Reed Switches:

Reed switches, described in below diagram, are more reliable than limit switches, because of their simplified construction.

The switches are built of flexible ferrous strips (reeds) and are placed near the intended travel of the valve stem or control rod extension.

Figure: Reed Switches

While using reed switches, the extension used is a permanent magnet. As the magnet approaches the reed switch then the switch shuts. While the magnet moves away, the reed switch opens. That ON/OFF indicator is same to mechanical limit switches. Through using a huge number of magnetic reed switches, incremental position could be measured. This method is sometimes used in monitoring a reactor's control rod position.

Failures are generally limited to a reed switch that is stuck open or stuck shut. The open (closed) indication will be continuously illuminated if a reed switch is stuck shut. The position indication for which switch remains extinguished regardless of valve position if a reed switch is stuck open.