A modern factory will have several electronic devices working, and most of them will have several sensors. Typically, these sensors connect to the devices using wires. The wires provide the sensor with a supply voltage, a ground connection, and the signal output. The application of power allows the sensor to function properly, whether the sensor is sensing the presence of a ferromagnetic metal nearby, or it is sending out a beam of light as a part of the security system. On the other hand, simple mechanical switches, like reed switches, require only two wires to trigger the sensors. These switches need magnetic fields to activate.
The reed switch was born and patented at the Bell Telephone Laboratories. The basic reed switch looks like a small glass capsule that has two protruding wires. Inside the capsule, the wires connect to two ferromagnetic blades with only a few microns separating them. If a magnet happens to approach the switch, the two blades attract each other, making a connection for electricity to flow through them. This is the NO type of reed switch, and it is a normally open circuit until a magnet approaches it. There is another type of reed switch, the NC type, and it has one blade as a non-ferromagnetic type. This switch is a normally closed type, allowing electric current to flow until a magnet approaches it. The approaching magnet makes the blades pull apart, breaking the contact.
Manufacturers use a variety of metals to construct the contacts. This includes rhodium and tungsten. Some switches also use mercury, but the switch must remain in a proper orientation for switching. The glass envelope typically has an inert atmosphere inside—commonly nitrogen—to seal the contacts at one atmospheric pressure. Sealing with an inert atmosphere ensures the contacts remain isolated, prevents corrosion, and quenches sparks that might result from current interruption due to contact movement.
Although there are solid state Hall effect sensors for detecting magnetic fields, the reed switch has its own advantages that are necessary for some applications. One of them is the superior electrical isolation that reed switches offer compared to what Hall effect sensors do. Moreover, the electrical resistance introduced is much lower for reed switches. Furthermore, reed switches are comfortable working with a range of voltages, variable loads, and frequencies, as they function simply as a switch to connect or disconnect two wires. On the other hand, Hall switches require supporting circuitry to function, which reed switches do not.
For a mechanical switch, reed switches have incredibly high reliability—they typically function for billions of cycles before failing. Moreover, because of their sealed construction, reed switches can function even in explosive environments, where a single spark could generate disastrous results. Although reed switches are older technology, they are far from obsolete. Reed switches are now available in surface mount technology for mounting on boards with automated pick-and-place machinery.
The functioning of reed switches does not require a permanent magnet to actuate them. Even electromagnets can turn them on. Initially, Bell labs used these switches abundantly in their telephone systems, until they changed over to digital electronics.