Reed Sensor

REED SWITCH:

                Using Reed Switches in a sensing environment, one generally uses a magnet for actuation. It is important  to understand this interaction clearly for proper sensor functioning. Sensors may operate in a normally open mode, a normally closed mode or a latching mode. In the normally open mode, when a magnet is brought toward the Reed Switch (or vice versa) the reed blades will close. When the magnet is withdrawn the reed blades will open. With the normally closed sensor, bringing a magnet to the Reed Switch the reed blades will open, and  withdrawing the magnet, the reed blades will re-close. In a latching mode the reed blades may be in either an open or closed state. When a magnet is brought close to the Reed Switch the contacts will change their state. If they were initially open, the contacts will close. Withdrawing the magnet the contacts will remain closed. When the magnet is again brought close to the Reed Switch, with a changed magnetic polarity, the contacts will now open. Withdrawing the magnet the contacts will remain open. Again, reversing the magnetic polarity, and bringing the magnet again close to the Reed Switch the contacts will again close and remain closed when the magnet is withdrawn. In this manner, one has a latching sensor or a bi-stable state sensor.

              One must be aware of when using a magnet. Please keep in mind the magnetic field is three-dimensional. A permanent magnet is the most common source for operating the Reed Switch. The methods used depend on the actual application. Some of these methods are the following: front to back motion

                                    

          

rotary motion  ring magnet with parallel motion

Before we investigate each of these approaches, it is important to understand the fields associated with the variousReed Switch vs. magnet positions and their on/off domain characteristics. The actual closure and opening points willvary considerably for different Reed Switches and different sizes and strengths of magnets.

                First consider the case where the magnet and Reed Switch are parallel. In Figure #24, the open andclosure domains are shown in the x and y-axis. These domains represent the physical positioning of the magnetrelative to the Reed Switch along the x-axis. The closure and opening points are relative to the movement of the ma- gnet along this x axis, where the magnet is fixed relative to the y-axis.  Here, three domains exist, wherein Reed Switch closure can take place. Keep in mind the center domain is much stronger and the graph gives a relative

                  Idea of the closure points on a distance basis along the y-axis. The hold areas shown, demonstrates thehysteresis of the Reed Switch and will vary considerably for different Reed Switches.  In fluid level controls,having a wider hold area can be beneficial, particularly if there is constant disruption to the fluid level as in a moving vehicle. the maximum distance away from the Reed Switch for closure is possible. This approach has thebest magnetic efficiency.