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Measuring Temperature and Relative Humidity U s in g Ch e a p A s ian El e c t r M o d u o nic l Par t 4e s The AM2302/DHT22 digital temperature and relative humidity (RH) sensing module provides about the simplest way to make a microcontroller project with temperature and RH sensing capabilities. by JIM ROWE L ow-cost modules capable of sensing and measuring both temperature and relative humidity (RH) have been available for a few years now. Initially these modules appeared as peripherals for Arduino and similar microcomputers but they soon became an almost standard add-on for just about any micro-based project. How humidity is measured Relative humidity is the ratio of the amount of water vapour per volume of air at a particular temperature to the maximum amount of water which can be contained by that volume of air at that same temperature without condensation. Another way to state this is that RH is approximately the ratio of the actual vapour pressure to the saturation vapour pressure. The saturation vapour pressure depends on the dew point temperature, which is the highest temperature for a given humidity level at which water vapour will condense and form dew. This means that RH depends on three factors: the amount of water vapour in the air, air temperature and atmospheric pressure at the time of measurement. Since the module described here measures both RH and temperature, if you assume a fixed barometric pressure (eg, at sea level it is typically close to 1 bar), you can compute the absolute humidity based on these two readings. Just about all of these temperature/ RH sensing modules are based on integrated digital sensors made by Chinese firm Aosong Electronics (based in Guangzhou), which also goes Fig.1: close-up of the humidity sensor, showing the two capacitor plates. Note the darker plate marked with red is much smaller than the gold one underneath.* Fig.2 (below): complete connection diagram for the AM2302/DHT22 sensor module. The 4.7kΩ pull-up resistor allows for bidirectional communication with a single DATA pin. AM2302/DHT22 RH & TEMPERATURE SENSOR MODULE 4.7kΩ 100nF DATA VCC GND 46  Silicon Chip 1 2 3 4 VDD DATA (NC) GND by the name MaxDetect Technology. What’s inside Most modules currently available use their improved AM2302 sensor, which has alternative names: DHT22 or RHT03. Aosong/MaxDetect say little about what's inside the AM2302/DHT22/ RHT03 but mention that it contains a dedicated 8-bit microcontroller (see Fig.5), a temperature sensor and one RH sensor, the latter being based on a special polymer capacitor. Curious to know more, I carefully cut away the slotted upper section of the plastic device body. All this achieved was to reveal the two sensors, fitted on the top of a very small PCB (18 x 14mm) which is potted inside the remaining part of the plastic body (see photo and Fig.6 at right). The polymer capacitor humidity sensor (Fig.1) and it works by measuring the relative change in the dielectric constant of the capacitor with varying humidity. Since the change in value differs between capacitors, sensor calibration is required to provide accurate results. A thermistor provides temperature sensing. The thermistor used is an NTC (negative temperature coefficient) type, made of a conductive material which decreases in resistance proportionally as the temperature rises. The microcontroller measures the RH sensor capacitance and the thermistor resistance, then converts the siliconchip.com.au BUSBUS RELEASED RELEASED FOR FOR 20µ20 s µs VCC VCC CODING CODING FORFOR DATA DATA BIT 'BIT 0' '0' s µs 80µ80 s µs 80µ80 START START SIGNAL SIGNAL FROM FROM MICRO MICRO (1ms (1ms RECOMMENDED) RECOMMENDED) CODING CODING FORFOR DATA DATA BIT 'BIT 1' '1' 28µ28 s µs 70µ70 s µs VCC VCC LOGIC LOGIC HIGH HIGH LOGIC HIGH LOGIC HIGH LOGIC LOW LOGIC LOW LOGIC LOW LOGIC LOW GND GND GND GND FORMAT OFOF START REQUEST SIGNAL FROM MICRO, FORMAT START REQUEST SIGNAL FROM MICRO, 'OK WILL START' RESPONSE FROM AM2302 SENSOR 'OK WILL START' RESPONSE FROM AM2302 SENSOR 50µ50 s µs SENSOR SENSOR RESPONSE RESPONSE SIGNAL SIGNAL 50µ50 s µs DATA DATA BITBIT CODING CODING FOR FOR 'READ' 'READ' SIGNALS SIGNALS FROM FROM AM2302 AM2302 Fig.3: to wake the sensor from standby mode, the micro pulls Fig.4: the micro differentiates between what type of bit it RH HIGH RH HIGH BYTE RH LOW RHaLOW BYTE BYTE PARITY BYTE BYTE TEMP TEMP HIGH HIGH BYTE BYTE TEMP TEMP LOW LOW BYTE BYTE has received based on the pulsePARITY time; a data bit of value the DATA line low for a minimum ofBYTE800µs and maximum zero has a pulse time of 78µs while a one has a pulse time of 20ms. The DATA line then goes high for 20µs. This is of 120µs. regarded as a start request sent to the AM2302. LSB LSB MSB MSB LSB LSB MSB MSB LSB LSB MSB MSB LSB LSB MSB uring range is from -40 to +80°C with capacitor from VCC to ground and a a resolution of 0.1°C and an accuracy 4.7kΩ pullup resistor between the digiSINGLE SINGLE 'READ 'READ FROM FROM AM2302' AM2302' TRANSACTION TRANSACTION DATA DATA FORMAT FORMAT of ±0.5°C. The long-term RH stability tal data bus line and VCC. is rated as ±0.5% per year. The reason for that resistor leads us The device is designed to run from to discuss the way the device commu3.3-5.5V DC, with operation from nicates with an external micro, over 5V recommended. It has a nominal that single-wire bus. current drain of 1.5mA when measuring, or 50µA when in standby. It How it handles data needs at least two seconds between Although it's poorly explained in measurements. the AM2302 data sheet, here's the T h e A M 2 3 0 2 / D H T 2 2 / R H T 0 3 basic idea: when the DATA line is module itself measures only 25.1 allowed to float at logic high levx 15.1 x 7.7mm, while the PCB for el (pulled high by the 4.7kΩ resisthe most common module using it tor), the sensor effectively sleeps in measures 39 x 23mm, as shown in standby mode. our picture. To wake it up, the external micro The sensor has four connection pins, must pull the DATA line down to logalthough one is labelled “NC” (no con- ic low for at least 800µs, but no more nection) in Aosong's data sheet. than 20ms. In fact, they recommend As you can see from Fig.2, there's that it be pulled down for 1ms. very little in a typical sensing modThen the micro should release the ule apart from the AM2302/DHT22/ DATA line, allowing it to float high RHT03 device itself. again for about 20µs. There are just two passive compoThis “1ms-low-followed-by-20µsnents on the board: a 100nF bypass high” sequence is regarded as the MSB LSB LSB MSB MSB analog readings to digital values. 'OK'OK WILL START START ' ' START SIGNAL SIGNAL ThisSTART micro and aWILL number of associRESPONSE RESPONSE FROM FROM MICRO MICRO FROM FROM Am2302 Am2302 ated components are mounted on the underside of the PCB; we can’t determine their exact configuration as it’s impossible to remove the potting without destroying most of the circuit. However, there is a YouTube video where someone has removed all the components from the device. Some of the pictures from that video are shown in this article, and the link to the video is at the end of this article. Aosong/MaxDetect state that every AM2302 sensor is temperature compensated and calibrated in an accurate calibration chamber, during or after which the calibration coefficients are saved in the micro's one-time programmable memory. Considering its low price, the claimed performance of the AM2302 is quite impressive. The RH measuring range is from 0 to 100%, with a resolution of 0.1% and an accuracy of ±2%, while the temperature meas- Fig.5 (above): the internal layout of the micro in the AM2302 sensor.* The module in question with the case still intact. The module has a fairly low profile, measuring only 7.7mm high. siliconchip.com.au Fig.6 (right): the sensor module with the top of the case removed. The bead type sensor is an NTC themistor and to the right is the capacitive humidity sensor.* February 2017  47 MSB LSB LSB PARITY BYTE TEMP LOW BYTE MSB MSB TEMP HIGH BYTE LSB MSB 'OK WILL START' RESPONSE FROM AM2302 RH LOW BYTE LSB START SIGNAL FROM MICRO MSB RH HIGH BYTE 50µs DATA BIT CODING FOR 'READ' SIGNALS FROM Am2302 LSB FORMAT OF START REQUEST SIGNAL FROM MICRO, 'OK WILL START' RESPONSE FROM AM2302 SENSOR 50µs SENSOR RESPONSE SIGNAL SINGLE 'READ FROM AM2302' TRANSACTION DATA FORMAT Fig.7: once there has been a start response from the sensor, the AM2302 sends out its measurement data in 40 bit sets. The first 16 bits is the relative humidity, the 16 bits after is the temperature and the final 8 bits are parity bits to pad the length of the data to 40 bits total. micro sending a start request signal to the AM2302. If the AM2302 responds to this wake up call, it pulls the DATA line down to logic low for 80µs, and then allows it to float high again for another 80µs. This is regarded as its “OK, will start” response. This “start request” and “OK will start” sequence is shown in Fig.3. Soon after this startup sequence, the AM2302 sends out its current measurement data as a sequence of 40 bits of data, grouped in five bytes as shown in Fig.7. The relative humidity reading is in the first two bytes (RH HIGH and RH LOW), followed by the temperature reading in the next two bytes (TEMP HIGH and TEMP LOW), and finally there's a checksum or parity byte to allow error checking. All of these bytes are sent MSB (most significant bit) first and LSB (least significant bit) last. It's also worth noting that both the RH and temperature readings have a resolution of 16 bits. While this single-wire-bus transaction may look fairly straightforward, it isn't quite that simple – because of the special encoding that Aosong uses for the data bits themselves. As shown in Fig.4, a binary zero is coded as a logic low of 50µs followed by a logic high of 28µs, whereas a binary one is coded as the same logic low of 50µs, but followed by a logic high of 70µs. So both a zero and a one begin with a logic low lasting for 50µs but a logic high that follows lasts for only 28µs in the case of a zero rather than 70µs in the case of a one. As a consequence, data bits with a value of 0 last for a total of 78µs, while those with a value of 1 last for 120µs. So the time taken by each of those data bytes as shown in Fig.7 will not 48  Silicon Chip be fixed but will vary, depending on the data bit values. For example, a byte consisting of all zeroes (00000000) will last for only 624µs, while a byte of all ones (11111111) will last for 960µs. So in practice, the duration of each data byte will vary between 624 and 960µs. The micro connected to the AM2302 needs to take this rather unusual coding system into account when it decodes RH and temperature data. How it's used You shouldn't have to worry about decoding the AM2302 measurement data yourself, because many people have already worked it out for most of the popular microcomputers. For example, if you want to hook up an AM2302-based module to a Maximite or Minimite, Geoff Graham has already solved this problem and provided a special command in his MMBasic programming language. It looks like this: HUMID pin, tVar, hVar Where HUMID is the command keyword and “pin” is the micro's I/O pin to which the module's DATA line is connected. “tVar” is the name of the floating-point variable you want to receive the returned temperature (in °C) and “hVar” is the name of a second floating-point variable to receive the returned relative humidity (as a percentage). It's that easy! If you're running the module from a 5V supply, you do have to make sure that you connect the module's DATA line to a Micromite pin that is 5V tolerant – ie, one of pins 14 to 18, 21 or 22 on the 28-pin Micromite. So if you have connected the module's DATA line to pin 18 of the Micromite and have declared the temperature and RH variables as say temp! and RH! respectively, you'll be able to read the sensor's data with this oneline command: HUMID 18, temp!, RH! If you want to take a sequence of say 10 readings spaced apart by the recommended minimum of two seconds and print them to the console, here's the kind of simple program you'll need: DIM nbr% = 10 DIM temp! = 0.0 DIM RH! = 0.0 PAUSE 1000 DO HUMID 18, temp!, RH! PRINT "Temperature = "temp! "C & humidity = " RH! "%" nbr% = nbr% - 1 PAUSE 2000 LOOP UNTIL nbr% = 0 If you want to hook up an AM2302based module to any of the Arduino versions, it's almost as easy. You have quite a choice when it comes to prewritten applications, some of which you'll find using these links: https://github.com/RobTillaart/ Arduino/tree/master/libraries/DHTlib https://github.com/nethoncho/ Arduino-DHT22 https://github.com/sparkfun/ SparkFun_RHT03_Particle_Library/ blob/master/firmware/examples/ RHT03-Example-Serial.ino There are also sample programs on both of these websites: www.aosong.com www.humidity.com So it's not at all difficult to use one of these low cost AM2302/DHT22/ RHT03 based modules with a readily available microcomputer. * these pictures have been taken from the video at: http://youtu.be/ C7uS1OJccKI by www.youtube.com/ SC user/electronupdate siliconchip.com.au