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Reed switch: working principle, Types, and applications

Reed switch

What is a reed switch?

Magnetic reed switch an electrical switch driven by a magnet. It consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. There are two types of configuration: Normally open (NO), closes when a magnetic field is brought near and normally close (NC) open when a magnetic field is brought near. When the magnetic field is removed, the contacts in the reed switch return to their original position.

It was invented at Bell Telephone Laboratories in 1936 by Walter B. Ellwood.

How a Reed Switch Work?

The switching mechanism of the reed switch consists of two ferromagnetic blades which are separated by a small gap. These ferromagnetic blades are lightweight, magnetic and flexible. When a magnet is brought close to these blades, the two blades pull towards each other. Once the blades are closed the normally open (NO) contacts that allow electricity to flow. Some reed switches do not have a ferromagnetic contact, such switches typically produce a closed (NC) output. When a magnet is brought close to these blades the contact is disconnected and the switch is removed from the contact.

Types of Reed switch:

Reed switches classified in two basic types: Normally Open and Normally Closed.

Normally Open (NO) Reed Switches:

In a Normally Open reed switch design, the switch defaults to an open position. When an outside magnet approaches, it polarizes the two contacts, allowing them to attract each other and closes the contacts. This closes the circuit, allows the electrical current to flow. When the outside magnet is removed, the stiff contacts spring back into their original position, thus breaking the circuit and stop the electrical current flow.

Normally close (NC) Reed Switches:

Normally Closed reed switches operate on the same principle, except that the switch uses a built-in magnet to keep its two contacts in a close state. This means that the switch’s default position is to keep the circuit closed, allowing the electrical current to flow continuously. When a magnet with reversed polarity approaches, it collapse out the magnetic field of the built-in magnet. This causes the two contacts to lose polarity, and they spring apart from each other to their default, non-magnetized positions. This breaks the circuit, stopping the electrical current flow.

Difference between Hall effect and reed switch:

A major difference between reed switches and Hall effect sensors is the proper orientation required for magnet.  Hall effect sensors activate when a magnetic field that is perpendicular to the solid-state sensor is applied.  Most look for the magnet's South Pole to be facing an indicated location on the sensor.  If you turn the magnet backwards or sideways the sensor won't activate.

Reed switches are an electrical switch with moving parts.  It consists of two ferromagnetic contact separated by a small gap.  In the presence of a magnetic field they will touch each other, making electrical contact.  In other words, the magnetic axis of the magnet should be parallel to the long axis of the reed switch.

Application examples of reed switches:

Reed Switches in Security Systems:

Reed switches can be used to signal the position of doors, elevators, windows, and gates. Reed switch sensors can limit access to equipment panels, prevent removal of valuable system components, sense the accumulation of water or other fluids or sense seismic movement.

Reed switches in home appliances:

Reed switches are used as liquid level sensors in coffee makers, dishwashers, washing machines and water heaters, or as heat sensors in cookers or steamers. They can also be used as proximity sensors on utility doors, access panels and bicycle odometers.

Reed Switches in Automobiles:

Reed switches are used in fluid level sensors to monitor brake fluid reservoirs and oil levels. They are also used in speed sensors for engine control and power steering. Automatic door locks, airbags, parking brakes, seat, door, and hood proximity sensors also use reed switches.

Reed Switches in Robotics and Automation:

Reed switches are used as proximity sensors to control robotic motion, speed, location and angle. They can sense whether valves, tracks, hoppers, and doors are open or closed, or if containers or conveyors are full or empty. Reed sensors report the position and movement of products in vending machines.

Reed Switches in very Harsh Environments:

Reed switches are hermetically sealed, durable, proven, reliable and precise. They can tolerate wide swings in temperature and to of shock. Reed switches tolerate high humidity, sea salt spray, toxic gases, dangerous chemicals, and dust or partial laden atmospheres. Reed switches do not need to be powered (battery) in the off position.

Advantages of reed switch:

  • Reliability and durability
  • Precision in magnetic sensitivity
  • Simple in design
  • As they are triggered by magnetism, there is no voltage required
  • As the contacts of a reed switch are sealed within a glass tube they are protected from atmospheric corrosion.

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