Reed switch

A reed switch is a small device that can be used to detect when something is near. It has two metal strips (called reeds) inside of it that are normally separated from each other. When something magnetic comes close to the reed switch, the reeds get pulled together and the switch will turn on. This is like a light switch - when the switch is on, something can happen, like a light turning on. Reed switches can be used in a lot of different ways, like when you open a door and the lights come on because of the reed switch detecting that the door opened.