Reed sensors - Functioning and appliance
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
Function: The structure is simple: a hermetically sealed reed consisting of iron-nickel alloy, forming a normally opened or normally closed contact. A mágnet approaching this ferromagnetic reed leads to swítching.
Reed - how does it work
Because of the hermetic seal and the contactless swítching the reed sensors have some obviously great advantages to the mechanical swítching. It´s difficult to find concrete specifications from manufacturers because of product liability, but the product life begins at about 1 million swítching cycles. The swítching time is about 0.2 ms.
Reed sensor animation
The applications for Reed's are varied. In the electropneumatics these magnetic swítches 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 solenoíd valve. Another possibility would be the use of a relay if the swítching power of the reed is not sufficient.
Reed example circuit