What is a pneumatic reed switch?

A magnetic reed switch is a type of magnetic sensor which responds to the presence of a magnetic field. Magnetic reed sensors are arguably the most common type of sensor used within pneumatic cylinders and are a proven technology within the industry.

How do reed switches work?

Magnetic reed switches feature magnetic proximity sensors which are switched on when an aligned magnetic field is applied. Magnetic poles of an aligned magnet sit next to each other within an axial plane. As the magnet approaches the reed switch, a magnetic field which is parallel to the reed switch is generated. Reed switches are composed of a pair of ferromagnetic metal reeds which are enclosed in sealed glass tubes. Without the presence of a magnetic field, the metal reeds separate and the sensor switches itself off. When used with pneumatic cylinders, pneumatic reed switches are mounted on cylinder tubes to detect the presence of a magnetic field generated by the magnet set on the cylinder piston. This action attracts the metal reeds together, signalling the electric circuits to switch on the magnetic sensor. A pneumatic reed switch sensor is a magnetic proximity sensor that is switched “ON” when an axially aligned magnetic field is applied to it. The magnetic poles of an axially aligned magnet are next to each other in the axial plane. As the axially aligned magnet approaches the reed sensor a magnetic field parallel to the reed switch is generated. A reed switch is composed of a pair of ferromagnetic metal reeds, which are enclosed in a sealed glass tube. Without the presence of a magnetic field the metal reeds will be separated, and the sensor will be switched “OFF”. When the cylinder piston passes the switch and applies a magnetic field strong enough to attract the reeds together, the sensor will become switched “ON”. Compared to other sensor options, reed switches are cost effective and can be operated with AC or DC voltages. Additionally, reed switch sensors have a low power consumption which makes them suitable for applications with power consumption requirements. Due to the mechanical nature of switching reed contacts, reed switch sensors do have limitations. First, the switching contacts have a finite number of switching cycles and will require maintenance over the lifespan of the machine. Next, reed switch sensors are not suitable for applications which are exposed to high vibration or shock. High shock and vibration could cause the reed contacts to chatter causing inaccurate signaling. The switching characteristics of a reed switch can also cause unintended double switching. Double switching is when the sensor output switches “ON” and “OFF” twice while the cylinder magnet passes the reed switch once. The false double switch of the sensor output is due to the non-uniform strength of a magnets force field. The strength of a magnet field is strongest at each pole of the magnet and weakest at the center between each pole. If the piston magnet is not strong enough, it may cause the switch output to double switch as it passes across the sensor. Finally, in compared to solid-state sensors, reed switches are relatively slow to activate which makes them unsuitable for applications which need fast response times. However, reed switch sensors for pneumatic cylinders are widely used due to them being relatively inexpensive compared to other sensors, require no standby power, can function with either DC or AC loads, and are a proven known solution. For 3 wire type pneumatic reed switch, click here.