Category Archives: Sensors

Sensor Nodes Based on the Raspberry Pi

Building sensor networks is economical if a microcontroller hosts the sensors. However, sometimes the computational power a microcontroller offers is not adequate. For instance, it may be necessary to convert the data to a different format, print a hard copy of the sensor data, or incorporate the data within an application. What you need is a computer that not only has more processing power, but also allows the use of common applications, affords access to peripherals, and permits the use of scripting languages.

Although the use of an inexpensive personal computer would be of great advantage here, using them as sensor nodes in the networks has its own disadvantages. The primary hurdle is most personal computers are built for use as servers or desktop computers, and almost no general-purpose input/output ports are available. Of course, a data collection card added to the personal computer will serve the purpose, but the cost of the computer added to that of the data-collection card makes the cost of the sensor node uneconomical.

Fortunately, single board computers such as the Raspberry Pi (RBPi) provide an easy solution to the above problem. With sufficient processing power and memory, use of standard peripherals, supported programmable I/O ports, and a small form factor, the RBPi is the most suited for building sensor networks economically.

Essentially, the RBPi is a single board computer that runs Linux as its operating system. To get started with the RBPi, you need a few additional things, such as a USB power supply rated at 2 A with a male micro-USB connector, an HDMI monitor, a keyboard, an optional mouse, and most importantly, an SD card to hold the OS.

The most commonly used operating system for the RBPi is the Raspbian image provided by the Raspberry Pi Foundation on their download page. Once you have downloaded the image, you will have to unzip it and write it into an SD card. The easiest way to do this with a Windows PC is to use the Win32 Disk Imager software. Those on the Mac OS X or the Linux PC may use the dd command.

Now it is time to boot up your RBPi board. Plug in the SD card holding the new image, plug in the keyboard, mouse, and the monitor. Once all the peripherals are in place, plug in the USB power and turn on the power. When prompted to enter a username and password, use Pi and raspberry respectively, and configure the system to your requirement.

For connecting and experimenting with sensors, you may use expansion boards, but using a simple prototyping board instead is more flexible. Using a Pi Cobble Breakout board or similar allows a simple ribbon cable from the GPIO connector on the RBPi to the prototyping board, with the pins arranged in the same order as those on the RBPi are.

Be careful to make or change connections with the RBPi powered off. Also double-check all connections are rightly connected. The GPIO on the RBPi is not protected against short circuits and high voltages, and is easily damaged.

Measuring 16 Temperatures Remotely

In several cases, one cannot access the area where the temperature needs to be monitored. For instance, the temperature inside a kiln may reach a few thousand degrees, which is beyond the tolerance of humans. Environment chambers may need to be completely sealed off when operating, which means monitoring the conditions within has to be remotely accomplished. Simpler cases may also be considered, where the computer logging the temperature is in a central location, whereas the monitored sites are spread out in different rooms.

The ideal instrument should allow measuring temperatures over a local or remote network, with a built-in web-server to access the instrument, requiring neither programming or app. Measurement Computing has just the instrument and it is the WebDaq-316, a stand-alone temperature logger that allows the user to measure temperatures on 16 channels using J, K, T, E, N. B, R, or S type thermocouples. The user can access the instrument through its web-server over a local or remote network.

With 3 GB of internal acquisition memory, the WebDaq-316 acquires samples at the rate of 75 samples/sec. However, if the memory is insufficient for the job on hand, the user can add more by inserting two USB flash drives or an internal SD memory card. The measurement data can be transferred from the SD card, flash drive, or the internal memory. Alternately, the user can download the acquired data from the web-server as well. It is easy to import the data to an analysis software or a spreadsheet as the WebDaq stores data in CSV format.

The user can operate the WebDaq-316 in two modes—normal or high resolution. In the normal mode, the instrument works at 75 samples/sec or 78 Hz maximum, whereas in high-resolution mode, it can scan at less than one sample/sec across all channels. In the high-resolution mode, WebDaq-316 drops its bandwidth to 14.4 Hz, which also lowers its noise and gain error. That allows the 24-bit delta-sigma ADC on the instrument to operate at its peak efficiency.

The web-server has the ability to send SMS texts or e-mail messages. Therefore, the user can receive an appropriate notification whenever a temperature moves out of limits. Additionally, there are four programmable digital IO channels on the WebDaq-316. The user can make use of these IO channels to operate some local activities such as trigger an alarm or shut down equipment. As the IO channels are programmable, they can be inputs or outputs. As inputs, they can act as trigger depending on external signal, and as outputs, they can trigger alarms. The channels are available on terminal strips on the front panel, which makes all T/C and DIO connections easier.

The user can assign measurement operations through jobs and schedules. For instance, the user may want to change jobs or sample rates whenever a temperature crosses the limits, or return to a schedule of lower rate when the temperature returns within the limits. The user may also want to schedule jobs for triggering alarms or receiving notifications of such conditions.

Based on the Raspberry Pi compute module, the WebDaq-316 operates on a DC power source of 6 to 16 V. This allows vehicular operation as well.

Measuring Air Quality with IoT Sensor

Bosch Sensortec is making an IoT environmental sensor for measuring air quality. The BME680 can measure the indoor air quality, relative humidity, barometric pressure, and ambient air temperature. It has four sensors housed within a single LGA package measuring 3x3x0.95 mm, and both mobile and stationary IoT applications can use the package for use in smart homes, offices, buildings, elder care, sports, and fitness wearables.

The BME680 measures the indoor air quality through its internal gas sensor by detecting a wide variety of gases in the range of parts per billion. The gases it can detect include hydrogen, carbon monoxide, and volatile organic compounds. While measuring altitude and pressure, the BME680 is accurate to within ±1 m and ±12 Pa respectively. Its temperature measurement capability extends from −40°C to +85°C, and it can measure relative humidity from 0% to 100%. In addition, the BME680 can measure an offset temperature coefficient of 1.5 Pa/K.

The BME680 consumes current according to its measuring parameter. While capable of operating from a supply voltage of 1.71 V to 3.6 V, it has a data refresh rate of 1 Hz. When measuring temperature and humidity, the BME680 consumes 2.1 µA, and 3.1 µA when measuring temperature and pressure. The current consumption goes up to 3.7 µA when measuring pressure, temperature, and humidity, while the maximum consumption is between 0.09 and 12 mA when the device is measuring gas, temperature, humidity, and pressure. Therefore, although the current consumption depends on its operating mode, its average current consumption in sleep mode goes down to 0.15 µA.

As an integrated environmental sensor, Bosch Sensortec has developed the BME680 specifically suited for mobile applications and wearables. As for both applications the size and low power consumption are key requirements, Bosch Sensortec has expanded its existing family of environmental sensors by adding the BME680 to its repertoire, while integrating the temperature, humidity, pressure and gas sensors, all of which are highly linear and highly accurate.

The BME680 comes in an 8-pin metal lid LGA package measuring only 3x3x0.95 mm. Bosch Sensortec has designed the sensor for optimized consumption that depends on its specific operating mode, high EMC robustness, and long-term stability. The specialty of the gas sensor within the BME680 is it can detect a wide spectrum of gases for assessing the indoor air quality for individual well-being. For instance, the BME680 can detect VOC or volatile organic compounds from alcohol, adhesives, glues, office equipment, furnishings, cleaning supplies, paint strippers, lacquers, and paints based on formaldehyde.

Applications for the BME680 are numerous. It can be used for altitude tracking as well as calorie expenditure for sports activities. It is sensitive enough for indoor navigation as it can detect change of floors and elevation. As GPS enhancement, it can improve time-to-first-fix, slope detection, and dead reckoning. As home automation control, the user can use the BME680 as an advanced HVAC control. Scientific experiments can use it for measuring volume and airflow, while agriculturists can use it as warning against dryness or high temperature. Sports enthusiasts can use it for monitoring fitness, well-being, detecting skin moisture, change in room, and for context awareness. BME680 is suitable for use as a personalized weather station and for indoor air quality measurement.

Sensing Temperature with NTC Thermistors

Temperature sensors using NTC thermistors are built from sintered semiconducting ceramic material. Such materials contain a mixture comprising several metal oxides. The specialty of these materials is they possess charge particles, which allow current to flow through the thermistor, and display large changes in its resistance value even when the change in temperature is rather small. The manufacturing process allows standard NTC thermistors to operate effectively in the temperature range between 50 and 150°C, with glass-encapsulation type of thermistors going up to 250°C.

Thermistors come in a large variety of sizes and styles. These include glass encapsulated, customizable probe assemblies, surface mount types,, disc types, and chip styles. The large variety is necessary as individual attributes of each style gives them the ability to perform effectively in several different industries, while adapting to various and different application requirements.

For instance, industries using NTC thermistors for measuring temperature include telecommunications, medical, healthcare, military, aerospace, automotive, industrial, HVAC, among many others. On the other hand, applications for NTC thermistors cover time delay, volume control, circuit protection, voltage regulation, temperature control, temperature measurement, temperature compensation, and more.

Now, the availability of precision interchangeable NTC thermistors eliminates the necessity of individual calibration for each thermistor. Capable of accuracies of ±0.05°C, the interchangeable thermistors are now gaining popularity as industry standard. Their standard resistance values usually range from 2.25 kilo-ohm to 100 kilo ohm, with temperature coefficients of -4.4% per °C or -4.7% per °C.

Interchangeable NTC thermistors offer extreme accuracy when sensing temperature. This makes these versatile sensors an excellent choice for use in industries focussing on temperature measurement and control. These industries include, among others, aerospace, HVAC, automotive, industrial, and medical. Applications for interchangeable NTC thermistors include temperature sensing, temperature measurement and control, temperature measurement, and control.

Using interchangeable NTC thermistors in the industry offers several benefits and features. These include beta of 3435 K to 4143 K, RoHS compliance, dissipation constant of 1 mW/°C, fast thermal response times, wide range of ohmic values, a thermal time constant of 7 secs, and fast measurement times. Aging is slow as these NTC thermistors show less than 1% change in resistance even after a span of 10 years. That means, there is no need to recalibrate the system when replacing the thermistor. This certainly reduces operating costs and system downtimes.

Glass encapsulated NTC thermistors are a special breed that are hermetically sealed. This allows the micro-sized sensors to eliminate reading errors from moisture penetration. As they are hermetically sealed, they function effectively in extreme conditions of temperature, pressure, and other severe environmental conditions. The extreme operating conditions allow glass encapsulated NTC thermistors to target markets such as industrial, automotive, medical, and HVAC.

Glass encapsulated NTC thermistors are good for applications involving outdoors such as infrared lighting systems, medical such as those relying on airflow/respirators, industrial such as those including monitoring of terminal temperatures of battery packs while charging, common household appliances such as in coffee makers, ovens, and refrigerators, HVAC such as for temperature measurement and control.

Apart from greater accuracy and faster response times, Glass encapsulated NTC thermistors offer high precision resistance and beta value with a huge operating temperature range.

How to Select Current Sensors?

To select an appropriate AC current sensor for an application, you must know the operational frequency range and the current rating the sensor will encounter. Additional considerations that you will need to decide are the type of the sensor, its mounting (through-hole or surface mount), turns ratio, and the overall dimensions.

Sensor type refers to a sensor only configuration, where a conductor integral to the application forms the primary. Another type could be a complete current transformer where the primary is included as a winding. Engineers typically use current sensors to measure and control the load current in control circuits, safety circuits, and power supplies. Power supplies usually require accurate control of current, and this requires sensing the magnitude of the current accurately.

Irrespective of whether you are using the sensor or transformer, the highest flux density handled is dependent on the worst-case current and frequency faced by the device. However, note that exceeding 2000 Gauss will mean most AC current sensors output will be non-linear. Therefore, the current through the sensor and its output voltage will no longer remain proportional, as the magnetic core of the sensor saturates at very high flux densities. To keep the flux density below the saturation limit, it is necessary to use higher secondary turns.

For instance, in wire-through-the-hole style of current transformers, looping additional primary turns through the hole can dramatically reduce the turns ratio, provided the wire diameter and the hole size permit. Increasing the primary turns allows the use of a higher input current transformer to provide higher output voltage across the terminating resistor on the secondary.

Manufacturers of current transformers offer online tools to help designers select the right current sensor or current transformer for specific application conditions. Initially, the user has to select the type of sensor—a transformer or a sensor only. The next selection is the preferred mounting style—SMT or Through-hole. The online tool also requires other parameters such as the maximum sensed current expected in amperes, the input frequency in kHz, the duty cycle of the primary current waveform as a percentage, and the desired output voltage corresponding to the expected maximum input current.

The tool then calculates the required terminating resistance based on the maximum input current, the number of secondary turns and the output voltage—basing the calculations on a single-turn primary. Next, the tool calculates the maximum flux density of the secondary, making sure it does not exceed 2000 Gauss. It does this by taking into account the output voltage, the duty cycle, secondary turns, and the frequency of operation.

The result lists all part numbers of the manufacturer that meet these input conditions, typically including a graph of the output voltage versus the sensed current for the calculated terminating resistance.

To select an appropriate current sense transformer for your application, you require knowledge of the maximum current, frequency, and duty cycle of the sensed current, including the output voltage you require. Using this information, the online selector tools will provide you with the appropriate terminating resistor value and a list of current sensors that meet the conditions of the application.

Inertial Sensing for Automation

In any type of industry, whether it is automotive, unmanned aerial vehicles, energy, logistics, agriculture, or manufacturing, automation brings increasing promises of great gains in terms of efficient utilization of resources, achieving accuracy, and safety. To achieve these gains it is necessary to identify the appropriate sensing technologies that will enhance the contextual knowledge of the equipment’s condition.

As the location or position of an equipment is a valuable input, precision inertial sensors provide accurate location information and help in maintaining accurate positioning. Where mobility is a factor, it is necessary to couple both the location and the contextual sensor information with the application. Operating in a harsh or complex environment often requires determination of position as a critical value. This is where inertial sensors help to make a difference.

Over the years, machinery has evolved from making simple passive movements, to functioning with embedded controls, and now it is moving towards fully autonomous operations, with sensors playing the enabling role. Earlier, for supporting offline analysis or process control, sensors working in isolation were adequate. However, obtaining real-time benefits requires increasingly sophisticated sensor types, while efficient processing requires important advances in sensor fusion. Therefore, increasingly intelligent sensor systems are coming up catering to complex systems on multiple platforms that require the knowledge of states the system has held in the past.

Inertial sensors used with smart machines serve two special functions, one for equipment stabilization or pointing, and other for equipment navigation or guidance. Most systems consider GPS as the most suitable for navigation. However, potential blockages cause significant concerns for many industrial systems. Some systems transition to inertial sensing when GPS is blocked, but this requires the inertial systems to be of sufficient quality to provide the same precision, as did the GPS.

Inertial sensors provide the feedback mechanism in case of servo loop or stabilization for maintaining a reliable positioning such as the antenna pointing angle, construction blade, crane platform, camera, UAV, or farming implement. For all these, the purpose is not only to provide a useful function, but also to deliver a safety mechanism or critical accuracy, even when the environment is incredibly difficult.

In reality, sensor quality matters when good performance is desired. Engineers use sensor fusion for making some correction, for instance, when correcting the temperature drift of sensors, or compensating an accelerometer when correcting for gravitational effects on a gyroscope. In such cases, this helps only in the calibration of the sensor to the environment, but does not improve the ability of the sensor to maintain performance between the calibration points. With a poor quality sensor, the accuracy falls off quickly, as the performance of the sensor rapidly drifts without expensive or extensive calibration points.

Even when using high quality sensors, some amount of calibration is desirable, especially when the aim is to extract the highest possible performance from the device. However, the most cost-effective method of calibration depends on the intricate details of the sensor, along with a deep knowledge of motion dynamics. This makes the compensation or calibration step an embedded necessity for the manufacturer of the sensor.

Specifying Fiber Optic Sensors

The industry prefers fiber optic sensors as they work well in tight spots and in applications that have a high degree of electrical noise. Fiber optic sensors are useful in machines, fixtures, and conveyors for sensing part presence as an important component of industrial automation. The industry often requires controlling sequence and error-proofing assembly based on the presence or absence of a part. In many cases, it is simply impossible to know whether a part is where it should be or the holder is empty as expected. Therefore, verification is only possible by using a presence sensor.

Sensors come in many varieties, including magnetic, capacitive, inductive, and photoelectric. Depending on the application, each type of sensor has its own merits and demerits. Among all the sensors available in the market, photoelectric sensors offer the broadest types and technologies, and suitable for the widest range of applications.

The family of photoelectric sensors includes a large variety of light-emission types that includes lasers of class 1 and 2, visible, and infrared. They also include different sensing technologies such as through-beam, reflective, suppression, background, and diffuse. Different housing configurations are also available such as fiber optics and photo eye. We will focus on specifying and applying fiber-optic sensors, as these offer the most advanced capabilities with options for configuration, and are most suitable for use in tight spots that the photo-eye sensor finds too small.

Fiber-optic sensors are also known as fiber photoelectric sensors, and comprise of two parts—the amplifier and the fiber cable. The amplifier is the electronic part and is actually a fiber photoelectric amplifier. The fiber-optic cable includes the optic sensor head and the fiber cable to transmit light to and from the amplifier.

All photoelectric sensors work with a simple technique. A light emitter produces the source signal and a receiver detects the signal. A large variety of technologies is available for sensing and measuring the light transmitted to the receiver. For instance, standard photo-eyes look for the presence or absence of light, whereas background suppression sensors sense the angle of the returning beam. Other type of sensors measure the time taken by the light to return, thereby providing a measure of distance it traveled.

Simple photo-eyes such as those used in reflective and diffuse units house the emitter and receiver in the same optical sensor head, while through-beam units house them in two optical sensor heads. On the other hand, fiber-optic sensors have all the electronics in a single housing, with a fiber cable connecting the separate emitter and receiver to the electronic housing. Light from the emitters and that coming to the receivers travels through the fiber cables, similar to high-speed data traveling through fiber-optic networks.

The above segregation means the technician has to mount only the sensor head on the machine, while routing the integrated fiber-optic cable and plugging it into the amplifier placed in a safe place such as a control enclosure to protect it from the harsh manufacturing environment.

A large variety of options is available for both fiber-optic cables and amplifiers. These range from basic to advanced, suitable for meeting the demands of increasing functionality, including advanced logic and communication capabilities.

GrovePi Kits for the Raspberry Pi

If you are looking to interface sensors to the Raspberry Pi (RBPi), the popular single board computer, GrovePi+ from Dexter Industries (SEED Studios) makes it very easy with their starter kit. The kit carries a GrovePi+ board, including more than 10 carefully selected sensors along with the necessary interfacing cables. The kit is very easy to use, as the user only has to plug the GrovePi+ board over your RBPi, and connect the necessary sensor to the board. GrovePi provides a powerful platform for any user to start playing with sensors and hardware.

The simplicity of the GrovePi+ board is evident, as you do not need any other hardware connection—only plug in the board atop the RBPi and initiate communications between the two boards over an I2C interface. The GrovePi+ board acts like a shield and the user can connect any of the Grove sensors from the kit to the universal Grove connector on the board, using the universal 4-pin connector cable available with the kit.

The GrovePi+ board has an ATMEGA328 micro-controller on it, and the Grove sensors, both analog and digital, connect to it directly. The RBPi also communicates with this micro-controller, which performs as an interpreter for the Grove sensors, sending, receiving, and executing commands the RBPi sends it. You can use any RBPi model with the GrovePi+, selecting from among RBPi A+, B, B+2, or B+3

GrovePi+ forms the hardware system for connecting, programming, and controlling sensors that help build your own smart devices. GrovePi+ is small—the size of a credit card—however, it is very powerful. You can think of the GrovePi+ kit as an Internet of Things kit for the RBPi—allowing you to connect numerous sensors to the RBPi—simply by connecting a cable from the GrovePi+ board to the sensor. The manufacturer’s website offers several software examples you can download and try. Alternately, you can write your own programs for the RBPi to control and automate any device.

GrovePi+ does away with the need for connecting sensors to the IoT using breadboards and soldering the sensors. Now it is only necessary to plug in the sensors and start programming directly. Therefore, GrovePi+ is and easy-to-use modular arrangement for hacking your hardware with the help of the RBPi and the Internet of Things.

Using the GrovePi+ system, one can connect over 100 types of sensors to the RBPi. The collection of sensors offered are all inexpensive and plug-n-play modules to sense and control inputs from the physical world. This provides countless possibilities of interacting with sensors, integrating them with the module and the RBPi to obtain unparalleled performance with ease.

For instance, Lime Microsystems and the SEED Studio have a new kit providing everything to start up a Software Defined Radio (SDR) with the RBPi and develop IoT applications for it. The LimeSDR Mini kit targets educational use and is meant for beginners. Lime has optimized the building block for use at 433/868/915 MHz and provides the necessary antennas in the kit. The kit also has an array of sensors from Grove and boards related to output from SEED Studios. The GrovePi+ board offers the computing power for the SDR, and you can use an RBPi 2, 3, or Z.

High Accuracy Digital Temperature Sensor

Analog Devices is offering a high accuracy digital temperature sensor that covers a wide industrial range. The tiny package also incorporates a humidity sensor. There is no necessity of adding a separate analog to digital converter to this sensor, as the device has one built into it, and provides a high-resolution digital output of 16 bits. With a wide operating voltage range, the device is suitable for industrial, domestic, and commercial use.

The ADT7420UCPZ-R2 from Analog Devices measures temperatures from -40°C to +150°C, while operating from a voltage range of 2.7 to 5.5 V. The device is available in a 4 mm x 4 mm package commonly known as Lead Frame Chip Scale Package (LFCSP). This wire bond plastic encapsulated near chip scale package has a substrate of copper lead frame within a leadless package format. Input/output copper pads are positioned on the perimeter edges of the package.

This allows the user to solder the perimeter pads and the exposed paddle available on the bottom surface of the package to the PCB. The exposed thermal pad on the bottom of the package conducts heat away from the package when it is soldered to the copper layer on the PCB. The thermal and perimeter pads are tin plated to provide good soldering.

Within the ADT7420 is an internal band gap reference, along with a temperature sensor. The 16-bit ADC within the device monitors the temperature and digitizes it to a resolution of 0.0078°C. By default, the ADC resolution is set to 13 bits or 0.0625°C, which should be adequate for most users. However, the user can change the ADC resolution via a programmable mode, to 16 bits. The programmable mode is accessible to the user through an I2C serial interface.

Analog Devices guarantees the ADT7420 will operate reliably when supplied from 2.7 V to 5.5 V. Typical current consumption by the device id 210 µA when operating from a supply voltage of 3.3 V. The user can optionally power down the device to make it enter a shutdown mode where the current consumption is typically 2.0 µA at 3.3 V. There is an additional power saving mode, where the user programs the device to read one sample per second. The temperature drift for ADT7420 is merely 0.0073°C.

The ADT7420 exhibits very high temperature accuracy of ±0.20°C between -10°C and +85°C, when working from a 3.0 V supply. When working from a wider supply voltage of 2.7 to 3.3V, the temperature accuracy of the device is ±0.25°C between -20°C and +105°C. As soon as the device powers up, the first temperature reading is available within 6 ms.

Implementing the ADT7420 is very easy, as it does not need any temperature calibration or correction by the user. The user also does not require any linearity correction for the usable temperature range. The user can program the device to produce an interrupt when it senses the temperature crossing a preset critical temperature.

Applications for the ADT7420 include replacement for RTD and thermistor, and compensation for thermocouple cold junction. Typically, the device is usable in medical equipment, and for industrial control and test, food transport and storage, environmental monitoring and HVAC, and Laser diode temperature control applications.

Using Reed Switches as Sensors

Any ordinary electrical switch has two contacts. Push-type switches are spring loaded so that pushing a button brings them together and they spring apart on releasing the button. Rocker switches have mechanical levers that close the contacts when in one position, while in the other position they pull apart.

In reed switches, the two contacts are in the shape of metal reeds, each coated with a metal that does not wear easily. The reeds are made from a ferromagnetic material, so they are easy to magnetize. The entire assembly is hermetically sealed within a thin glass envelope containing a nonreactive gas such as nitrogen. For extra protection, sometimes the glass envelope may have a plastic casing.

The ferromagnetic material making up the reeds is typically a nickel-iron alloy that shows high magnetic permeability but low magnetic retentivity. That means, when brought close to a magnet, it magnetizes the reeds, which come together in contact. On moving the switch away from the magnetic field, the reeds lose their magnetic property and separate. Their movement has high hysteresis, that is to say they close and open slowly and smoothly. The reeds have a flat area where they contact each other, and this helps to extend the life and reliability of the switch.

Although reed switches typically have two ferromagnetic contacts, some variants may have only one ferromagnetic contact, while the other is non-magnetic. Others may have three contacts, with two non-magnetic and the central one as ferromagnetic.

Like ordinary switches, reed switches also come as two major variants—normally open type and normally closed type. This refers to the position of the reeds when there is no magnetic influence on them. Therefore, the normally open type has its reeds separated from each other, and they close when a magnet is brought close enough. The normally closed type of reed switch has its reeds in contact with each other, and they move apart when a magnet is brought close enough.

As the magnet comes close to a normally open reed switch, the two contacts become magnetized as opposite magnetic poles, and they attract each other to close. In this position, the switch can pass an electric current. This magnetizing of the reeds is independent of the pole of the magnet coming close to them. As the magnet moves away, the reeds lose their magnetism, and their stiff and springy nature makes them spring apart in their original position.

Reed switches are very useful as sensors such as for sensing level of liquids. A sealed stem holds the reed switches at different heights. A float containing a permanent magnet rides on the stem, going up and down as the liquid level changes. When the float magnet comes close to one of the reed switches, it snaps close, changing its electrical status that any electronic circuit can sense. Automotive, marine, and industrial applications use reed switches for level sensing.

A float switch in a dishwasher controls the level of water in the machine. The shaft containing the reed switch is positioned at the water fill limit of the pan. As the water rises, so does a float containing the magnet. When the magnet comes close to the reed switch, it closes, and signals the ECU.