Which Sensor Should You Choose: Hall Effect Sensor or Reed Switch?

Reed Switches and Hall Effect Sensors

Magnetic sensors are crucial in a wide range of devices, from automobiles to smartphones. If you're choosing a magnet for your magnetic sensor, you might wonder: Should I go for a Hall effect sensor or a reed switch? How should I orient the magnet? What tolerances should I keep in mind? This guide provides a detailed explanation of how to select the right magnet-sensor combination.

What is a Reed Switch?

A reed switch is a type of electrical switch that is triggered by a magnetic field. It consists of two contacts located on ferrous metal reeds, enclosed in an airtight glass casing. Normally, these contacts remain open, meaning there is no electrical connection. When a magnet is brought close to the reed switch, it causes the contacts to close, completing the circuit. Once the magnet is removed, the reed switch returns to its original state.

What is a Hall Effect Sensor?

A Hall effect sensor is a solid-state device that changes its output voltage in response to a magnetic field. Unlike reed switches, Hall effect sensors have no moving parts, making them ideal for digital applications. While they can serve a similar function to reed switches, they work without any mechanical action.

Which sensor is right for your application depends on several factors, including cost, magnet orientation, frequency range (reed switches typically have a limit of 10 kHz), signal bounce, and the design of the accompanying circuitry.

Magnet - Sensor Orientation

A key distinction between reed switches and Hall effect sensors is the orientation of the activating magnet. Hall effect sensors activate when a magnetic field is perpendicular to the sensor. Most Hall effect sensors are designed to detect the south pole of a magnet facing a specific location on the sensor. Always check your sensor's datasheet, as improper magnet orientation may prevent activation.

Reed switches, on the other hand, are mechanical devices with moving parts. The two ferromagnetic reeds inside are separated by a small gap. When a magnetic field is applied along the axis of the reeds, they close, creating an electrical connection. In other words, the magnet’s axis should be aligned with the long axis of the reed switch. Manufacturers like Hamlin provide useful application notes that detail the proper magnet orientations.

Other configurations are possible, such as using Hall effect sensors to detect steel blades in a spinning "fan" setup. In this case, when the blades pass between a stationary magnet and sensor, the magnetic field is blocked, causing the switch to open or close.

What Size Magnet Do I Need?

Your sensor's datasheet will specify the magnetic field strength required for activation. This is usually between 10 and 100 gauss, but be sure to verify the specific value for your sensor. Some sensors use AT (Ampere Turns) instead of gauss, but you can roughly convert between the two (1 gauss ≈ 1 AT).

For Hall effect sensors, you can calculate the distance from the magnet’s surface where the minimum magnetic strength is met. If you’re unsure, tools like a Magnet Calculator can help estimate the appropriate distance based on the magnet's size and field strength.

For reed switches, you need to know the strength of the magnetic field at a given distance from the side of the magnet. While there’s no simple formula for this, you can approximate by using the calculator's results and adjusting based on your sensor's requirements.

Engineering Tolerance Issues

A common question is the tolerance of magnetic field strength, especially for neodymium magnets. The field strength of axially magnetized disc or cylinder magnets depends on their residual flux density (Br). In general, magnets fall within 97% to 100% of the specified Br value, meaning a 3% tolerance.

In real-world applications, mechanical tolerances in the system, such as variations in the distance between the magnet and sensor, can have a larger impact than minor variations in magnetic strength. For instance, a 0.03" tolerance in the magnet-sensor distance can lead to a significant change in the field strength at the sensor. This means that when selecting a magnet, it’s important to account for mechanical tolerances, and designers often opt for a stronger magnet to compensate for these variations.