Reed Sensor Uses Magnetic Switch For Zero Power Solutions

A reed sensor uses magnetic fields to open or close a switch. Unlike solid state, or other mechanical switches, a reed sensor consumes zero power as it operates based on the flux of the magnetic field.

The reed sensor was originally developed at Bell Labs in the 1930’s by Dr. Ellwood. The first devices were made using two flattened pieces of ferromagnetic wires that resemble reeds, hence the name. The reeds are hermetically sealed in an enclosure, typically glass.  Modern reeds are made from plated wires which can be used to make switches as small as 0.025 inches up to 3 inches.

The most common implementation is for a reed switch that is normally open. In this configuration, the two reeds are separated by an air gap and close to make contact only in the presence of a magnetic field. This type of switch is called a “Single Pole Single Throw Normally Open” (SPST-NO) switch or “Form A” switch.

You can get a pack of 10 switches for about $2 on Amazon: 10W Low Voltage Current Normally Open Magnetic Switch 2x14mm. These are great to experiment with for Arduino projects.

It’s also possible to create a switch element that is electrically conductive (e.g., closed) in the absence of a magnetic field and which opens when the magnetic field is applied. This configuration requires the addition of a nonmagnetic contact. This type of switch is called a “Single Pole Double Throw” (SPDT) switch or “Changeover” switch. It’s important to note that this is a “break-before-make” switch – meaning that the closed contact opens before the open contact closes.

Also through Amazon, Reed Switch Magnetic Induction Switch Electromagnetic N/O N/C SPDT Glass Seal CYT1023 10-pack for about $14.

In both cases above, the state or position of the switch returns to its original starting state when the magnetic field is removed. The restoring spring force inherent in the reeds is sufficient for the reeds to resume their original orientation.

The benefits for selecting a reed switch for an application include:

• Sealed unit so can be used for liquid / fluid level monitoring and measuring
• No wearing parts, so can open and close in the billions of operations
• Low current, Low voltage switch activation
• Zero Power consumption operation
• Rated for hazardous environments

Related Article – Sensors For Liquid Level Monitoring

The most common applications include:

• security and alarm (e.g., alarm contacts on doors and windows)
• proximity sensing  (these are sometimes referred to as NAMUR sensor)
• sensing movement
• counting
• fluid level measurement
• test and measurement
• household appliances (e.g., door open / closed position)
• automotive
• medical devices (e.g., hearing aids and sensors which are swallowed)
• telecommunication
• industrial

The main manufacturers and suppliers of reed switches are :

Hamlin

Omron Automation And Safety

Standex-Meder Electronics

Reed sensor designs are finding more applications as engineers look to methods to reduce power usage when possible. This technology makes it possible to develop new products which consume zero power.

(Photo Credit – eSolutions)

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