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 ...