Andy, Hampshire asked:
In public toilets, there are often automatic hot air hand driers, which come on when you put your hand under them and go off again when you move away. How does the hand drier know when to switch on/off?
This type of hand drier contains a proximity sensor. If you look underneath the drier, near where the air outlet is, you can see a small glass section, almost like a mini window pane. If you were able to open this up and look inside, you would see an emitter which usually sends out an infra red pulse. Next to the emitter is an infra red receiver. The infra red source is too weak to bounce back from the floor to the receiver, but when you put your hand in the way the infra red beam bounces off your hands back into the receiver and it turns the hot air on. Others are designed with just a receiver to pick up the infra red radiation emitted by your hands.