Intro to Reed Switches
What is a reed switch?
A reed switch consists of two ferrous leads sealed inside a glass tube. With an appropriate magnetic field applied, the leads will attract each other and snap together. Please note that this article will be expanded later.
Reed Switch Operation Patterns
Magnet Pole Facing Reed Switch Axis
These views give an idea of the activate regions near a reed switch. There will also be a hysteresis region (not shown). The magnet will need to be pulled a certain distance outside the activate regions before the switch will deactivate.
The actual shapes will depend on how the wire leads have been trimmed and formed. Note that trimming and forming the leads will change the shapes for a particular switch. Reed switches should be tested for applications after the leads have been modified.
As a rough guideline, the activate regions will shrink as leads are trimmed to shorter lengths.
How do Reed Switches Operate?
Reed switch magnetization in normal operation
This animation shows the basic idea of how a reed switch works. An approaching magnet causes the leads to magnetize. At the center of the switch, the leads have opposite magnetic polarities and attract each other.
Reed Switch Magnetization in Biased Configuration
Reed switches are sometimes used in a biased configuration. A bias magnet holds the switch normally closed. An approaching magnet will cause the switch to open.
With a single magnet, the leads are magnetized so that they attract. The second magnet causes the leads to have the same polarity at the center. With the same polarity, they repel each other so the switch opens.
Please note that this article is incomplete. There are other reed switch configurations not shown. The reason for this is the difficulty involved with simulating reed switch behavior. The majority of the graphics in the articles about Hall-effect and other sensors were generated through actual simulation work. The graphics in this article are purely schematic. Some aspects of reed switch behavior would be best shown via simulation or graphing of experimental data. At some point in the future, I'll put in the time necessary to produce quality and accurate graphics. For now, I'm putting in my time generating quality content on other topics.