The operation principle of the magnetic sensor is based on the use of reed contacts, which are formed by two plates of low-reluctance, magnetic, iron material (iron/nickel) located inside a glass bulb filled with an inert gas. By exposure to a magnetic filed, the plates are attracted towards each other by effect of magnetic induction, and thus establish an electric contact. The reed contact is actuated by an external field generated by a permanent magnet (magnetic unit). The reed contacts differ from each other with for to their format (standard or miniature) and type (NO, NC, Change-Over):
 
Normally Open Contact (NO) the reed contact, which is open in the rest state, closes when the magnet approaches.
 
Normally Closed Contact (NC) the reed contact, which is closed in the rest state, opens when the magnet approaches.
 
Change-Over Contact (S) both the N.O. and N.C. functions are obtained in a single bulb. When the actuating magnet gets closer and moves away, the contact switches from the rest state to the working state and vice versa.
 
Bistable Contact (BI) an internal magnet pre-magnetized the plates, but without closing them. When a magnet with the same polarity approaches, the magnetic filed is reinforced. In this case, the contact closes and will remain closed also when the actuating magnet leaves the influence zone.
To open the contact again the magnet has to approach, with an opposite polarity to the premagnetized
field.
 
Compared to traditional, mechanical, proximity devices, the use of reed contacts offers many advantages:
· Protection of the contact against dust or humidity, thanks to the hermetic sealing.
· Easy installation.
· High switching frequency (up to 300 Hz).
· Long life of the contact (10⁸ operations).
· maintenance free and very small dimensions.
· Impact resistance with acceleration equal to 50 times the gravity acceleration.
· resistance to vibration whit acceleration equal to 35 times the acceleration of gravity.

For correct voltage and current values, the working life of a magnetic sensor depends on the mechanical characteristics of the contact. Inversely, for high values the life is dependent on the load characteristics. In this case, the best solution is to install an external protection at the sensor output.