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Using Cheap Asian Electronic Modules By Jim Rowe MOS metal oxide semiconductor Air Quality Sensors In the last few years we’ve noticed the rise of low-cost air quality sensors and sensing modules. In this article we’ll investigate some of the currently available MOS (metal oxide semiconductor) type sensor modules, what they offer and how to use/interface them with an Arduino Uno. M OS type air quality sensors (sometimes called ‘MOx sensors’) 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 is shown in Fig.1, which depicts a cross-section of a typical 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 volatile organic chemicals (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 36 the reducing gas level, the higher the current. Therefore, the basic MOS sensor essentially behaves as a reducinggas-to-DC-analogue-current transducer. The Hanwei MQ-135 Probably the most common of the low-cost MOS sensors currently available is the Hanwei MQ-135, which is designed to be sensitive to ammonia (NH3), nitrous oxides (NOx), carbon dioxide (CO2), alcohol, benzene and smoke. Like the other sensors in the Hanwei series, the MQ-135 sensor comes in a cylindrical 6-pin package, 19mm in diameter and 15mm high. Most modules using the MQ-135 simply take the current output from the sensor and convert it to a proportional voltage using a fixed load resistor. The output voltage can then be measured using a DMM, or fed into one of the ADC inputs of a microcontroller unit (MCU). Fig.2 shows the circuit of Hanwei’s own air sensor module using the MQ-135. The MQ-135’s heater pins (H) are connected between the +5V (Vcc) line and the GND line via a 5.1Ω series current-limiting resistor. One end of the tin oxide sensing resistor (Rs) is connected to the +5V line via the two A pins, and the other end goes to the GND line via the two B pins and a 1kΩ load resistor. The two B pins are also connected to the A0 analogue output pin, to allow the voltage across the load resistor to be sent to a DMM or an MCU’s ADC input. The rest of the components are so that the module can also be used as a simple gas level alarm. One half of the LM393 dual comparator (IC1b) compares the voltage across the 1kΩ load resistor with a reference voltage set using trimpot VR1, so whenever the A0 voltage rises slightly above the reference voltage, the output of IC1b Fig.1: shows the cross-section of a MOS (metal oxide semiconductor) sensor and how it works. Practical Electronics | July | 2023 Fig.2: the circuit of Hanwei’s MQ-135 air sensor module. The lead photo shows a group of MQ-model sensors. (pin 7) drops to near ground level, causing the D0 LED to begin glowing. The voltage level at the D0 output pin is pulled down simultaneously. One change should ideally be made to the module if you want to use it with an MCU for monitoring the gas level, rather than simply using it as a gas level alarm. This involves replacing the sensor’s 1kΩ load resistor with a 22kΩ resistor to give a higher output voltage swing and improve reading accuracy. This resistor is an M2012/0805size (2.0 x 1.2mm) SMD component, so you’ll need a fine-tipped soldering iron and either a magnifying glass or a microscope. Fig.3 shows how to hook it up to an Arduino Uno or a compatible MCU after making that change. You just need to connect the module’s Vcc and GND pins to the corresponding pins on the Arduino, plus the module’s A0 MQ-135 Sketch Program void setup() { Serial.begin(9600); Serial.println(“Practical Electronics MQ-135 demo!”); } void loop() { int reading = analogRead(A2); Serial.println(reading); delay(1000); } Sample Output Practical Electronics MQ-135 demo! 696 694 694 691 692 710 Practical Electronics | July | 2023 pin to one of the Arduino’s ADC input pins; in this case, we’re using A2. There are quite a few Arduino libraries and sketches available to work with the MQ-135 module. You’ll find links to some of them in the list of links at the end of this article. However, I found many of them a bit tricky to negotiate. But I did find some very helpful information on Rob’s blog at: https://blog.robberg.net/mq-135-arduino Then I came across an elementary sketch using no libraries, but just showing the current analogue voltage provided at the module’s A0 pin, see: https://arduinolearning.com/amp/code I adapted this sketch slightly, and its listing is replicated below along with some of the sample output from when this sketch is running. When I breathed on the MQ-135, that caused the voltage reading to rise from under 700 to about 728 before falling back down again. As you can see, there’s no attempt to convert the A0 voltage readings to equivalent gas levels – for that, you would need one of the fancier sketches relying on their dedicated libraries. SGX Sensortech MiCS-5524 Another MOS sensor found in low-cost air/gas sensing modules is the MiCS5524, made by SGX Sensortech (an Amphenol company) in Switzerland. This is much smaller than the MQ-135, coming in an SMD package measuring only 7 x 5 x 1.6mm. The MiCS-5524 detects CO, ethanol, hydrogen, ammonia and methane. It is used in an 18 x 13mm gas sensing module with the same name available from various internet suppliers, including ALiExpress, Mouser and PiHut for around £10 to £15). The circuit of the MiCS-5524 module is shown in Fig.4. It is essentially just the sensor itself with an 82Ω current-limiting resistor for the sensor’s heater and a 91kΩ load resistor for its sensing resistor Rs, with a 100nF capacitor across the latter to provide noise reduction. P-channel MOSFET Q1 is included so that the power to the sensor can be controlled using the module’s EN pin. This pin can be left floating if the module is to operate continuously. Fig.5 shows how easy it is to connect the MiCS-5524 module to an Arduino Uno, while the sketch is shown overleaf with the sample output. The sketch is almost identical to the MQ-135 program and is similarly based on: https://arduinolearning.com/amp/code SGX Sensortech MiCS-VZ-89TE SGX Sensortech also makes a fancier and slightly larger module (23 x 14mm) called the MiCS-VZ-89TE, available from suppliers like element14, Digi-Key and Mouser for around £20 plus delivery. This module incorporates its own dedicated MCU with ADCs (analogue-to-digital converters) and embedded conversion algorithms. As a result, this module can provide both PWM and I2C digital outputs for CO2 equivalent and TVOC (isobutylene equivalent). I couldn’t find any circuit diagram for the MiCS-VZ-89TE module, but its layout is shown in Fig.6. Fig.3: the connection diagram for the MQ-135 sensor module with an Arduino Uno or similar. 37 Fig.4: (left) the circuit diagram for the MiCS-5524 module, which is simpler than the previous MQ-135 sensor and detects fewer gases. Next to the circuit are two different modules that use this chip. I found it fairly easy to connect to this module. I used two 5-pin sections of SIL header strip, with the top of the second and fourth pins of each strip cut short, allowing the tops of the remaining three pins to be soldered to the notches on one side of the module. You can then plug the complete assembly into a small breadboard for testing and use. Fig.7 shows how the MiCS-VZ-89TE module can be connected to an Arduino Uno or equivalent MCU. The GND connection goes to one of the Arduino’s GND pins, while the module’s 3.3V power connection goes to the Arduino’s +3.3V pin. The module’s I2C connections SDA and SCL are wired to the Arduino’s pins A4/SDA and A5/SCL, respectively. Each of these pins needs an external 4.7kΩ pull-up resistor connected to the +3.3V pin, because the MiCS-VZ-89TE module doesn’t provide pull-ups. I found an Arduino sketch and library to read the CO2 and VOC levels from a MiCS-VZ-89TE, written by H Grabas – it is available on his website at: https://bit.ly/pe-jul23-grab This sketch and its library worked so well that I adapted it to produce the sketch listed opposite, along with a sample of the output from the Arduino IDE Serial Monitor. For this to work, you need to download Mr Grabas’ library from his website and install it as a library in the Arduino IDE. When running, it gives you a VOC reading and a CO2 reading approximately once per second. I tried breathing on the module’s sensor, causing the VOC readings to rise to around 270.4ppb (parts per billion), while the CO2 reading barely moved from about 414 ppm (parts per million). Next, I sprayed a tiny amount of isopropanol (spectacle cleaning fluid) a few centimetres above the sensor, causing the VOC reading to jump up to its maximum figure of 1000ppb. So the MiCS-VZ-89TE and the sketch and library certainly seem to be working. The ScioSense CCS811 Another MOS sensor found in several low-cost air/gas sensing modules is the CCS811, made by ScioSense BV in Eindhoven, The Netherlands. The CCS811 is in a tiny SMD package, measuring just 4 x 3 x 1.2mm. Fig.6: the layout diagram for the MiCS-VZ-89TE module. MiCS-5524 Sketch void setup() { Serial.begin(9600); Serial.println(“Practical Electronics MiCs-5524 demo!”); } void loop() { int reading = analogRead(A0); Serial.println(reading); delay(1000); } Sample Output Fig.5: MiCS-5524 connection diagram. 38 Practical Electronics MiCs5524 demo! 40 39 40 39 Fig.7: MiCS-VZ-89TE connection diagram to an Arduino Uno. Practical Electronics | July | 2023 MiCS-VZ-89TE Sketch: Fig.8: the block diagram for the CCS811 module. One type of this module is shown below, with a larger variant shown below. #include <MICS-VZ-89TE.h> #include <Wire.h> MICS_VZ_89TE voc; void setup() { voc.begin(); Serial.begin(9600); Serial.println(“Reading the MiCS-VZ-89TE sensor”); } void loop() { voc.readSensor(); Serial.print(“VOC =”); Serial.print(voc.getVOC()); Serial.print(“ | ”); Serial.print(“CO2 = ”); Serial.println(voc. getCO2()); delay(1000); } Sample Output Reading the MiCS-VZ-89TE sensor VOC = 0.00 | CO2 = 413.97 VOC = 135.37 | CO2 = 413.97 VOC = 270.74 | CO2 = 413.97 VOC = 1000.00 | CO2 = 420.96 Despite its miniscule size, this sensor 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 like an Arduino or a Micromite. It’s described by ScioSense as an ‘ultra-low power digital gas sensor’ and detects a range of VOCs and provide both eTVOC (equivalent total VOC) and eCO 2 (equivalent CO2) levels. Fig.8 is a block diagram of the CCS811. Pin 4 (PWM) and pin 5 (SENSE) must be connected together for correct operation of the MOX sensor’s heater control circuit. Pin 1 (ADDR) is to allow the CCS811’s I2C address to be set to either 90d/5Ah (ADDR pin low) or 91d/5Bh (ADDR pin high), while the AUX pin (8) has no internal connection. The CCS811 sensor is used in many air quality sensing modules, including the Keyestudio KS0457 CO2 Air Quality module, the Duinotech SENCCS811 Air Quality Sensor module (Jaycar Cat XC3782), the Adafruit CCS811 Air Quality Sensor and the CJMCU-811 CO2, Temperature and Humidity Sensor from Banggood. Fig.9 shows the circuit for many of these CCS811 sensor modules. Along with the CCS811 sensor itself, there’s voltage regulator REG1, which steps down the incoming +5V power to provide the 3.3V needed by the CCS811, plus MOSFETs Q1 and Q2 which, together with four 10kΩ pull-up resistors, perform logic-level conversion for the I2C digital communication lines (SDA and SCL). Diodes D1 and D2, together with the two 100kΩ pull-up resistors, allow the WAKE and RST pins of the CCS811 to be pulled low. The WAKE pin must be pulled to ground to allow the chip to operate. Note that pin 1 of the CCS811 is pulled low by a 100kΩ resistor to set the I2C address to 90d/5Ah. Also, as mentioned earlier, pins 4 and 5 are tied together and pulled high via two more 100kΩ resistors. Incidentally, some CCS811-based modules (such as the CJMCU-811) have an additional pin on the I/O connector, with the extra pin connected to pin 1 of the CCS811 and labelled ‘ADD’. This allows the I2C address of the module to be changed to 91d/5Bh by pulling the pin high. It’s quite easy to connect the SENCCS811 and most of the other CCS811based air quality modules to an MCU like an Arduino Uno, as shown in Fig.10. The Vcc and GND pins connect to the +5V and GND pins of the Fig.9: the circuit diagram for the CCS811 module. Practical Electronics | July | 2023 39 www.poscope.com/epe Arduino, while the SDA pin goes to the Arduino’s A4/SDA pin and the SCL pin to the Arduino’s A5/SCL pin. Finally, the module’s WAKE pin connects to another GND pin on the Arduino. Some modules have the pins in a different order, so make sure you check the connections for the module you are using. Several Arduino libraries are available to support a sketch communicating with these modules. I found the easiest one to use was the Keyestudio KS0457 library (CCS811.h and CCS811.cpp), available from: https://fs.keyestudio.com/KS0457 I also downloaded Keyestudio’s readData.ino sketch and adapted it to produce my own version of the sketch: read_CCS811_data.ino. It’s a bit too long to reproduce the listing here, but you can download it from the July 2023 page of the PE website at: https://bit.ly/pe-downloads - USB - Ethernet - Web server - Modbus - CNC (Mach3/4) - IO - PWM - Encoders - LCD - Analog inputs - Compact PLC - up to 256 - up to 32 microsteps microsteps - 50 V / 6 A - 30 V / 2.5 A - USB configuration - Isolated PoScope Mega1+ PoScope Mega50 Shown below is the output of that sketch. The Arduino provides a stream of measurements for both the eCO2 level in ppm and eVOC in ppb. At one point, I blew in the direction of the CCS811 sensor to give it some extra CO2. That’s the reason for the sudden rise in eCO2 and eTVOC readings, from around 400ppm and 1-2ppb up to peaks of 1743ppm and 384ppb a second later. Then the readings fell slowly after that. Summary After trying several of these modules, I’m less keen on those based on the MQ-135 sensor than on the Sensortech MiCS sensors or the ScioSense CCS811 sensor. That’s mainly because of the scarcity of easy-to-­ understand software if you want to do more than simply ‘raise the alarm’ if the CO2/VOC level rises above a preset ‘safe’ level. I’m also not that keen on modules based on the SGX Sensortech MiCS5524 sensor for much the same reason. Overall, I prefer the ‘smarter’ modules like the SGX Sensortech MiCSVZ-89TE or most of those using the CCS811 sensor. These modules are all much easier to get going with an MCU like an Arduino as a reliable CO2/VOC sensor. Sample Output - up to 50MS/s - resolution up to 12bit - Lowest power consumption - Smallest and lightest - 7 in 1: Oscilloscope, FFT, X/Y, Recorder, Logic Analyzer, Protocol decoder, Signal generator 40 Getting data from the CCS811... eCO2: 400ppm, eTVOC: 0ppb eCO2: 410ppm, eTVOC: 1ppb eCO2: 414ppm, eTVOC: 2ppb eCO2: 1743ppm, eTVOC: 384ppb eCO2: 1345ppm, eTVOC: 143ppb eCO2: 977ppm, eTVOC: 87ppb Fig.10: CCS811 connection diagram. I would give first prize to the MiCS-VZ-89TE module (element14 2925865). But second prize would go to any of the modules based on the ScioSense CCS811 sensor, like the Duinotech SEN-CCS811 from Jaycar (Cat XC3782), the CJMCU-811 from Banggood (ID 1157216), the Keyestudio KS0457 or the Adafruit CCS811: www.adafruit.com/product/3566 I will describe some of the NDIR and PAS type air quality sensor modules in a future article. Useful Links MQ-135: • www.arduinolibraries.info/ libraries/mq135 • https://github.com/ Phoenix1747/MQ-135 • siliconchip.com.au/link/abct • https://blog.robberg.net/ mq-135-arduino MiCS-5524: • www.sgxsensortech.com • siliconchip.com.au/link/abcu • https://bit.ly/pe-jul23-grab CCS811: • https://fs.keyestudio.com/KS0457 • https://bit.ly/pe-jul23-811 Reproduced by arrangement with SILICON CHIP magazine 2023. www.siliconchip.com.au Practical Electronics | July | 2023