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Bathroom Fan Controller


Many bathrooms are fitted with a fan to vent  excess humidity while someone is showering. This fan can be connected to the light  switch, but then it runs even if you only want  to brush your teeth. A better solution is to  equip the fan with a humidity sensor. A disadvantage of this approach is that by the time  the humidity sensor switches on the fan, the  room is already too humid. Consequently, we decided to build a circuit  that operates by sensing the temperature of  the hot water line to the shower. The fan runs  as soon as the water line becomes hot. It continues to run for a few minutes after the line  cools down, so that you have considerably  fewer problems with humidity in the bathroom without having the fan run for no reason.

Naturally, this is only possible if you can  fit a temperature sensor somewhere on the  hot water line and the line does not become  warm if hot water is used somewhere else. We use an LM335 as the temperature sensor.  It generates an output voltage of 10 mV per  Kelvin. The output voltage is 3.03 V at 30 °C,  3.13 V at 40 °C, 3.23 V at 50 °C, and so on.  We want to have the fan switch on at a temperature somewhere between 40 and 50 °C (approx.100–150 °F). To do this accurately,we first use the opamps in IC2 to improve  the control range. Otherwise we would have  an unstable circuit because the voltage differences at the output of IC1 are relatively  small. IC2a subtracts a voltage of exactly 3.0 V from  the output voltage of IC1.


Bathroom-Fan-Controller-Circuit -Diagram
 Bathroom Fan Controller Circuit Diagram

It uses Zener diode  D1 for this purpose, so this is not dependent on the value of the supply voltage. The  value of R2 must be selected according to  the actual supply voltage so that the current through D1 is approximately 5 mA. It is  600 Ω with a 6-V supply (560 Ω is also okay),  or 2400 Ω (2.2 kΩ) with a 15-V supply. If you  have to choose between two values, use the  lower value. IC2b amplifies the output voltage of IC2a  by a factor of 16 ((R7 + R8) ÷ R8). As a result,  the voltage at the output of IC2b is 0.48 V at  30 °C, 2.08 V at 40 °C (104 °F), and 3.68 V at  50 °C (122 °F). Comparator IC3a compares this  voltage to a reference voltage set by P1. Due  to variations resulting from the tolerances of  the resistor values, the setting of P1 is best  determined experimentally. A voltage of 2.5 V  on the wiper should be a good starting point  (in theory, this corresponds to 42.6 °C).

When  the water line is warm enough, the output of IC3 goes Low. R10  provides  hysteresis  at  the  output  of  IC3a by pulling the voltage on the wiper of  the setting potentiometer down a bit when  the output of IC3a goes Low. IC3b acts as an  inverter so that relay Re1 is energised via T1,  which causes the fan to start running. After  the water line cools down, the relay is de-energised and the fan stops. If this happens  too quickly, you can reduce the value of R11  (to 33 kΩ, for example). This increases the  hysteresis. The circuit does not draw much current, and  the supply voltage is noncritical. A charging  adapter from a discarded mobile phone can  thus be used to power the circuit. If the supply voltage drops slightly when the relay is  energised, this will not create any problem.  In this case the voltage on the wiper of P1 will  also drop slightly, which provides a bit more  hysteresis on IC3a.

Author : Heino Peters - Copyright : Elektor

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