Bench Talk

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You may have heard the expression “opposites attract.” This universally relatable phrase is derived from magnetism, where opposite poles, north (N) and south (S), are attracted to each other. You may remember this from playing with magnets as a kid. But, many people don’t realize that magnets are used in many systems and products in their everyday life.

Today’s buildings and homes are equipped with intelligent technologies for safer, more convenient, and energy-efficient living. Littelfuse is a global leader in magnetic sensing solutions. Its Building Automation solutions help designers create Internet of Things (IoT) devices that improve the safety, reliability, convenience, and energy efficiency of buildings. Sensor solutions provide information related to tampering, position, speed, and temperature, among other variables. The following will cover three magnetic sensor technologies: reed, Hall effect, and tunneling magnetoresistance (TMR) related to applications in building automation.

Three leading magnetic sensor approaches include reed sensors, Hall effect devices, and TMR sensors.

Reed sensors are reed switches packaged within an external housing for simplified mounting/connecting and additional protection against environmental influences (Figure 1). These sensors are typically mounted in mechanical systems. A bare reed switch can easily be mounted on circuit boards. However, for an application such as a door-security sensor, the reed switch needs a protective shell/housing for handling and mounting. These packages offer resistance to mechanical stress by protecting the bare glass of the reed switch. Reed sensors require zero power to operate. They can carry a load up to 100W and are available in NO, NC, CO in various sizes. The switches operate in X, Y, and Z sensing directions and are suitable for ultra-low-power battery applications. They can switch power level loads, incorporate the most mounting flexibility and exhibit stable operation over the widest range of temperatures.

Figure 1: Littelfuse reed switches operate in X, Y, & Z sensing directions and consume zero power to operate, making them suitable for ultra-low-power- battery applications. (Source: Mouser Electronics)

A Hall effect device is a semiconductor-based integrated circuit with Hall plates that respond to magnetic fields (Figure 2). Hall effect technology enables sophisticated sensors used in various automotive, electronic, and industrial product applications. Hall effect devices provide digital or analog output signals used for proximity and continuous rotary or linear positioning. Unlike a reed switch, a Hall effect device contains active solid-state circuitry, so it draws current at all times. These devices are available in digital, analog, and programmable configurations and are well-suited for high-shock or high-vibration environments. They are reliable to billions of operations and suitable for angular position sensing and high-speed switching.

Figure 2: The image shows an example of a hall effect device, the Littelfuse 55110 Flange Mount Hall Effect Sensor. Hall effect devices draw current at all times and are suitable for angular position sensing and high-speed switching. (Source: Mouser Electronics)

Tunneling magnetoresistance (TMR) elements are a new type of magnetoresistance sensor. TMR sensors have better linearity, better temperature stability, higher sensitivity, and lower power consumption compared to Hall effect sensing technology available today (Figure 3). The sensors are solid-state devices with no moving parts. Like Hall effect, the sensors are available in digital and analog and require very little power to operate (lower even than that required of Hall effect sensors). Of the three magnetic sensors covered, they provide the highest level of sensitivity. They are eminently suitable for high-shock and high-vibration environments and offer reliability up to billions of operations. They also require the most miniature magnet needed to operate.

Figure 3: The Littelfuse TMR Switch series has better linearity, better temperature stability, higher sensitivity, and lower power consumption compared to Hall effect technology. (Source: Mouser Electronics)

High-noise environments might require over-voltage (OV) protection. Both Hall effect and TMR sensors show sensitivity to electromagnetic interference (EMI) and electrostatic discharge (ESD). Proximity to motors, generators, reactive circuit components, or long cable harnesses (>1m) can cause damage. Design engineers are cautioned to consider the appropriate device protections, including options such as Littelfuse Transient Voltage Suppressor (TVS) Diodes for high-noise or Littelfuse SD05 Uni/Bidirectional TVS Diodes for low-noise situations.

Table 1: Magnetic Sensing Application Examples

Reed sensors have a clear advantage for anti-tamper applications. Reed sensors do not draw power and thus enable longer system battery life and provide 3D anti-tamper protection. In meter counting, reed is presently in the lead; however, TMR may make sense in some cases where battery life is not as important. Reed has a proven track record with 100M+ units in the field (and counting). Reed is the straightforward winner of indoor security applications. A changeover to reed sensors pays for itself in battery life and no service calls.

Today’s buildings and homes are equipped with intelligent technologies for safer, more convenient, and energy-efficient living. Littelfuse magnetic sensors are an attractive choice for control in building automation.