Reed Switches and Hall Effect Sensors

Reed Switches and Hall Effect Sensors

Reed Switches and Hall Effect Sensors
Magnetic sensors are used in everything from cars to cellphones. What magnet should I use with my magnetic sensor? Should I use a Hall effect sensor or a reed switch? How should the magnet be oriented to the sensor? What tolerances should I be concerned with? Learn more with a K&J walk-through of specifying a magnet-sensor combination.

What is a Reed Switch?

Two Hall effect sensors and a reed switch. The reed switch is on the right.
The reed switch is an electrical switch operated by an applied magnetic field. It consists of a pair of contacts on ferrous metal reeds in an airtight glass envelope. The contacts are normally open, making no electrical contact. The switch is actuated (closed) by bringing a magnet near the switch. Once the magnet is pulled away, the reed switch will go back to its original position.

What is a Hall Effect Sensor?
A Hall effect sensor is a transducer that varies its output voltage in response to changes in magnetic field. In some ways, Hall effect sensors can ultimately perform a similar function as a reed switch, but with no moving parts. Think of it as a solid-state component, good for digital applications.

Which of these two sensors is right for your application depends on a number of things. Factors include cost, magnet orientation, frequency range (reed switches typically aren’t usable over 10 kHz), signal bounce and the design of the associated logic circuitry.

Magnet – Sensor Orientation
A key difference between reed switches and Hall effect sensors is the proper orientation required for an activating 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, but check the specification sheet of your sensor. If you turn the magnet backwards or sideways, the sensor won’t activate.

Reed switches are a mechanical device with moving parts. It consists of two ferromagnetic wires separated by a small gap. In the presence of a magnetic field that is parallel to those wires, 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. Hamlin, a manufacturer of reed switches, has an excellent application note on the subject. It includes great diagrams showing the areas and orientations in which the sensor will activate.
Proper Magnet Orientation: A Hall effect sensor (left) vs. a reed switch (right)
It should be noted that other configurations are possible and often used. For example, Hall effect sensors can detect steel blades of a spinning “fan.” The steel blades of the fan pass between a stationary magnet and stationary sensor. When the steel is between the two, the magnetic field is redirected away from the sensor (blocked) and the switch opens. When the steel moves away, the magnet closes the switch