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What are RTDs?

RTDs or Resistance Temperature Detectors are the simplest way to measure temperature. RTD sensors work on the principle that a metal’s electrical resistance changes with temperature. For instance, the electrical resistance of pure metals typically increases with an increase in temperature, that is, they exhibit a positive temperature coefficient. RTDs operate over a huge temperature range, starting from -200 °C, right up to +850 °C. They offer excellent long-term stability, high accuracy, and repeatability.

An RTD sensor, being a passive device, does not produce a signal by itself. An electronic circuit is necessary to send an excitation current through the sensor. This produces a voltage across the RTD, proportional to the excitation current and the resistance of the RTD. Further electronic circuitry amplifies the voltage across the RTD and delivers it to an analog-to-digital converter, whose output produces a digital output, a representative of the temperature of the RTD.

The electronics in an RTD circuit have some basic trade-offs. For instance, the excitation current must be small enough to prevent self-heating in the RTD element. Any excitation current produces Joule or I2R heating in the RTD. This self-heating effect can raise the sensor’s temperature to a value higher than that of the environment that the RTD is measuring. By keeping the excitation current low, it is possible to keep the self-heating low to a great extent. Moreover, the amount of self-heating also depends on the medium surrounding the RTD sensor, and how effectively it allows heat to accumulate. For instance, placing the RTD element in still air produces a more pronounced self-heating effect than immersing it in moving water.

System noise, offsets, and drift of different system parameters also affect the minimum detectable change in temperature. Therefore, the RTD voltage must be large enough to overcome them. As the excitation current must be low enough to prevent self-heating, it is necessary to use an RTD sensor with sufficiently large resistance, so that it will produce a relatively large voltage. Although it is necessary to use a large RTD resistance to reduce measurement errors, it is not advisable to arbitrarily increase the resistance. This is because a large RTD resistance leads to an increase in the response time.

Theoretically, any metal should work for constructing an RTD. In fact, Siemens used copper wire for constructing the first RTD in 1860. However, he soon discovered that by using platinum, he could produce RTDs that were more accurate over a wider temperature range.

Precision thermometry typically uses platinum RTDs as the temperature sensor. This is because platinum has a linear resistance-temperature relationship, higher repeatability, and a wider temperature range. Moreover, platinum does not react with most of the contaminant gases in the environment. However, the industry also uses two other materials for making RTDs: nickel and copper. Among the three metals, copper offers the highest linearity and the lowest cost. However, as copper has the highest conductivity of the three, it offers a lower resistance. A copper RTD, therefore, produces a relatively lower voltage and can be difficult to use for measuring small temperature changes.

What is Thermorite?

Kemet has developed and patented a type of ferrite material that is sensitive to temperature. They use this material to make various types of thermal sensors. The electronic industry uses thermal sensors and switches for monitoring temperature and maintaining various applications in stable modes of operation. A reed switch inside the thermal sensor makes or breaks the current flow.

Kemet offers three basic types of thermal sensors. The first type is a Kemet sensor using a reed switch inside a thermos-ferrite body. The second type is a thermal sensor using a bi-metal switch. The third type is thermistors.

The Kemet thermos-ferrite and reed switch combination works by sensing the temperature of the surrounding environment. The thermos-ferrite has a curie point and as the temperature crosses this curie point, the magnetic reed switch switches on or off.

The bi-metal switch contains two metal plates that have different thermal expansion coefficients. As the temperature changes, the two metals deform and disconnect at a particular temperature.

Thermistors are semiconductor sensors and may have a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). They change their resistance in accordance with changes in temperature.

Thermorite is a ferromagnetic material with soft magnetic characteristics when under curie temperature. As the temperature goes up, the saturated magnetic flux density of the material decreases, and the material becomes paramagnetic. That is, the material loses its magnetic property rapidly as its temperature reaches the curie point, becoming close to zero. Time does not affect curie temperature as it is a function of the compounding ratio of the material. That means Kemet can make Thermorites with different curie points by changing the material’s permeability.

Kemet offers two types of thermos-ferrite thermal sensors—the Break type and the Make type. The Break type thermal sensors consist of a reed switch surrounded with a Thermorite jacket around the switch part and two permanent magnet cylinders on each end. The Break type switch remains ON when the operating temperature is lower than the trigger temperature. The switch opens as the temperature rises and reaches the trigger temperature or crosses it. When the temperature goes down, the switch closes again only when the temperature goes below the recovery temperature.

When the temperature is below the trigger temperature, the Thermorite jacket is magnetic, and it generates an annular magnetic field. The magnetic field induces the N tip and the S tip of the reed switch to touch due to magnetic attraction. This turns the switch to its ON position. As the temperature rises and reaches the trigger temperature, the Thermorite jacket loses its magnetic flux. This allows the tips of the reed switch to pull apart, and the switch turns OFF.

Construction of the Make type thermal switch is similar to the Break type. The only difference is the former has two Thermorite jackets with a gap in between. The two Thermorite jackets create annular magnetic fields when the temperature is below curie point. This causes the reed switch poles to stay apart, and the switch remains OFF. As the temperature rises to the curie point, the Thermorite jackets lose their magnetic flux. This allows the two reed switch tips to close, and the switch turns ON.

What is the Pyroelectric Effect?

With the electronic industry trending more toward automated devices, their safety and reliability are assuming the utmost importance. Pyroelectric sensors help to make these devices work properly, by indicating changes that require specific types of reactions. Many types of ceramic materials can absorb infrared rays and generate an electrical signal in response.

Certain crystalline materials demonstrate Pyroelectricity. These materials, which are electrically polarized, demonstrate a change in their polarization when they undergo a change in temperature. The change in polarization of the crystal material generates a temporary but detectable voltage across it. Different materials exhibit differences in pyroelectric coefficients that show their sensitivity to temperature.

Infrared radiation heats pyroelectric ceramic crystals to generate a detectable voltage. It is possible to detect the infrared rays the object is generating by using passive infrared sensors. The sensor can detect the wavelengths that the pyroelectric ceramic crystal absorbed when it is in position between the hot object and the sensor. Pyroelectricity has several applications.

Motion Sensors—Typically, there are two types of infrared motion sensors, active and passive. Active infrared sensors have a long range of operation, and the emitter and sensor can be far apart. A garage door safety sensor is a good example of an active sensor. Anything blocking the infrared beam across the opening of the garage door generates a signal to prevent the garage door from moving.

Passive infrared sensors can also detect motion by sensing infrared radiation or heat direct from a source. Such sensors can detect the presence, or absence, of an object emitting heat, such as a human body.

Pyroelectric motion sensors can be surface-mount devices and are highly sensitive. Manufacturers offer them in single-pixel configuration or as a 2×2 pixel configuration, allowing users to determine the direction of the motion it has detected. The sensors have a high dynamic range and a fast response time that ensures rapid and accurate motion detection.

Gas Sensors—Infrared pyroelectric sensors can detect and monitor gases. In fact, this is one of their most popular applications. The sensors operate by directing infrared radiation from an emitter through a sample of the gas. The detector senses if a certain IR wavelength is present on the other side. If the sensor does not detect that wavelength, it means the gas that absorbs this wavelength is present in the sample. Optical IR filters allow fine-tuning the sensor to a specific wavelength, thereby permitting only the desired wavelength to pass through to the sensing element.

Pyroelectric gas sensors are available in small SMD packages and most have a digital I2C output, although analog outputs are also available. The sensor consumes very low power but offers high sensitivity and extremely fast response times.

Food Sensors—Similar to gas sensors, infrared pyroelectric food sensors can detect food-related substances like sugar, lactose, or fat. These are typically general IR spectroscopy sensors for monitoring commercial, medical, or industrial substances or processes.

Flame Sensors—With pyroelectric elements, it is easy to construct sensors for detecting flames. As flames are strong, flame sensors, apart from detecting the presence of the flame, can also discriminate the source of the flame. Typically, they compare three specific IR wavelengths and their interrelated ratios. This allows them to detect flames with a high degree of accuracy.

Underwater, Battery-less, Wireless Camera

At the Massachusetts Institute of Technology, engineers have built a wireless camera that does not require a battery to operate underwater. The necessity arose when scientists wanted to observe life under the oceans. They realized they knew less about earth’s oceans than the surface of Mars or the far side of the moon.

An underwater camera must remain tethered to a research vessel for receiving power or sent to a ship periodically to recharge its batteries. This limitation is a big challenge, preventing easy and widespread explorations underwater.

MIT engineers took up the challenge of overcoming the problem. They came up with a camera that does not require batteries, works underwater, and transmits wirelessly. Compared to other underwater cameras, the new camera is more than 1000,000 times more energy-efficient. The device even takes color photos, transmitting them wirelessly through the water.

Sound powers the new autonomous camera—converting the mechanical energy of sound waves into electrical energy for powering its imaging and communications circuitry. After capturing the image, the camera encodes the data and uses underwater waves to transmit it to a receiver for reconstructing the image.

As the camera does not need a rechargeable power source, it can run for a long time before retrieval. This enables scientists to explore remote areas under the ocean. The camera is helpful in capturing images of ocean pollution and monitoring the health and growth of fish.

For a camera that can operate autonomously underwater for long periods, engineers required a mechanism for harvesting underwater energy by itself while using up very little power internally.

The camera uses transducers made of piezoelectric materials that the engineers placed around its exterior. The transducers produce an electrical signal when sound waves hit them. The sound waves may come from any source, such as from marine life or a passing ship. The camera then stores the energy it has harvested, until it has enough for powering its electronics.

The camera has ultra-low-power imaging sensors to keep its power consumption at the lowest possible levels—but these sensors capture only gray-scale images. Moreover, underwater environments are mostly dark, so the camera also needs a low-power flash.

MIT engineers solved both problems simultaneously by using three LEDs of red, blue, and green colors. For capturing an image, the camera first uses the red LED, then repeats the process with a blue LED, and finally with the green LED.

Although each image is black and white, the white part of each photo has the reflection of its respective colored light. Combining the image data during post-processing reconstructs the color image.

The engineers use an underwater backscatter process to transmit the captured image data after encoding it as bits. A nearby receiver transmits sound waves through the water to the camera, reflecting it back just as a mirror would. The camera can choose to either reflect the sound back to the receiver or act as an absorber and not reflect it.

The transmitter has a hydrophone next to it. If it senses a reflected signal from the camera, it treats it as a bit 1. If there is no signal, then it is a bit 0. The receiver uses this binary information for communicating with the camera.

What are Radar Sensors?

Autonomous driving requires the car to have radar sensors as its ears. Originally, the military and avionics developed radar for their applications. Automobiles typically use millimeter wave radar, with a working frequency range of 30-300 GHz, and wavelengths nearer to centimeter waves. These millimeter wave radar offer advantages of photoelectric and microwave guidance to automobiles, because of their significant penetration power.

Automobile collision avoidance mainly uses 24 GHz and 77 GHz radar sensors. In comparison with centimeter wave radar, millimeter-wave radar offers a smaller size, higher spatial resolution, and easier integration. Compared to optical sensors, infrared, and lasers, millimeter wave radar has a significantly stronger ability to penetrate smoke, fog, and dust, along with a good anti-interference ability. Although the millimeter band radar is essential for autonomous driving, heavy rain can significantly reduce the performance of radar sensors, as it produces a large interference. 

Automobiles first used radar sensors in a research project about 40 years ago. Commercial vehicle projects started using radar sensors only in 1998. Initially, they were useful only for adaptive cruise control. Later, radar sensors have developed to provide collision warnings also.

Radar sensors are available in diverse types, and they have a wide range of applications. Automobile applications typically use them as FMCW or frequency-modulated continuous wave radars. FMCW radars measure the air travel time and frequency difference between the transmitted and received signals to provide indirect ranging.

The FMCW radar transmits a frequency-modulated continuous wave. The frequency of this wave changes with time, depending on another triangular wave. After reflection from the object, the echo received by the radar has the same nature of frequency as the emitted wave. However, there is a time difference, and this tiny time difference represents the target distance.

Another radar in common use is the CW Doppler radar sensor. These sensors use the principle of the Doppler effect for measuring the speed of targets at various distances. The radar transmits a microwave signal towards the target, analyzing the frequency change of the reflected signal. The difference between the two frequencies accurately represents the target’s speed relative to the vehicle.

Autonomous vehicles use radar sensors as their basic but critical technical accessories. The radar sensor helps the vehicle to sense objects surrounding it, such as other vehicles, trees, or pedestrians, and determine their relative positions. Then the car can use other sensors to take corresponding measures. Radar sensors provide warnings like front vehicle collisions and the initial adaptive cruise. Vehicles with autopilot radars require more advanced radar sensors such as LIDARs that offer significantly faster response speeds.

Autonomous vehicles must develop technologically. Autonomous driving basically requires an autonomous vehicle to quickly understand and perceive its surrounding environment. This requires the coordination of various sensors, allowing the car to see six directions and hear all. Reliable and decisive driving by an autonomous vehicle requires timely and accurate sensing of roads, other vehicles, pedestrians, and other objects around the vehicle.

Automotive electronics mainly uses radar sensors to avoid forward collisions, sideways collisions, backward collisions, automatic cruises, automatic start and stop, blind spot monitoring, pedestrian detection, and automatic driving of vehicles.

What are Multilayer Chip Capacitors?

The electronics industry uses various types of capacitors in its circuits. These capacitors provide different capabilities and functionality depending on the type and construction. One of the most prevalent types of capacitors is the MLCC or multilayer ceramic capacitor.

Most MLCCs are applicable to circuits that require small-value capacitance. They are preferably useful as filters, in op-amp circuits, and bypass capacitors. This is because MLCC offers small parasitic inductance as compared to aluminum electrolytic capacitors. Therefore, MLCC offers better stability over temperature, subject to their temperature coefficient.

MLCC is available in three categories or classes. The Class I type of ceramic capacitors offer low losses and high stability in resonant circuits. Although they do not require aging corrections, their volumetric efficiency is low. Class II and Class III offer high volumetric efficiency, but their stability is not as good as that of Class I capacitors. Once outside the referee time of the manufacturer, Class II and Class III capacitors may require aging corrections. Manufacturers specify the referee time during which the capacitor will remain within the tolerance range.

Alternating layers of dielectric ceramic and metallic electrodes make up an MLCC. This structure makes them physically small but does not provide them with volumetric efficiency. Design engineers selecting MLCC for electronic applications look for two important parameters—voltage rating and temperature coefficient.

The voltage rating of the MLCC indicates the maximum safe voltage the circuit can apply across the capacitor terminals. For enhanced reliability, designers use a capacitor with a voltage rating higher than it will experience in the circuit. One advantage over electrolytic capacitors is that MLCCs are non-polarized. Therefore, it is possible to connect MLCC in any position without damage.

The temperature coefficient of an MLCC depends on its Class category. If the capacitor contains Class I ceramic material, it will have a very low-temperature coefficient, which means, a change in temperature will minimally affect the capacitance. Class I MLCC also tends to have low dielectric constants, which means the material offers very small capacitance per volume. For instance, C0G and NP0 type Class I MLCC feature a 0 temperature coefficient with a tolerance of ±30 ppm.

Class II MLCC, although less stable over temperature, contains ceramic material with a higher dielectric constant. That means Class II MLCC can have more capacitance in the same volume compared to that of Class I. Class II MLCC are available in X, Y, and Z temperature coefficients. For instance, X7R is a common Class II MLCC, and can operate within a temperature range of -55 °C and +125 °C with a tolerance of ±15%. X5R MLCC can operate within a temperature range of -55 °C and +85 °C with a tolerance of ±15%. Y5V MLCC can operate within a temperature range of -30 °C and +85 °C with a tolerance of +22/-82%. MLCCs with wider temperature ranges are also available with the higher stability of temperature characteristics. However, these capacitors tend to cost more.

Engineers use several capacitors with various values in parallel or series for providing the requisite impedance over a wide range of frequencies.

Three-Phase Monitor Relays Protect Expensive Machinery

Three-phase motors power many industrial and commercial machines. One can find these machines in material handling, water treatment, air conditioning systems, ventilation, heating, marine, machine tools, and aviation applications. However, a range of fault conditions can damage these reliable devices when not addressed quickly. This can lead to a shortened operating lifetime or even a failure, resulting in significant repair costs and downtime.

Phase monitoring relays can detect these faults, notify the operators, and stop the machinery before it develops permanent damage. These relays detect the presence of all three phases, their correct sequence, and that all phase voltages are within the specified range. Should an error develop, the relay opens a set of contacts, initiating an alarm condition, and powers down the machine. There are many types of phase-sensing relays. They can handle a wide range of phase configurations, voltages, and errors.

Among the common failure modes of three-phase motors, are those related to their three-phase power source and their effects on the motor. An imbalance in the phase voltages, or a loss in one of the three phases, can result in the remaining phases driving higher-than-normal currents into the motor. This can lead to a loss of rotational power and excessive vibrations. Likewise, over-voltages and under-voltages can force the motor to draw excess current for driving the same load, and this can shorten the life of the motor. An incorrect phase sequence may cause the motor to reverse the direction of rotation. This can have significantly disastrous results on the load connected to the motor.

Phase monitoring relays monitor the state of the three-phase power source. The three-phase lines that they monitor also power them. Apart from the phase sequence, they also monitor the loss of any phase voltage. Only when all the phases are present, and are in the correct sequence, do the relays activate. Whenever there is a loss of any phase, or the phase sequence is incorrect, the relays de-energize.

Some phase monitoring relays also have the capability to monitor the voltage levels of all three phases. This typically uses a true root-mean-square measurement. The relay deactivates whenever the voltage drops below a preset threshold. Some relays also offer adjustable limit settings along with voltage detection. Other relays monitor phase asymmetry along with tolerance. Typically, phase monitoring relays offer a delay before actuation. This prevents spurious activation from temporary voltage levels or asymmetry issues. In some models, the delay is adjustable.

The DPA01CM44 is an example of a three-phase monitoring relay meant for three-wire configurations. The three-phase source powers the relay. Relay models available operate at voltages of 208, 230, 400, 600, and 690 VAC. Although relays for mounting on DIN rails are typical, plugin models are also available. The relay output configuration can be single or dual SPDT contacts.

Normal voltage and phase conditions allow the relay to remain activated. That means, the normally open contacts of the relay output remain closed. Abnormal conditions make the relay operate within 100 milliseconds. The front panel on the relay has status LEDs to indicate relay activation and power on.

What is Capacitance to Digital Converter Technology?

The healthcare industry has witnessed many advancements, innovations, and improvements in electronic technology in recent years. Healthcare equipment faced challenges like developing new treatment methods and diagnoses, home healthcare, remote monitoring, enhancing flexibility, improving quality and reliability, and improving ease of use.

A comprehensive portfolio of these technologies includes digital signal processing, MEMS, mixed-signal, and linear technologies that have helped to make a difference in healthcare instrumentation in areas such as patient monitoring and imaging. Another is the capacitance to digital converter technology that offers the use of highly sensitive capacitance sensing in healthcare applications. For instance, a capacitive touch sensor is a novel user input method that can be in the form of a slider bar, a push button, a scroll wheel, or other similar forms.

In a typical touch sensor layout, a printed circuit board may have a geometric area representing a sensor electrode. This area forms one plate of a virtual capacitor, while the user’s finger forms the other plate. For this system to work, the user must essentially be grounded with respect to the sensor electrode.

Analog Devices has designed their CapTouch controller family of ICs, the AD7147/ AD7148, to activate and interface with capacitance touch sensors. The controller ICs measure capacitance changes from single-electrode sensors by generating excitation signals to charge the plate of the capacitor. When another object, like the user’s finger, approaches the sensor, it creates a virtual capacitance, with the user acting as the second plate of the capacitor. A CDC or capacitance to digital converter in the ICs measures the change in capacitance.

The CDC can measure changes in the capacitance of the external sensors and uses this information to activate a sensor. The AD7147 has 13 capacitance sensor inputs, while the AD7148 has eight. Both have on-chip calibration logic for compensating for measurement changes due to temperature and humidity variations in the ambient environment, thereby ensuring no false alarms from such changes.

Both CDCs offer many operational modes, very flexible control features, and user-programmable conversion sequences. With these features, the CDCs are highly suitable for touch sensors of high resolution, acting as scroll wheels or slider bars, requiring minimum software support. Likewise, no software support is necessary for implementing button-sensor applications with on-chip digital logic.

The CDCs function by applying an excitation signal to one plate of the virtual capacitor, while measuring the charge stored in it. They also make the digital result available to the external host. The CDCs can differentiate four types of capacitance sensors by changing the way they apply the excitation.

By varying the values of these parameters, and/or observing the variations in their values, the CDC technology directly measures the capacitance values. The distance between the two electrodes affects the output of the CDCs in inverse proportions.

The family of Analog Device CDCs, the AD714x, AD715x, and AD774x, are suitable for applications involving a wide range of functions. These involve various input sensor types, input ranges, resolutions, and sample rates. Applications involve liquid level monitoring, sweat detection, respiratory rate measurement, blood pressure measurement, and more.

What are Piezoelectric Audio Devices?

The piezoelectric effect is a versatile and extremely useful phenomenon. Engineers have adopted this phenomenon in various transducer applications. Some of these applications involve transforming the applied voltage to mechanical strain output, for use as a basic source of sound. In a complementary mode, the application of mechanical stress to the Piezo material causes the rugged sensor to produce a voltage. Piezoelectric devices are low-cost, reliable, and rugged, and this allows engineers to exploit their unique properties.

Piezo-based speakers offer many attributes as sound sources. Unlike electrodynamic speakers, piezo-based speakers can be relatively thin, yet create very high sound pressure levels. However, mechanical and physical material issues can limit their audio quality. Now, a team at MIT is changing all this. They have developed a dense array of tiny dome speakers that they have based on Piezo technology. They have significantly transformed the classic analog function of loudspeakers. Their new loudspeakers are paper-thin, very flexible, and fully capable of turning any surface into an active audio source.

Although there are conventional thin-film loudspeakers, the basic requirement is the film must be free to bend to produce sounds. Firmly mounting such thin-film loudspeakers to a surface would attenuate their output and dampen the vibrations, while limiting their frequency response tremendously.

However, the ingenious approach of the MIT team has solved the problem in a rather unique way. Their new loudspeaker does not have to vibrate the entire material surface. Rather, they have fabricated tiny domes on a thin layer of piezoelectric material, such that each dome can vibrate independently. Each dome is about 15 µm in height, and they move up and down by only half a micron when vibrating. As each dome forms a single sound-generating unit, it requires thousands of these tiny domes to vibrate together to produce audible sounds. While the basic loudspeaker is only 120-µm thick, it weighs only 2 grams. Only standard processes are necessary to manufacture this loudspeaker at low costs.

Spacer layers surround the domes on the bottom and top of the film. This helps to protect the domes from the mounting surface, and at the same time allows them to freely vibrate. These spacer layers also protect the domes from impact and abrasion during daily handling, thereby enhancing the durability of the loudspeaker.

To make the film loudspeakers, the researchers used a thin sheet of PET or polyethylene terephthalate. This is a standard plastic used for a variety of applications. They used a laser to cut tiny holes in the sheet while laminating its underside with an 8-µm thick film of PVDF or polyvinylidene fluoride. This is a common industrial and commercial coating. Then they applied vacuum and heat to bond the two sheets.

As the PVDF layer is very thin, the pressure difference that the vacuum creates together with the heat causes it to bulge, but it cannot force its way through the PET layer. This makes the tiny domes protrude through the holes. The researchers laminated the free side of the PVDF layer with another layer of PET and this acts like a spacer between the bonding surface and the domes. Regardless of the rigid bonding surface, the film loudspeaker could generate a sound pressure level of 66 dB at 30 cm.

What are Reed Switches?

A modern factory will have several electronic devices working, and most of them will have several sensors. Typically, these sensors connect to the devices using wires. The wires provide the sensor with a supply voltage, a ground connection, and the signal output. The application of power allows the sensor to function properly, whether the sensor is sensing the presence of a ferromagnetic metal nearby, or it is sending out a beam of light as a part of the security system. On the other hand, simple mechanical switches, like reed switches, require only two wires to trigger the sensors. These switches need magnetic fields to activate.

The reed switch was born and patented at the Bell Telephone Laboratories. The basic reed switch looks like a small glass capsule that has two protruding wires. Inside the capsule, the wires connect to two ferromagnetic blades with only a few microns separating them. If a magnet happens to approach the switch, the two blades attract each other, making a connection for electricity to flow through them. This is the NO type of reed switch, and it is a normally open circuit until a magnet approaches it. There is another type of reed switch, the NC type, and it has one blade as a non-ferromagnetic type. This switch is a normally closed type, allowing electric current to flow until a magnet approaches it. The approaching magnet makes the blades pull apart, breaking the contact.

Manufacturers use a variety of metals to construct the contacts. This includes rhodium and tungsten. Some switches also use mercury, but the switch must remain in a proper orientation for switching. The glass envelope typically has an inert atmosphere inside—commonly nitrogen—to seal the contacts at one atmospheric pressure. Sealing with an inert atmosphere ensures the contacts remain isolated,  prevents corrosion, and quenches sparks that might result from current interruption due to contact movement.

Although there are solid state Hall effect sensors for detecting magnetic fields, the reed switch has its own advantages that are necessary for some applications. One of them is the superior electrical isolation that reed switches offer compared to what Hall effect sensors do. Moreover, the electrical resistance introduced is much lower for reed switches. Furthermore, reed switches are comfortable working with a range of voltages, variable loads, and frequencies, as they function simply as a switch to connect or disconnect two wires. On the other hand, Hall switches require supporting circuitry to function, which reed switches do not.

For a mechanical switch, reed switches have incredibly high reliability—they typically function for billions of cycles before failing. Moreover, because of their sealed construction, reed switches can function even in explosive environments, where a single spark could generate disastrous results. Although reed switches are older technology, they are far from obsolete. Reed switches are now available in surface mount technology for mounting on boards with automated pick-and-place machinery.

The functioning of reed switches does not require a permanent magnet to actuate them. Even electromagnets can turn them on. Initially, Bell labs used these switches abundantly in their telephone systems, until they changed over to digital electronics.