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Air Quality Sensors Many different air quality sensors and sensing modules have appeared on the market, some of them surprisingly low in cost. Here’s a quick rundown of what they do and how they work. By Jim Rowe I Image Source: www.pexels.com/photo/white-clouds-and-blue-sky-907485/ nterest in air quality sensors and monitors has grown steadily, especially during bushfires when there’s a lot of smoke in the air, or for people who live in countries with factories near urban areas that cause poor air quality. Air filters and air quality sensors are now an essential part of the air conditioning systems in office buildings, hospitals and factories. But the filters and sensors developed for these ‘large scale’ applications are generally rather expensive. Then when the COVID-19 virus and its growing family of mutants appeared in late 2019 and were soon found capable of spreading via aerosol droplets, interest in air quality sensors almost exploded. It soon became apparent that smaller and lower-cost sensors were needed to sense and control the air quality in ‘smaller scale’ environments like homes, retail stores and schools. To meet this challenge, designers worldwide soon came up with many different kinds of low-cost air quality sensors and modules. There are so many that it can be daunting to pick the sensor or module best suited for your particular application. This article will describe the main types of low-cost air quality sensors and explain what each type does and how they work. There are quite a few acronyms commonly used in this area, and you’ll find the more common ones explained in the Glossary sidebar. Before getting to the sensors, let’s look at the undesired matter that can be in the air we breathe. What’s in the air There are three main types of harmful components in the air we breathe: particulate matter, volatile organic compounds and toxic gases like sulfur dioxide, nitrogen dioxide, ozone, carbon monoxide and carbon dioxide – the last of which we exhale ourselves. Particulate matter includes smoke and smog particles, which have long been recognised as a health risk. It also includes liquid aerosol droplets, which may contain things like viruses and bacteria. Currently, there are three official categories of airborne particulate matter, specified according to particle size and diameter: PM10, PM2.5 and PM1.0. PM10 refers to particles less than 10 micrometres (μm) in diameter, PM2.5 to particles less than 2.5μm in diameter and PM1.0 to particles less than 1μm in diameter. To put these numbers in perspective, the diameter of human hair is typically between 50μm and 70μm. Particles with a diameter of less than 10μm are small enough to pass through our nostrils and throat and enter our lungs. Once inhaled, these particles can remain in our lungs and contribute to serious health problems like emphysema and lung cancer. Even smaller particles with a diameter of less than 2.5μm can pass through the lung tissues and enter our bloodstream, where they can cause even more serious problems in organs like Nine of the low-cost MQ-series MOS gas sensors made by Hanwei Electronics in Henan, China and widely available on the internet. 70 Silicon Chip Australia's electronics magazine siliconchip.com.au the heart, liver and kidneys. This also applies to particles with a diameter of less than 1μm. PM1.0 is arguably a less useful criterion than the other two as the effect of these particles is similar to PM2.5. It’s almost impossible to have totally clean air, especially in an urban environment. So what levels of airborne particulate matter are regarded as relatively ‘safe’? The current guidelines are: • PM10 particles should not exceed 20μg per cubic metre (μg/m3) averaged over a year, or 50μg/m3 mean over 24 hours. • PM2.5 particles should not exceed 10μg/m3 averaged over a year, or 25μg/m3 mean over 24 hours. As for volatile organic compounds (VOCs), these are vapours emitted by many of the materials used in building our homes and offices, and many of the products we have and use in them. Common VOCs that may be present in the indoor air are benzene, ethylene glycol, formaldehyde, methylene chloride, tetrachloroethylene, toluene, xylene and 1,3-butadiene. By the way, “organic” means that they contain carbon molecules (like our organs, hence the name), not that they have been grown without synthetic fertiliser or pesticides. VOCs come from paints, varnishes, vinyl flooring, adhesives and composite wood products. Many can cause health problems in people with asthma and similar breathing problems, as well as people with specific allergies. Currently, there aren’t many ‘safe level’ guidelines for VOCs, though, and the general advice seems to be that they should be kept as low as possible – especially over the long term. Now we come to toxic gases. The most common of these in our homes and offices is carbon dioxide (CO2) because we exhale this ourselves. The best way to keep the CO2 level reasonable is to provide adequate ventilation. Still, it is also the major component of combustion gases, along with water vapour (but water is generally harmless). Other examples of toxic gases are sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO) and ozone (O3). Luckily, since CO is produced mainly by imperfect combustion, there shouldn’t be much of it in the air inside our homes and offices. But if you work in or adjacent to a siliconchip.com.au vehicle repair facility or parking garage or have an unflued gas heater, it may well be of concern. Historically, SO 2 pollution has been associated with the combustion of wood or fossil fuels like coal. So nowadays, in urban areas, this should not be a serious problem – unless you live near a coal-fired power station or prefer an old-fashioned wood fire to heat your home. Like CO, NO2 is generally produced as a result of combustion. Motor vehicles are the main source outdoors. Indoors, the primary sources are gas, wood, oil, kerosene, coal-burning fires and heaters, and tobacco smoke. Ozone can be emitted by office equipment involving high voltage, like laser printers and photocopiers. It is also generated by arcing within brushed motors. Safety guidelines for some of these gases are currently: • SO2: less than 40μg/m3 averaged over one hour. • NO2: less than 10μg/m3 average over a year, or 200μg/m3 over one hour. • O3: less than 60μg/m3 mean over eight hours. Types of air quality sensor Currently, there are four main types of air quality sensor: the metal oxide semiconductor (MOS) type, the non-dispersive IR sensor (NDIR) type, the photo-acoustic spectroscope (PAS) type and the particulate matter counter (PMC) type. Let’s now look at how these work. MOS sensors Sometimes called MOx sensors, these rely on the behaviour of particles of a metal oxide (usually tin oxide) when heated in the presence of air and/ or other gases. The basic principle of a MOS sensor is shown in Fig.1, which shows a cross-section of a MOS sensor. The silicon substrate of the sensing chip has a thin layer of tin oxide on the top, placed there by chemical vapour deposition. Electrodes at each end allow its resistance to be measured. On the underside of the chip is a heater element, used to heat the oxide layer to around 200-250°C, to speed up the sensor’s response. When the oxide layer is heated in the presence of clean air, donor electrons in the oxide attract oxygen molecules from the air and they are ‘captured’ by the oxide particles. As a result, a depletion layer forms on the surface of the oxide layer, and its electrical resistance rises. But if reducing gases such as carbon monoxide (CO) and some VOCs are present in the air, oxygen molecules in the surface of the oxide are released, and the depletion layer becomes thinner. As a result, the effective resistance of the oxide layer is reduced. So the current passed by the oxide layer varies proportionally with the amount of reducing gas in the air surrounding the oxide layer. The higher the reducing gas level, the higher the current. MOS sensors can detect specific VOCs by ‘doping’ the oxide layer with various chemicals. This is done in the MQ-series of sensors made by Hanwei Electronics Group in Henan, China. For example, their MQ-3 sensor is designed to detect alcohol vapour, so it’s suitable for use in a ‘breathalyser’. On the other hand, their MQ-5 sensor is designed to detect natural gas, LPG and coal gas, so it’s suitable for use in gas leak detectors. The other sensors in this series are designed for sensing: • MQ-4: methane gas • MQ-6: LPG, iso-butane & propane Fig.1: the basic principle of a MOS sensor. Australia's electronics magazine May 2022  71 • MQ-7: carbon monoxide (CO) • MQ-8: hydrogen (H2) • MQ-9: methane (CH4), LPG & CO • MQ-135: ammonia (NH3), nitrous oxides (NOx), carbon dioxide (CO2), alcohol, benzene and smoke Many of the Hanwei MQ-series sensors are used in many low-cost gas sensing modules available on the internet. They are all in a cylindrical six-pin package, either 17mm or 20mm in diameter and 10mm or 15mm high. Most of these modules simply take the analog current output from the sensor and convert it to a proportional voltage using an op amp buffer. The output voltage can then be measured using a digital multimeter (DMM) or fed into one of the ADC inputs of a microcontroller unit (MCU). SGX Sensortech Other MOS sensors found in lowcost air/gas sensing modules are the MiCS-5524 and the MiCS-4514, both made by SGX Sensortech (an Amphenol company) in Switzerland. These are much smaller than the MQ-series sensors, being in an SMD package measuring only 7 x 5 x 1.6mm. The MiCS-5524 detects CO, ethanol, hydrogen, ammonia and methane, while the MiCS-4514 has a second MOS sensor that detects nitrogen dioxide (NO2). The MiCS-5524 sensor is used in a gas sensing module with the same name, available from various internet suppliers, including Banggood, which currently has it priced at US$11.00 plus free shipping. This module measures only 18 x 13mm. The MiCS-4514 sensor is used in a fancier and slightly larger module (23 x 14mm) called the MiCS-VZ89TE, provided by SGX Sensortech itself and available from suppliers like Two more low-cost modules using the CCS811 MOS sensor made by ScioSense BV in Eindhoven. The one on the left is the Geekcreit CJMCU-811, available from Banggood, while the one on the right is Duinotech SEN-CCS811, available from Jaycar (Cat XC3782). element14 for around $25 plus shipping. This module incorporates its own ADCs (analog-to-digital converters), together with a dedicated MCU with detection algorithms. This module can provide CO2 equivalent and TVOC (isobutylene equivalent) readings via both PWM outputs and over an I2C serial bus. ScioSense BV Yet another MOS sensor found in low-cost air/gas sensing modules is the CCS811, made by ScioSense BV in Eindhoven, The Netherlands. Like the MiCS devices, the CCS811 is in a tiny SMD package, but it’s even smaller at just 4 x 3 x 1.2mm. Despite this, the CCS811 incorporates both an ADC and a dedicated MCU with built-in conversion algorithms, plus an I2C digital interface to link directly to a PC or an MCU. ScioSense describes it as an “ultralow-power digital gas sensor” and is claimed to detect a range of VOCs, providing both eTVOC (equivalent total VOC) and eCO2 (equivalent CO2) levels. The CCS811 sensor is used in several air quality sensing modules, including the Keyestudio KS0457 Two low-cost modules using the MiCS5524 MOS gas sensor made by Swiss firm SGX Sensortech. The module on the left is available from Banggood (probably made by Geekcreit), while the one on the right is the MiCS-VZ-89TE provided by SGX Sensortech itself, with a built-in MCU. 72 Silicon Chip Australia's electronics magazine CO2 Air Quality module, the Duinotech SEN-CCS811 Air Quality Sensor module (Jaycar Cat XC3782), the Adafruit CCS811 Air Quality Sensor and the CJMCU-811 CO2, Temperature and Humidity Sensor from Banggood. We’ll have a lot more details on MOS/MOx air quality sensors in a follow-up article next month, which will also show how to hook them up to microcontroller modules. NDIR sensors Another type of gas sensor is the non-dispersive infrared (NDIR) type, which, as the name suggests, makes use of IR light. It’s a simple kind of spectrophotometer that does not use any ‘dispersive’ elements like a prism or diffraction grating to separate the various wavelengths. Instead, it uses optical filters and/or a narrow-band infrared light source like LEDs or a semiconductor laser. It was discovered some time ago that molecules of different gases absorb light of specific IR (and near-IR) wavelengths. Pierre Bouguer discovered the general principle before 1729, and it was later elaborated on by Johann Lambert in 1760 and August Beer in 1852. Nowadays, it’s known as the Beer-Lambert law or the Beer-­LambertBouguer law. So by passing light of a specific wavelength through an air/gas mixture, the degree to which the light is attenuated indicates the amount of that gas present. The absorption spectra of various gases are shown in Fig.2. Carbon dioxide (CO2) absorbs light with a wavelength of 4.26μm (red lines) and also at a group of wavelengths around 15μm. Similarly, ozone (O3) absorbs light at wavelengths between 9.4-10μm (dark green lines), while carbon monoxide (CO) absorbs light at wavelengths between siliconchip.com.au Fig.2: the absorption spectra of various gases that can be detected by some of the sensor modules. 4.6-4.8μm (purple lines) and nitrogen dioxide (NO2) absorbs light between 6.17-6.43μm (light green lines). The operating principle of a simple NDIR sensor is shown in Fig.3. The IR light comes from the LED on the left, while there are two IR detectors on the right, behind separate optical filters. One filter passes only light of the wavelength corresponding to the gas to be detected. In contrast, the other filter passes either all other wavelengths or else the wavelength absorbed by a gas like nitrogen, which is the major component of air. By comparing the output of the two IR detectors, it can determine the proportion of the gas you want to detect in the chamber. NDIR detectors have been used in heating, ventilation, and air conditioning (HVAC) systems for years. However, they have tended to be large and relatively expensive – until recently, when IR LEDs and IR detectors based on micro-electromechanical systems (MEMS) have allowed them to be made smaller and for somewhat lower in cost. They still haven’t appeared widely in the low-cost air quality sensor (LCAQS) market, however. wavelengths in sunlight (like IR and ultraviolet or UV) also emit sound. The basic structure of a PAS sensor is shown in Fig.4. On the left again is the pulsed IR light source (generally a MEMS LED array), with an optical filter to its right passing only light of the wavelength absorbed by the gas to be detected; in this example, the wavelength of 4.2μm for detection of CO2. Then at the far end of the chamber, there’s a MEMS microphone, optimised to detect low audio frequencies. When the detected sound level is amplified, it can be converted into a figure corresponding to the amount of CO2 present in the cell. Note that the sensor as a whole is enclosed in an acoustic insulation layer, to reduce the influence of external sound. LCAQS sensors using the PAS principle have only appeared in the last couple of years because their development has depended on MEMS technology. The only one currently available is the XENSIV PAS CO2 sensor from Infineon Technologies (an offshoot of Siemens in Munich, Germany). This comes in a very compact PCB ‘mini board’ module measuring only 14 x 13.8 x 7.5mm, which combines the PAS sensor with a dedicated MCU running advanced compensation algorithms. The Infineon XENSIV PAS CO2 sensor mini-board module is currently available from suppliers like element14 and Mouser Electronics for around $48. Particulate matter sensors The fourth kind of air quality sensor is particulate matter or ‘PM’ sensors or counters. These can fall into three groups depending on the size of the particles they are designed to detect: less than 10μm (PM10), less than 2.5μm (PM2.5) and less than 1μm (PM1.0). However, some of them provide several ‘channels’ to deal with particles of different sizes. Currently, the PM2.5 type is the most common in the low-cost section of the market, so we will concentrate on this type. The basic principle of this type of PM sensor is shown in Fig.5. A small fan pulls air from the environment into a channel which passes through a sensing chamber. A small Fig.3: how a simple NDIR (non-dispersive infrared) sensor works. PAS sensors Another kind of gas sensor is the Photo-Acoustic Spectroscopy or PAS sensor, which again makes use of the way specific IR wavelengths can be absorbed by molecules of a particular gas (according to the Beer-Lambert law). But in PAS sensors, the degree of absorption is not measured directly. Instead, they make use of a phenomenon first discovered by Alexander Graham Bell in 1880: that when a thin disc is exposed to pulses of sunlight (using a rotating slotted wheel), it emits sound. Later, Bell showed that materials exposed to the non-visible siliconchip.com.au Fig.4: the basic structure of a PAS (photoacoustic spectroscopy) sensor. Australia's electronics magazine May 2022  73 laser sends a focused beam of light through the chamber, where any particles of matter in the air will scatter the light towards the sides. One or more photodiodes in the sides of the chamber detect this scattered light. Any light that is not scattered by PM particles passes through the chamber to be absorbed by the ‘beam dump’. By controlling the fan speed and thus moving the air through the sensing chamber at a known rate of volume, together with measuring the output of the photodiodes, the concentration of particles in the air can be calculated. The result is in units of μg/m3 (micrograms per cubic metre). Note that the traditional and most accurate way of measuring PM is the ‘gravimetric’ method, using a preweighed clean filter to collect particles from the air over a 24-hour sampling period, then weighing the filter again to determine the total mass of the accumulated particles, in micrograms. The concentration is obtained by dividing this figure by the volume of air that passed through the filter during the 24-hour sampling period. There are several low-cost PM2.5 sensors currently available, including the Grove-Laser PM2.5 Sensor module based on the Seeed Studio HM3301 sensor from Shenzen, China; the SN-GCJA5 sensor made by Panasonic Photo and Lighting Co. in Osaka, Japan; and the SPS30 PM sensor from Sensirion in Staefa, Switzerland. Fig.5: the basic operating principle of a PM (particulate matter) sensor. The Seeed Studio HM3301 sensor is inside a compact plastic and metal case measuring 38 x 40 x 15mm. In addition to the fan, laser and photodiodes, it has built-in electronics which provide fan control, photodiode signal amplification, filtering, multichannel data acquisition and an MCU for data processing. The output is via an I2C interface. In the Grove-Laser PM2.5 module, the HM3301 sensor is mounted on a PCB measuring 80 x 40mm, with a four-pin connector at one end for connection to a 3.3-5V power supply and the I2C lines for connection to a PC or external MCU. The effective PM2.5 The Grove Laser PM2.5 air sensor module is based on the Seeed HM3301 particulate matter sensor. The sensor itself measures only 38 x 40 x 15mm, and the module comes with a cable to connect to an Arduino or similar MCU. 74 Silicon Chip measuring range of the module is 1-500μg/m3, although it can measure up to a maximum level of 1000μg/m3. This module is available from Australian distributor Pakronics in Rosanna, Victoria for around $50 plus shipping. The Panasonic SN-GCJA5 PM2.5 sensor is again inside a compact moulded plastic box, measuring 37 x 37 x 12mm and weighing 13g. Like the HM3301 sensor, it includes all electronics to control the fan speed, amplify and filter the signals from the photodiodes, and an MCU for data processing. The Panasonic SN-GCJA5 PM2.5 particulate matter sensor comes in a small moulded plastic case measuring 37 x 37 x 12mm. In addition to the fan, laser and photodetector, it contains all electronics and provides both I2C and UART digital outputs. Australia's electronics magazine siliconchip.com.au The output is via either an I2C or a UART TX (serial) terminal. The effective measuring range of this module is 0-2000μg/m3. The Panasonic SN-GCJA5 PM2.5 sensor is currently available in Australia from element14 for around $37 plus delivery. It comes in a compact plastic-and-metal case measuring 41.2 x 41.2 x 12.3mm and weighing only 26.3g. As with the other two, it includes all the electronics to control the fan speed, amplify and filter the photodiode signals, together with an MCU for data processing. The output is via either an I2C interface or a UART TX/RX interface (selectable). The effective PM2.5 measuring range is 0-1000μg/m3. By the time you read this article, the Sensirion SPS30 PM sensor should also be available in Australia from element14, for around $60 plus shipping. I will review some of the sensors described here, and show how to use them in a future series of articles. Glossary of terms ADC Analog-to-Digital Converter – a device that converts a current or voltage into a digital value (usually an integer) eCO2 A concentration of CO2 in the air inferred by measuring the concentration of VOCs (see below) COPD Chronic Obstructive Pulmonary Disease – includes asthma, emphysema, asbestosis, etc IAQ Indoor Air Quality LCAQS Low-Cost Air Quality Sensors – officially defined as sensors costing less than US$500(!) MCU Microcontroller Unit – a small processor with onboard memory and peripherals MEMS Micro ElectroMechanical Systems – devices fabricated like an IC but with mechanical elements. See our November 2020 feature article (siliconchip.com.au/ siliconchip.com.au/ Article/14635) for details Article/14635 MOS Metal Oxide Semiconductor – a type of semiconductor that varies its resistance depending on the concentration of reducing gases it is exposed to, allowing it to detect CO and some VOCs MOx Another name for MOS NDIR Non-Dispersive Infrared (IR) sensor NOx The oxides of nitrogen, NO2 & NO3, generally created when air is heated to very high temperatures (eg, inside an internal combustion engine, especially diesel engines) PAS Photo Acoustic Spectroscopy – gas molecules exposed to IR pulses produce sound which can be used to determine the gas concentration Manufacturers: www.infineon.com www.sgxsensortech.com siliconchip.com.au/link/abcv siliconchip.com.au/link/abcw PMC Particulate Matter Counter – a device which counts the number of particles in an air sample PM10 Particulate matter in the air, including only particles less than 10μm in diameter PM2.5 Particulate matter in the air, including only particles less than 2.5μm in diameter Retailers: www.jaycar.com.au https://au.element14.com/3523840 www.pakronics.com.au www.banggood.com siliconchip.com.au/link/abcx SC PM1.0 Particulate matter in the air, including only particles less than 1μm in diameter tVOC A VOC reading (see below) equivalent to a reference concentration of isobutylene (a VOC) VOCs Volatile Organic Compounds – a large group of chemicals within many of the products we have in our homes and offices; their vapours can form a health risk if breathed in U Cable Tester S B Test just about any USB cable! USB-A (2.0/3.2) USB-B (2.0/3.2) USB-C Mini-B Micro-B (2.0/3.2) Reports faults with individual cable ends, short circuits, open circuits, voltage drops and cable resistance etc November & December 2021 issue siliconchip.com.au/Series/374 DIY kit for $110 SC5966 – siliconchip.com.au/Shop/20/5966 Everything included except the case and batteries. Postage is $10 within Australia, see our website for overseas & express post rates siliconchip.com.au Australia's electronics magazine May 2022  75