Magnetic Reed switch: an amazing addition to your design toolbox

Reed switches are simple, passive, electromechanical components that were invented circa the 1940s. Reed switches aren’t mentioned much in electronic literature or forums that much, but they have some amazing capabilities. Reed switches are sometimes called magnetic switches. Reed switches open or close when exposed to a magnetic field. The reed switch consists of a couple of contacts made of magnetically sensitive materials (reeds) that are hermetically encapsulated in a glass envelope. The reed switch opens or closes when a magnet is near. When the magnet is removed, the reed switch returns to its normal position. The strength of the magnet required to actuate the reed switch is measured in units of gauss, found in the datasheet for the component.

Figure 1: The magnetic reed switch is both a sensor and load-conducting contacts (reeds) in one simple device. A magnetic field causes the reeds to pull in, or meet. Shown: Form A type reed switch. (Credit: Meder Electronics)

Why not use a Hall effect sensor?

Hall effect sensors also operate when exposed to a magnetic field. However, there are some interesting differences. Hall effect sensors are solid-state (semiconductor) devices that can trigger an external relay with a low voltage output signal. Hall effect sensors require constant power to operate. Even when the Hall Effect sensor is not detecting a magnetic field, it needs to be powered on so it can be ready to sense a magnetic presence. A reed switch is both the sensor and the contacts in one element and needs no power to do its job. Reed switches conduct electricity through the reeds that act as contacts and also move upon detecting magnetic field, thus requiring no external parts; the reed sensor is the switch, too. Reed switches can be used over a wide temperature range, in all types of harsh and dirty environments, and come in a wide variety of shapes and packages (the glass surface mount type is shown in Figure 1.) Hall Effect Sensors are polarity sensitive, whereas Reed switches are not. Both reed and Hall sensors offer contactless operation with a magnetic field as a trigger. The ON state resistance of a Hall Effect sensor is at least 10 times that of Reed switches. Reed switches can withstand a three foot drop test, similar to that of Hall Effect sensors.

magnetic/reed switch — Figure 2: The axis of the magnetic field is parallel to the reed switch to cause actuation. (Credit: Hamlin, Inc.)

Some reed switches can operate at a greater distance from the magnetic source than a hall effect sensor, given the same magnetic flux as a trigger. If positioning is a big deal, you should know that Hall Effect devices need to be placed orthogonal to the magnetization axis of the magnet, whereas for a reed switch, you need to place the magnet’s field axis parallel to the reed switch. The simple reed switch is hardy and requires no power, but the Hall Effect sensor is more sophisticated in that it can determine the direction, as well as the rate at which it passes the magnetic field. Many Hall Effect sensors are packaged together with a relay or transducer and circuitry to provide an all-in-one contactless solution. However, sometimes the reed switch might be all that is necessary. Reed switches are used in medical devices such as defibrillators, in automotive safety systems, and in automatic test equipment for integrated circuits.

Table 1: A comparison of Reed Switches vs. Hall Effect Sensors

Parameter	Reed Switch	Hall Effect Sensor
Form Factor	Moving parts/electromechanical	Solid State
Physical size	< 4 mm long (glass encapsulate)	IC package can be < 2 mm plus leads
Touch-free sensing	Yes	Yes
Voltage Switching Range	0 – 1000 V	Signal output in milli-volts
Switching time	< 100 µs to operate	< 10 µs to operate
Load switching	Switches loads directly	Sensor needs external circuitry to switch a load
Switch operations in a lifetime	Billions	Billions
Voltage output offset	Not applicable	Required to adjust for temperature or thermal stress
ESD Protection Required	No	Yes
Vibration limits	10 g	> 50 g
Power required to sense & operate	No	Yes
Operating Temperature	0° – 70°C, outside of which temperatures will affect performance.	-55° – 150°C
Polarity sensitive	No	Yes
External circuitry that may be required	None	If not integrated into the Hall Effect sensor chip, it may require the following: amplifier, chopper, voltage regulator, ESD protection, external filter and switch
Senses both rate and direction of a magnetic field	No	Yes. Less likely to lose count of revolutions.
Latching capability	External circuit required	Additional circuit required that may be included in the integrated chip package