Functioning and appliance of reed sensors

Reed-sensors, also known as magnetic sensor, in shorted Reed1, are binary non-contact sensors, which have a wide range of applications in addition to the electric pneumatics. The Reeds were developed by the 'Bell Laps'-company at the end of 1930. From about 1940 the industrial use began.

Symbol - Reed switch

Symbol - Reed switch

Function:  The structure is simple: a hermetically sealed reed consisting of iron-nickel alloy, forming a normally opened or normally closed contact. A magnet approaching this ferromagnetic reed leads to switching.

Reed - how it works

Reed - how it works

Because of the hermetic seal and the contactless switching the reed sensors have some obviously great advantages to the mechanical switches. It´s difficult to find concrete specifications from manufacturers because of product liability, but the product life begins at about 1 million switching cycles. The switching time is about 0.2 ms.

Detecting end-position of a cylinder

Reed sensor animation

The applications for Reed's are varied. In the electropneumatics these magnetic switches are used primarily for detecting the end position of cylinders:

Example circuit: Pressing the S1 button the cylinder extends and automatically retracts upon reaching the front end position. In the animation below, the reed switches directly a solenoid valve. Another possibility would be the use of a relay if the switching power of the reed is not sufficient.

Reed example circuit

Reed example circuit

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