circuit presented here automatically determines whether there is really a need to start the fan and reacts appropriately. The circuit is powered from an AC mains adaptor that is capable of delivering an output supply of 6V DC.
The heart of the circuit (Fig.1) is an ordinary reed switch (S1) that is mounted on the cistern. The reed switch is controlled by a small magnet fitted on the toilet lid. When the lid is upright, the reed switch is closed by the magnet. One input of gate N1 of IC1 swings high. The resulting pulse at the output of gate N2 is inverted by gate N3, triggering the timer 555 (IC2). As a result, the relay (RL1) energises, causing the exhaust fan to run for a time period determined by the combination of VR1 and capacitor C3. After this time is over, the exhaust fan automatically switches off.
Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Mount the 5V relay (refer Fig. 2) on the PCB and connect relay contacts to the exhaust fan through external wires.
Fig. 3 shows the proposed fitting of the magnet on the lid and the reed switch on the cistern.