Switches are one of the most basic electrical components and as such they are included in just about every electrical circuit in existence. Along with resistors, capacitors and inductors, switches are one of the fundamental building blocks in electronics. However, while toggle switches (like those in the familiar light switch) are one of the most often used switching devices, there is a slightly more exotic and very useful variety of switch called the reed switches.
The way that reed switches operate is somewhat different than those conventional switches that most toggle everyday. A reed switch is usually meant to open or close based indirectly on human interaction whereas a toggle switch interacts directly with the user of the device. For example, one area in which reed switches has seen prolific use is the cell phone industry. Nowadays, the trend is towards smartphones, but not long ago flip phones were the rage and still make up a large portion of that market. When a flip phone is closed the action will either close or open a reed switch which signals the processor to shut down the cell phone. In that case, the consumer is not actually toggling the switch directly, but rather one side of the phone case has a magnet which activates the reed switch upon open or close.
That is precisely how a reed switch mechanism operates. A “normally open” reed switch will be configured so that the contacts close when placed in the correct magnetic field. There are also “normally closed” varieties which break current flow when subjected to a magnetic field. The magnetic sensitivity of reed switches makes them especially useful as sensors in a wide range of applications as well as making them quite interesting to hobbyists in general. One such area is home burglar alarms. The window or door sensors traditionally would have reed switches incorporated into the schematic in order to tell if there was a break in the perimeter. Even old-school keyboards would have reed switches that were activated by the pressing of an individual key. A laptop may incorporate that type of switch to determine when to go to sleep or hibernate. However, with recent improvements in solid state device technology, the reed switch has been overtaken in many instances by newer hall effect sensors which can perform the same tasks as reed switches, but with a longer life expectancy and a greater reliability.