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Contents
1. 4 2 Sensor PCB 4 3 Casing and airflow PROGRAMMING 5 1 Structure of the code 5 2 Sensor functions 5 2 1 Humidity and temperature functions 5 2 2 CO function 5 2 3 NO NO Os CO and anemometer functions 5 2 4 Particle sensor function 5 2 5 Connection through internet 5 3 Web application TESTING 6 1 Final testing results CONCLUSION SOURCES Nn A A 00 10 11 13 14 18 21 22 23 26 29 29 31 31 33 35 35 37 39 41 41 43 44 GLOSSARY Anemometer Wind speed sensor Asynchronous communication transmission of data without external clock signal AT Command A set of commands to control Sim900 GPRS shield C Low level programming language CO Carbon monoxide molecule CO Carbon dioxide molecule EAGLE Cadsoft Eagle PCB design software GPRS General Packet Radio Service LED Light emitting diode electronic component MBED MBED LPC1728 microcontroller NO Nitric monoxide molecule NO Natrium dioxide molecule 03 Ozone molecule PCB Printed Circuit Board PPB Parts per billion air quality measure PPM Part per million air quality measure QSORT Quick Sort Algorithm to sort data C STD C standard function library UART Universal Asynchronous Receiver Transmitter USB Universal Serial Bus 1 INTRODUCTION The client of this batchelor s thesis is Aarhus University Herning The researchers want to know how well the Danish THOR modeling system app2. st ilmasta l mp tilaa ilmankosteutta p ly hiilidioksidia sek muita haitallisia kaasuja Laitetta ohjataan mikrokontrollerilla ja mittaustulokset l het n puhelinverkon v lityksell palvelimelle josta k ytt j voi tutkia mittaustuloksia web sivujen kautta Prototyypin piirilevy on suunniteltu ja rakennettu itse Piirilevy on valmistettu asiakkaan toimesta Elektroniset sensorit ohjainpiirit ja kotelo on valittu asiakkaan toimesta ennen projektin alkua Mikropiiri on ohjelmoitu C ohjelmointikielell Ty n tavoitteet saavutettiin prototyyppi saatiin rakennettua ja j rjestelm toimii oletetusti Projektissa k ytetyt ilmanlaatusensorit olivat yhteensopivia vaikkakin sensoreiden mittaustapa vaikutti muiden sensoreiden mittaustuloksiin Ongelma pystyttiin kiert m n sijoittamalla sensorit eri tavalla kuin alun perin oli suunniteltu Prototyypin mittaama tieto oli tarpeeksi tarkkaa vaikka kalibrointia ei suoritettu Suunnittelussa otettiin huomioon osien k ytt ik sek niiden vaihdettavuus Asiasanat mikro ohjaimet prototyypit ilmanlaatu ilmastoteknologia TABLE OF CONTENTS 7 INTRODUCTION PROJECT GOAL 2 1 Used technologies ZEE IC 2 1 2 RS 232 and UART CONTROLS AND SENSORS 3 1 MBED LPC1768 microcontroller 3 2 GSM GPRS module 3 3 Anemometer 3 4 Temperature and humidity sensor 3 5 CO sensor 3 6 Particle sensor 3 7 CO NO NO and O sensors HARDWARE DESIGN AND MANUFACTURING 4 1 MBED PCB
3. The module stores SMS messages and phonebook data into a SIM card There is a built in implementation of the Internet protocol suite TCP IP networking model The communication is done with serial communication and it has a maximum buffer size of 556 bytes The module supports an antenna and an audio interface which can be seen in Figure 6 ElecFreaks 2014 The SIM900 module is controlled via AT commands based on GSM 07 07 07 05 and EFCOM enchanced AT commands The AT commands originate from a Hayes command set and they are pre programmed inside the SIM900 by the manufacturer The AT Commands are series of short text strings which combined together forms commands for operations such as changing parameters establishing connection into internet or sending an SMS message The commands are created to ease the use of the microchip Shanghai SIMCom Wireless Solutions 2009 10 T N SON FIGURE 6 GSM GPRS module ElecFreaks 2014 3 3 Anemometer Anemometer Figure 7 is manufactured by Chinese Shaanxi Enjoy Imp amp Exp Co Ltd The sensor does not need any external power because the voltage is formed by the rotation of the sensor The direction of the rotation affects the output of the sensor if the sensor rotates backwards then the voltage will be negative The output voltage corresponds to the wind level wind speed as shown in Figure 8 The sensor outputs a maximum of five volts where one meter per second is
4. TxD and the received exchange data RxD and a common ground line TxD pin sends data into the receiving device s RxD pin When the voltage is between three and 15 volts the line logic is set to zero Space and when the voltage is between minus three and 15 the line logic is set to one Mark The area between minus three and plus three volts is called the transition region where the line does not know which logic level it is on The logic levels can be seen in Figure 4 When the line is free it is set to logic level zero Campbell 1989 50 15V Space 0 3V Transition region 3V Mark 1 15V FIGURE 4 Logical levels Campbell 1989 49 Universal asynchronous receiver transmitter UART is a piece of hardware which is used in serial communication When sending UART splits a byte into individual bits and forms a bit transmission by adding a start bit and one or two stop bits before sending the message through the communication lines While receiving UART saves the data bits into a buffer and resamples the bits back into bytes When the buffer overloads it discards the incoming data bits or replaces the old data bits with the new Campbell 1989 25 Oo CONTROLS AND SENSORS Oo 1 MBED LPC1768 microcontroller MBED LPC 1768 was chosen to be the microcontroller of the prototype The main reasons for this were the amount of serial and PC ports and the manufacturer s high level C C API The manufacturer is NX
5. and 36 eee Calculated low ratio 0 8 49405 v 0 are 3 Slope 1 6 FIGURE 35 Calculation when low ratio percentage is egual or under 5 6 m 5 6 0 8 Calculated low ratio 5 6 Particle K 000 x 0 2 0 2 00 0 2 Slope 1 6 Slope when changed 1 2 g FIGURE 36 Calcultion when low ratio percentage is over 5 6 5 2 5 Connection through internet The connection between the server and the devices is done through the SIM900 GSM GPRS shield The shield opens a connection to the server s PHP script and it is controlled with AT commands The AT commands are passed as a printf function through a serial connection When all settings are established connection through HTTP protocol is initialized After establishing the start commands shown in Figure 37 the connection is working until the GSM GRPS shield is rebooted After every command the GSM GPRS shield generates return statements which are read with a getc function so the buffer does not overload There are a set of commands which block return statements but they are not used because return statements are needed in the final product to confirm that the connection is established 38 GPRS printf AT SAPBR 3 1 CONTYPE GPRS r n wait 1 while GPRS readable GPRS getc GPRS printf AT SAPBR 3 1 APN internet r n wait 4 while GPRS readable GPRS getc GPRS printf AT SAPBR 1 1 r n w
6. lt lt 8 Temp L T dat T dat gt gt 2 Assign floats to pointers pHumidity float H dat 6 10e 3 pTemperature float T dat 1 007e 2 40 0 FIGURE 30 Changing humidity and temperature data into correct values 5 2 2 CO function The CO sensor uses a serial port to communicate with the MBED At the beginning of the function a list of command bytes are sent into the sensor Command bytes inform the sensor which setup it has to use The bytes are explained in Table 5 After sending the command bytes the program needs to wait for 15 seconds for the sensor to measure the amount of CO2 The CO value reguest can be seen in Figure 31 34 TABLE 5 CO command bytes Serial Address Command PC Address How many Checksum 1 Byte 1 Byte 2 Bytes bytes to read 2 Bytes 1 Byte OxFE Ox44 0x00 0x08 0x02 Command Bytes 0x46 EEPROM Read 0x44 RAM Read unsigned char readC02 7 10xFE 0x44 0x00 0x08 0x02 0x9F 0x253 for unsigned char i 0 i lt 7 i co2Serial putc readC02 i wait ms 50 amp co2Serial readable N S wait 15 Figure 31 Request CO value When the CO is measured the program starts to read the data from the serial buffer The CO sensor returns a set of bytes where the fourth and fifth bytes are the ones holding the measurement data Other bytes are not needed in this project The high and the low bytes are
7. 201 2a One or more devices usually microcontrollers are working as a master device One PC bus has a maximum of 128 devices because of the address range which is the byte s last seven bits The master can communicate with one slave device at a time and its duty is to start data transfer generate clock signal and to end data transfer All devices are connected by the same two wires Serial Clock Line SCL sends the clock signal and Serial Data Line SDA sends data Identification of the device is made with byte address which needs to be sent when the connection is opened The first bit of the slave s byte address specifies the device either into read 1 or write 0 mode The latest version of the PC bus needs pull up resistors from the power line to both SCL and SDA wires as shown in Figure 2 NXP Semiconductor 2012a FIGURE 2 Structure of PC line NXP Semiconductor 2012a When the voltage level is less than 80 percent from the total voltage pushed into SDA and SCL lines the voltage is LOW If the voltage is 80 percent or over then the voltage is HIGH When the PC bus is free both lines are running HIGH Before the transfer of the data can begin a start condition must be set The master device sets the SDA line to LOW which tells the devices that the start condition is set The data is transferred one byte at a time and the ninth tick is reserved for the receiving device to send a bit back to the sending device The nin
8. DAN K LAHDEN AMMATTIKORKEAKOULU N Lahti University of Applied Sciences lt Design construction and programming of an air guality measurement device LAHTI UNIVERSITY OF APPLIED SCIENCES Faculty of Technology Degree Programme in Information Technology Software engineering Bachelor s Thesis 2014 Miikka Juomoja Lahti University of Applied Sciences Degree Programme in Information Technology JUOMOJA MIIKKA Design construction and programming of an air guality measurement device Bachelor s Thesis in Software Engineering 45 pages Spring 2014 ABSTRACT The objective of this thesis was to design and build a prototype air guality measurement device and to create a web application to support it The main focus in this thesis was to design and building the prototype Another objective was to test the compatibility of the given sensors and to determine if they are accurate enough The thesis was done for Aarhus University and it is meant to be a tool in the university s research projects The air guality measurement device measures temperature humidity particles carbon dioxide and other toxic gasses from the surrounding air The device is controlled with a microcontroller Measurement data is sent through a mobile communication network into a web server from where the user can analyze measurement data with a web application The prototype s circuit boards were designed and built from the scratch The cir
9. P Semiconductor which is the same company that owns the rights to PC bus The microcontroller is based on a 32 bit ARM CortexTM M3 design The microcontroller includes a built in USB programming interface which is as simple as using a USB Flash Drive NXP Semiconductor 2014b The core is running at 96MHz and the microcontroller has 32KB of RAM memory and 512KB of programmable FLASH memory The microcontroller s 40 pins consist of 26 digital input and output pins three serial ports two PC ports and other I O interfaces as shown in Figure 5 The Microcontroller works within the limits of 4 5 to 9 volts and it can give 3 3 or 5 volts out The built in USB programming interface works by dragging and dropping files from the computer An online compiler is included with purchase of the microcontroller NXP Semiconductor 2014b GND C 5 ONH anvon e 5 r i sera feet 11 T5 Te a m x ta s a mbDec FIGURE 5 MBED LPC1768 microcontroller s design and features NXP Semiconductor 2014b 3 2 GSM GPRS module The GMS GPRS shield is manufactured by an open hardware facilitation studio ElecFreaks The shield uses a SIM900 GPRS GSM module manufactured by SIMCom Wireless Solutions Co Ltd The SIM900 module is working with five volts and it has low power consumption of 1 5mA in the sleep mode The module is using a quad band engine which means that the module is compatible with four main GSM freguencies
10. The second timer measurementTimer runs inside a while loop The while loop does not do anything else but wait until 30 seconds has passed 30 second is the time needed the sensor to measure the true amount of particles in the air Measuring process is show in the lines 1 19 in Figure 34 void pauseTimer lowlimer stop mSeconds mSeconds lowlimer read ms lowlimer reset inline void startTimer lowlimer start float readDust dustAnin2 mode PullUp measurementTimer reset lowlimer reset n DOON OU amp U N dustAnin2 fall amp startTimer dustAnin2 rise amp pauselimer measurementTimer start Start measurement timer mSeconds 0 while measurementTimer read lt 30 0 1 float tempSeconds float mSeconds 300 0 if tempSeconds lt 5 6 tempSeconds tempSeconds 0 8 1 6 0 2 0 2 else tempSeconds 5 6 0 8 1 6 0 2 0 2 tempSeconds 5 6 1 2 0 2 return tempSeconds FIGURE 34 Calculation of particles from particle sensor The measured milliseconds are changed into a low ratio percentage and calculated This is shown in lines 21 to 25 of Figure 34 There are two different calculations because the slope changes at the value of 5 6 In line 21 the amount 37 of seconds is changed into a low time percentage and the amount of particles are calculated in lines 23 and 25 The calculations are done from Figure 16 and they are presented in Figures 35
11. ait 2 while GPRS readable GPRS getc GPRS printf AT HTTPINIT r n wait 2 1 2 3 4 6 7 8 9 i jo O FIGURE 37 Waking up GRPS module The communication is done with the AT HTTPPARA and AT HTTPACTION commands The first command needs the URL of the server with the measurement data at the end of the URL The data is sent using a HTTP GET request so all parameters are included in the URL The GET request is used instead of a HTTP POST request because the GET request makes thecode shorter faster and readable There are two different cases where there is a need to send data When the device is powered it informs the server that the device is online and after every hour the device sends measurement data The AT HTTPPARA command fills the buffer and the AT HTTPACTION sends the created request to the web server In Figure 38 the device s ID is sent into the server so the server knows that the prototype is connected and will send measurement data every hour GPRS printf AT HTTPPARA URL Ffi erver E i d dV deviceld wait 1 3 while GPRS readable GPRS getc GPRS printf AT HTTPACTION 0 r n wait 8 while GPRS readable GPRS getc GPRS printf AT HTTPREAD r n wait 8 FIGURE 38 Sending data using HTTP GET reguest 39 5 3 Web application The prototype was only one part of the project The data needs to be presented for the user through a web applic
12. ard size into 100 x 80 mm One board was too small to hold all electronic components so design was changed from one PCB to two PCBs The first board has only a MBED microcontroller screw terminals and powerjacks The second board contains all measurement sensors except the CO2 sensor and the anemometer The anemometer was placed outside the casing with screws and its wires are connected into the sensor PCB The overall design can be seen in Figure 19 Sensors were not soldered straight into the board Headers are soldered into the PCB s and sensors are attached to them so the sensors and microcontroller can be removed or replaced easily The device gets its power from two power adapters which are connected to the electrical grid The CO sensor and the GSM GPRS shield already had existing PCB layouts so they are not built into the MBED or the sensor PCBs Two small PCBs were needed to enable connection between the CO sensor and the GSM GPRS shield with MBED PCB These two PCBs hold headers for the sensor pins the headers lead into screw terminals from where they are connected into the two main PCBs This way the CO2 sensor and GSM GPRS shield are repleacable and both are connected into MBED PCB with wires The anemometer is connected into the Sensor PCB s screw terminals 22 Anemometer FIGURE 19 Prototypes overall design 4 1 MBED PCB The MBED board is simple It holds a MBED microcontroller 24 screw terminals and two pow
13. aser trimmed thermoset polymer capacitive sensing element which provides resistance to most application hazards such as condensation dust dirt oil and common environmental chemicals The polymer helps to provide stability and reliability and it works well in indoor and outdoor environments Every sensor goes through a 12 hour rehydration process at 75 RH and a five hour rehydration process in conditions of 50 RH to correct the temperature offset This process provides long term stability and removes the need of recalibration of the sensor Honeywell International 2013 3 5 CQO sensor The CO sensor is called K 30 Figure 10 and it is manufactured by CO2Meter Inc The Sensor measures CO values with infrared light CO absorbs the infrared light and the sensor measures the amount of light which is passes through the gas The sensor is works with five volts and it communicates either through PC or serial communication The maximum measurement range is 10 000 ppm of CO it needs one minute to warm up it stabilizes itself after the first hour CO2Meter Inc 2014 SE 0018 FIGURE 10 CO sensor CO2Meter Inc 2014 The sensor includes a built in self correcting algorithm which tracks the lowest reading for the last seven days and corrects it comparing to an expected fresh air 14 value of zero or 400 ppm of CO2 The calibration is useful when the sensor is used outdoors The tuning speed is limited for 30 ppm per wee
14. ation The web server holds a small PHP script which receives data from the prototype and adds it into a MySOL database The web application reguests measurement data from the PHP script and presents it for the user The website is built from two sections The first section includes a data table where the user can see accurate measurement data from each hour This was done by using DataTables javascript library DataTables creates automatic table pages from the HTML table data DataTables add table pages sorting and searching which makes the table versatile The DataTables output can be seen in Figure 39 The user needs to retreive data from the website so they can reference to them when needed This is done by using TableTools which is a plugin for DataTables TableTools is an Adobe Flash element which is added above the search field in Figure 39 This button toolbar allows the user to copy selected data into the clipboard print or download data as a PDF XLSX or CVS file DataTables and TableTools are jOuery plugins ESTEA Prototype UU 2014 01 01 fey 2014 01 31 Load data Graph Settings Copy CSV Excel PDF Print FIGURE 39 Datatable in the website The second section of the website is a page which represents data as a graph This is done with Flot javascript library Flot is a jOuery plugin which creates 40 automatic graphs from the given data The graphs can be retrieved from the webpage as pictures Additional to
15. be seen in Figure 33 The Alphasense sensor functions are compressed to increase the speed of the program This solution makes the calculations smaller faster and more readable All the measurement functions are built as inline functions to increase speed readCO return coAnin read 3300 readCOFixed return coAnin read readNO2 return no2Anin read 3 readNO2Fixed return no2Anin read 3306 readNO return noAnin read 3300 0 696 1 readNOFixed return noAnin read aR A 711 8 0 7264 N read03 return o3Anin read 3300 512 0 0 7189 read03Fixed freturn o3Anin read 0 512 0 0 668 readWind return 3 3 windAnin read 0 4 1 2 3 4 6 7 8 9 FIGURE 33 Inline functions 5 2 4 Particle sensor function The particle sensor is also connected through an analog input and it measures half of the one minute measurement cycle The calculation of the voltage low time is 36 done with an interrupt class Two timer classes are needed from the MBED SDK one for measuring the total time and the second to measure the low time The interrupt class detects when a voltage is on a falling edge line 14 and it starts a function startTimer which counts the amount of milliseconds of the downtime When interrupt is on the rising edge line 15 class runs a pauseTimer function which adds counted milliseconds into a total measurement variable tempSeconds and resets the timer
16. ced FIGURE 23 Alphasense sensor design Alphasense Ltd 2012 a The biggest problem with the design was the placement of the humidity and temperature sensor The sensor is placed next to the particle sensor and when it is measuring it heats the air next to it and the heated air affects the humidity and temperature sensor values The Alphasense sensors measure air quality by burning reaction and they also affect the humidity and temperature sensor A simple fix for this is presented in Chapter 4 3 There is also a problem with the anemometer A 26 negative voltage breaks up the MBED microcontroller and it must be changed into apositive voltage with an operational amplifier 4 33 Casing and airflow A plastic casing was provided at the beginning of the project It needed to work only as testing environment before the final layout was confirmed The original idea was to assemble all sensors inside drill holes on two sides of the casing and let air flow naturally The sensors were assembled into the casing and a set of problems were noticed The particle sensor heats the air for half of the one minute measurement cycle and the Alphasense sensors use burning reaction to measure toxic gasses The air is not changing inside the casing When the casing s cover was closed the CO values rose slowly and the temperature and the humidity values were different from the values measured outside the casing The device needs airflow and thi
17. combined and returned into the main loop Reading CO data is shown in Figure 32 while readIndex lt 7 while co2Serial readable response readIndex co2Serial getc readIndex if readIndex lt 7 readIndex Q wait ms 50 x J 1 2 4 6 8 9 pat lt gt return int response 3 lt lt 8 int response 4 FIGURE 32 Read CO data from buffer 35 5 2 3 NO NO O3 CO and anemometer functions The NO NO Os CO and wind speed values are received as analog inputs The analog inputs are pins that read the amount of voltage coming into the MBED microcontroller The MBED can only read voltages from 0 0 to 3 3 volts and the value can be read from the pin with two different MBED SDK s built in functions The normal read function changes the voltage difference into a float value from zero to one A more accurate way is to use function read_u16 This function changes the voltage into an unsigned integer from zero to 65535 The program uses a basic read function because it makes calculations more readable and there is no need for accuracy that the read_u16 function provides The analog input works with one function call so all the sensor functions are small and simple The functions return voltage values which need to be multiplied by 3 3 volts and to be calculated by correct calculations found in Chapters 3 3 and calculations which Alphasense Ltd provided These calculations can
18. cuit boards are manufactured by the client The electronical sensors microcontroller and casing were chosen by the client before the project started The microcontroller was programmed with the C programming language The objectives were met the prototype was built and it works as expected Air measurement sensors were compatible although the measurement of the sensors affected measurement data from other sensors The problem was solved by relocating the sensors The prototype s measurement data was accurate enough although calibration was not accomplished The lifespan and replaceability of measurement sensors were taken into account Key words microcontroller prototype air quality weather technology Lahden ammattikorkeakoulu Tietotekniikan koulutusohjelma JUOMOJA MIIKKA Ilmanlaadunmittausj rjestelm n suunnittelu rakentaminen ja ohjelmointi Ohjelmistotekniikan opinn ytety 45 sivua Kev t 2014 TIIVISTELM T m n opinn ytety n tavoitteena oli suunnitella ja toteuttaa prototyyppi ilmanlaadunmittausj rjestelm st sek siihen liittyv st web toteutuksesta Ty ss keskityt n prototyyppiin erityisesti suunnitteluun ja toteutukseen Tavoitteena on kartoittaa k ytett vien ilmanlaatusensoreiden yhteensopivuusongelmat sek niiden tarkkuus T m opinn ytety on tehty Aarhusin yliopistolle ja on tarkoitettu ty kaluksi yliopiston tutkimusprojekteja varten Ilmanlaadunmittausj rjestelm mittaa ymp r iv
19. d referred 23 April 2014 Available at http www alphasense com index php air faqs Alphasense Ltd 2014c How Electrochemical Gas Sensors Work Alphasense Ltd referred 23 April 2014 Available at http www alphasense com WEB 1213 wp content uploads 2013 07 AAN_104 pdf Alphasense Ltd 2014d NO B4 Nitric Oxide Sensor Alphasense Ltd referred 23 April 2014 Available at http www alphasense com WEB1213 wp content uploads 2014 04 NOB4 pdf Brandt J 2012 THOR an Intergrated Air Pollution Forecasting and Scenario Management System Aarhus University referred 23 April 2014 Available at http www2 dmu dk 1 Viden 2 Miljoe tilstand 3 luft 4 spredningsmodeller 5 Thor default en asp Campbell J 1989 The RS 232 solution 2 edition Hoboken Sybex inc CO2Meter Inc 2014 Datasheet K 30 Sensor CO2Meter Inc referred 23 April 2014 Available at http co2meters com Documentation Datasheets DS30 01 20 20K30 pdf ElecFreaks 2014 EFCom Pro GPRS GSM Module ElecFreaks referred 23 April 2014 Available at http www elecfreaks com wiki index php title EFCom_Pro_GPRS GSM_Modu le 45 Schildt H 2003 C from the Ground Up 3 edition Berkeley McGraw Hill Osborne Honeywell International 2013 Honeywell HumidIcon Digital Humidity Temperature Sensors Honeywell International referred 23 April 2014 Available at http sensing honeywell com honeywell sensing humidicon hih6100 series product she
20. e total amount of time when the voltage is low in the 30 second measurement cycle The sensor s low voltage is from 0 7 to 1 0 volts and the width of the low voltage pulse changes from 10 to 90 milliseconds as shown in Figure 14 The measured time is translated into a low ratio percentage with equation shown in Figure 15 The low time percentage has to be converted into the amount of particles which is shown in Figure 16 Samyoung S amp C Co 2012 17 Low pulse width 10ms 90ms Hi 4 5V Low 0 7V sum time of Low t sec measuring time 30 sec FIGURE 14 Particle sensor s PWM signal Samyoung S amp C Co 2012 t s x 100 Low ratio 30 FIGURE 15 Low time percentage calculation Samyoung S amp C Co 2012 Low Ratio W o 0 2 0 4 0 6 0 8 1 2 2 1 4 1 6 Particle mg m3 FIGURE 16 Relation between the amount of particles and low ratio percentage Samyoung S amp C Co 2012 18 3 7 CO NO NO and O sensors The CO O3 NO and NO sensors are manufactured by Alphasense Ltd Figure 17 All four sensors are electrochemical and they have three electrodes in them which are connected to the three pins coming out from the porcelain casing These electrodes are shown in Figure 18 All three electrodes are in contact with each other through hydrophilic separators labeled as wetting filters in Fi
21. e code structure and solutions and finally reveals the test results and outcome 2 PROJECT GOAL The client of the project was Aarhus University Before the project began they had chosen and purchased all measurement sensors MBED microcontroller and prototype s casing They provided all necessary instruments server space and software The main goal was to create an air quality control device prototype a PHP server script and a webclient for the user The project was built over repository architecture In repository architecture devices communicate through a shared data server and in this project air quality control devices later referred as the prototype only send data into the web server The users read the measured data from the repository via the webclient Figure 1 shows all entities and their relations in this project The prototype was only one part of the project The measurement data must be stored into a unified data storage from where it can be rerieved The second part of the project was to develop a PHP server script which handles data storing and retrieving with MySOL database gueries The third part of the project was to create the webclient which generates tables and graphs from the sensor data to the user Air quality control devices e P Write data Data repository PE Shared data server MySQL database Users and Admins FIGURE 1 Project topography 2 1 Used technologi
22. e device was measured for a long period of time Secondary focus was on MBED s memory control and the amount of data sent to the server With this test the amount of data sent through the GSM GPRS shield was measured The last test took place in varied locations indoors and outdoors 6 1 Final testing results The last test was done in a controlled indoor environment where the environment was stabile After 14 hours of testing no memory leaks were found and all sensors measured air quality Nine hours of measurement data can be seen in Table 6 From the first five hours values humidity and the amount of particles were realistic Humidity was compared to thereadings from a household humidity meter 42 and the amount of particles was compared to graph provided by manufacturer Figure 16 Temperature was a bit higher than the real room temperature because of the airflow problem in the casing The anemometer was not attached to the device and therefore it gave random values from the analog input When the sensor is attached it gives 0 8 25 meters per second as it should measure The CO value was bigger than normal Normal CO value indoors is 1500ppm The CO values are high because inside the casing the air did not move in or out The Alphasense and particle sensors increase the amount of CO inside the casing In the end the value is near the normal values so with changes to the layout this should be fixed The Alphase
23. e program sends median values 29th value to the server The median values are used because occasionally there will be incorrect measurement values The measurement values can differ from the normal measurements and those error values should be filtered from the true data In the background a timer object is measuring how long it takes to run all the functions in microseconds The timer value is reduced from one minute and the outcome is the value which the program needs to wait until the next loop can start The sensor functions are never coherent and this way the sensor loop of the application runs exactly one minute The last loop runs for two minutes and in that time window the last data is retrieved data is sorted and sent into the web server The structure of C code is presented in Figure 25 30 Q Power on Create 13 Arrays Establishe connection to the web server JS hour loop Infinite Start minute loop Wait X Microseconds Start Timer Run all 13 Measurement functions 7 Timer No Aa 59th Minute Loop ee lt End minute loop Start Timer QSort 13 Arrays Send data into the web server Stop Timer End Hour Loop Yes Power on FIGURE 25 Structure of C code To add more speed to the program some functions are done by doing them inline Inline function is a small function whose code is expanded in line rather than called Every time a function is called a
24. egual to 0 4 volts MBED can read only the maximum of 3 3 volts so this sensor can measure up to 8 25 meters per second The sensor must be placed in an open space where the wind comes straight into the sensor The measurement value is lower if wind is obstructed from any angle The sensor does not reguire any calibration 11 FIGURE 7 Anemometer FIGURE 8 Graph for the relationship between the output voltage and wind level 3 4 Temperature and humidity sensor Temperature and humidity values are measured from one sensor Honeywell Humidlcon digital humidity temperature sensor HIH6130 Figure 9 The sensor is operating with 3 3 volts and in the sleep mode it only requires one 12 microampere of power It can be controlled either with SPI or PC buses The sensor works in the range of 5 to 50 degrees of Celsius and from 10 to 90 percent of relative humidity RH The true accuracy level of humidity and temperature can be seen in Table 1 The accuracy of the humidity changes at the maximum of 1 2 RH in five years The accuracy of the temperature changes at 0 05 degrees of Celsius per year in five years Honeywell International 2013 FIGURE 9 Humidity and temperature sensor SparkFun Electronics 2014 TABLE 1 Total error band of temperature humidity sensor Honeywell International 2013 90 past 8 fae a s s mis s s 2 3 43 3 3 2 Relative Humidity RH g 13 The sensor uses a l
25. erjacks The communications between PCBs are done with screw terminals and wires The powerjacks are connected with five and six volt adapters Five volt powerjack is connected with a screw terminal where the line continues into the GSM GPRS shield the CO sensor and the sensor PCB Six volt powerjack is connected with the MBED voltage input to provide power to the MBED The line continues into the sensor board through screw terminals The third voltage comes out from the MBED a 3 3 volt line is connected with the sensor PCB through screw terminals One side of the PCB is a ground plate covered with copper All ground pins are connected to this copper layer 23 The MBED is connected to the board through a 40 pin header so it can be replaced easily Most of the MBED pins are connected into a screw terminal from where they continue into corresponding sensors or into the sensor PCB All pins are not needed and they are not connected The main purpose was to add pins which would also be useful in the future All PC and serial ports analog inputs and a few digital outputs are connected In addition the pulse width modulation output pins are connected because they can control fans or work as digital outputs The PCB design can be seen in Figure 20 FIGURE 20 MBED PCB 4 2 Sensor PCB There are three different voltage inputs coming from the MBED PCB five volt for the particle sensor six volt for the Alphasense sensors and 3 3 volt
26. es The prototype s building process can be split into two parts The first part is the designing and manufacturing of the device The design of the device s printed circuit boards PCB was done with Cadsoft Eagle PCB design software Cadsoft Eagle is one of the most used PCB design software in the market The project was done with a freeware version of the software which limited the maximum size of the PCB board The second part of the project was programming The prototype was programmed with C programming language using Google s C styling guide The only library needed is MBED microcontroller s own software development kit MBED SDK MBED SDK is an open source C C library which gives the programmer low level and high level tools to control their own MBED microcontroller for example an inter intergrated circuit PC or serial connection controls which are also used in this project NXP Semiconductor 2012b 2 1 1 PC 1 2C is a databus originally manufactured by Philips Semiconductor nowadays known as NXP Semiconductor FC is designed to be a bidirectional 2 wire bus The communication is 8 bit oriented and it can make up to 100 kbit s in the standard mode up to 400 kbit s in the Fast mode up to 1 Mbit s in Fast mode plus or up to 3 4 Mbit s in the High speed mode The databus has high noise immunity wide operating temperature and supply voltage range and it has extremely low current consumption NXP Semiconductor
27. et 009059 6 en pdf name HIH6 130 021 001 Honeywell International 2011 PC Communication with the Honeywell HumidIcon Digital Humidity Temperature Sensor HIH 6130 6131 Series Honeywell International referred 23 April 2014 Available at http www phanderson com arduino I2CCommunications pdf NXP Semiconductors 2014a PC bus specification and user manual NXP Semiconductors referred 23 April 2014 Available at http www nxp com documents user manual UM10204 pdf NXP Semiconductors 2014b LPC1769 68 67 66 65 64 63 NXP Semiconductors referred 23 April 2014 Available at http www nxp com documents data sheet LPC1769 68 67 66 65 64 63 pdf Samyoung S amp C Co 2012 Dust Sensor Module P N DSM501 Samyoung S amp C Co referred 23 April 2014 Available at http www samyoungsnc com products 3 1 20S pecification 20DSM501 pdf Shanghai SIMCom Wireless Solutions 2009 SIM900 Hardware Design Shanghai SIMCom Wireless Solutions referred referred 23 April 2014 Available at http www simcom us act admin supportfile SIM900 HD VI1 01 091226 pdf SparkFun Electronics 2014 Humidity and Temperature Sensor HIH6130 Breakout SparkFun Electronics referred 23 April 2014 Available at https www sparkfun com products 11295
28. for the humidity and temperature sensor The backside of the sensor PCB works as a ground layer similar to the MBED PCB The sensor PCB holds all the measurement sensors except the CO sensor The board size limitations of Eagle forces the Alphasense sensors to be placed on the edge of the board All sensor pins are attached to screw terminals The design is seen in Figure 21 There are also two screw terminals reserved for the anemometer which is attached outside the casing 24 FIGURE 21 Sensor PCB design Figure 22 shows the electric design of the humidity and temperature sensor The SDA and SCL lines are powered from the same power source as the humidity and temperature sensor Two 3 3kQ pull up resistors are connected between the SDA and SCL lines and the power source The alarm output high AL_H and alarm output low AL_L pins are not connected because they are not used 3 3V Veci Hum Temp sensor GND FIGURE 22 Humidity and temperature sensor design 25 Figure 23 shows how the circuit boards of the Alphasense sensors are built All four sensors were pre assembled into circuit boards by the manufacturer The Alphasense PCBs have six pins where three are ground pins The voltage input six volts is common and the measurement signal measurement value comes from the ICI pin Vout1 The IC2 pin Vout2 is reserved for calibration and normally it will not be needed When the sensor is old enough it will be repla
29. gure 18 which allows the capillary transport of the electrolyte which is usually sulfuric acid Alphasense Ltd 2012 c FIGURE 17 NO sensor Alphasense Ltd 2012 d Gas diffusion barrier Working Electrode ee T Wettin Reference Electrode niers E G Counter Electrode Electrolyte Reservoir FIGURE 18 Schematic diagram of the sensors Alphasense Ltd 2012 c Gas comes into contact with a working electrode through a gas diffusion barrier In the working electrode electrochemical oxidation CO NO or reduction NO32 occurs The working electrode is directly exposed to all gases in the air 19 Therefore the electrode may absorb wrong gases from the air and become poisoned which breaks the sensor The sensor might be recovered if it is infused with other gases like sulfur to reverse the poisoning Alphasense Ltd 2012 c Alphasense Ltd 2012 describes the functioning of the counter electrode in this way The counter electrode balances the reaction of the working electrode if the working electrode oxidises the gas then the counter electrode must reduce some other molecule to generate an equivalent current in the opposite sense The counter electrode must keep up with the working electrode The amount of current is equal to the amount of reactions balanced by the counter electrode The most common reaction is the reduction of oxyden if the c
30. h draws outside air into the sensor while a light emiditing diode LED blinks A built in detector measures particles from the amount of light passing through the air The detector sends data about the amount of particles through an amplifier circuit into two output circuits as shown in Figure 13 Output circuit two is factory calibrated and always gives a pulse width modulation PWM output from all particles which are over one micrometer Output circuit one s PWM signal can be selected This pin can send the amount of particles which are bigger than 2 5 or 1 75 micrometer The measurement size is controlled with a resistor between control and ground pins Table 2 shows the relation between the resistor size and measured particle size Samyoung S amp C Co 2012 16 Dust or 3Vcc Smoke particle LED 5 GND K fare Output 7 1 Control 1 AMP circuit 1 4 Vout 4 Detector ts circuit Output l 2Vout2 Heater circuit 2 FIGURE 13 Particle detect process inside particle sensor Samyoung S amp C Co 2012 TABLE 2 Relation between resistor size and measured particle size Samyoung S amp C Co 2012 Resistor value a Description Ohms Open no resistor Preset sensitivity over 2 5 micrometer 100K Half sensitivity over 1 75 micrometer Equal sensitivity of Output circuit two over one 27K micrometer The measurement is done by listening to incoming PWM signal and counting th
31. k by the manufacturer The automatic calibration can be added by adding a jumper wire in Din1 400 ppm or Din2 0 ppm as shown in Figure 11 Indoors the calibration must be disabled CO2Meter Inc 2014 30 48 1 20 O 15 i AGND oe JN ground Jumper awitch Array bind Dins Dinz Din1 Bind IC AGND DGND and GO are connected together O 49 53 1 95 DGND Digital ground 2 54 0 10 1 91 0 075 2 54 0 10 4 0 107 i j X IDC terminal s A 375 U x Main terminal 9 53 0 375 L A kd 10 16 0 400 Hi FIGURE 11 Schematics of CO Sensor CO2Meter Inc 2014 3 6 Particle sensor The particle sensor module is called DSM 501 Figure 12 and it is manufactured by Samyoung S amp C Co Ltd The sensor is designed for rooms or casings smaller than 30m to measure particles bigger than one micrometer The sensor is a cost efficient solution to measure a guantity of floating particles which can cause 15 allergy or respiratory disease The sensor doesn t need calibration and it is accurate after one minute from the startup The sensor must undergo maintenance every six months lens must be wiped clean to prevent measurement errors Samyoung S amp C Co 2012 45mm 59mm FIGURE 12 Particle sensor Samyoung S amp C Co 2012 The particle sensor uses a resistor to heat air for 30 seconds The heat creates an updraft whic
32. knowledged by sending a zero bit and closing the PC line This whole process can be seen in Figure 29 The bytes are loaded directly into unsigned char values and acknowledgements are handled with MBED SDK s read functions parameters i2c start i2c write address lt lt 1 0x01 Hum H i2c read 1 Hum L i2c read 1 Temp H i2c read 1 Temp L i2c readf 0 i2c stop 1 2 4 6 7 FIGURE 29 Reading data using PC The received four bytes need to be edited that they will present the correct 14 bit data The two highest bits of the humidity s high byte are command bits and 33 those bits are not part of the calculation They need to be changed into zero before thehigh and low bytes are combined This is done by using a bitwise operator AND with the hexadecimal value 0x3F The last two bits of the temperature s low byte are zeroes and those are not part of the calculation either After combining temperatures high and low bytes the formed value is shifted to the right by two bits which corrects the value The data is calculated and returned out from the function by using pointer values given as formal parameters in the function call The data transfer and the assignment code can be seen in Figure 30 Calculation in lines seven and eight are compressed calculations from Figures 27 and 28 Hum H Hum H amp 0x3f H dat unsigned int Hum H lt lt 8 Hum L T dat unsigned int Temp H
33. lies in campus and nearby areas Since 1996 the National Environmental Research Institute NERI Denmark has developed a comprehensive and unique integrated air pollution model system THOR The model system includes several meteorological and air pollution models capable of operating for different applications and different scales The system is capable of accurate and high resolutions three days forecasting of weather and air pollution from regional scale over urban background scale and down to individual street canyons in cities Brandt 2012 The air quality may be influenced by a power plant located nearby and some concerns have been shown about the pollution which is comes from neighbor countries with the wind Professional accurate air quality sensors are expensive and the nearest professional sensors are located 32 kilometers from the university The easiest solution is to create a small and cheap device which gives fairly accurate measurement values This device will be a tool to determinate if the modeling system is working as it should Also it will be used to determinate University s indoor air quality The objective was to create a prototype of an air quality measurement device with the help of another exchange student The project included a server which handles the incoming data and a website which helps researchers in analyzing the data The device should measure air molecules particles humidity temperature and wind s
34. me sensors is short The Alphasense sensors needs to be changed frequently and the lens of the particle sensor must be cleaned once in six months This is not a problem if the sensor is located in a place which is easy to access The GSM GPRS shield was able to connect to the internet from every measurement site outdoors and indoors The program sent an average of 200 bytes of data every hour so one sensor sends 1 7 megabytes of data in a year So an ordinary prepaid mobile card could be used for several years In the future if the device is set up indoors the device should make a connection to the web server through a WLAN network The project continues in Aarhus University The device was only a prototype and the next step is to create a fully working device and to distribute it to nearby areas The next version will have a fixed layout of the sensors and a smaller casing The fans will be attached and the code will support proper fan control The sensors will be calibrated and the overall code will try to fix itself if a communication error occurs If the device is taken outdoors it must draw its power from a different power source for example from solar panels 44 SOURCES Alphasense Ltd 2014a Designing a potentiostatic circuit Alphasense Ltd referred 23 April 2014 Available at http www alphasense com WEB 1213 wp content uploads 2013 07 AAN_105 03 pdf Alphasense Ltd 2014b Frequently asked quations Alphasense Lt
35. mometer are located on the opposite side of PCB s other screw terminals The reason for this layout is the length of the anemometers s cord which is only ten centimeters long This is not a problem in the final device because the casing will be smaller The last thing which needs to be considered is the attaching of the board The boards can be located in places where normal people cannot reach and the persons attaching the boards might not be familiar with the technology There has to be one or two LEDs attached into the casing The LEDs inform startup errors to the attacher If the errors are not solved automatically by the program then a spare device must be attached and the malfunctioning device must be returned to the maintenance department Mostly these errors will occur with the GSM GPRS shield and they usually consist of a bad connection or a simcard malfunctioning 29 5 PROGRAMMING 5 1 Structure of the code In the beginning of the program the device generates arrays for every measurement value and three test calibration values for CO NO and NO gt There are a total of 13 arrays Once a minute the program runs all the measurement functions and saves the returned measurement data into a corresponding array The main loop runs once an hour One loop runs through all sensor functions 59 times and sorts all arrays from the lowest value to the highest using the Qsort function of the C standard library After the sorting th
36. nse sensor values were incorrect The calculations provided by the manufacturer gave false values which were lethal for living persons The sensors were decided to leave untouched until the first realease version of the device was completed TABLE 6 Nine hours of preasure test results Hum Tem wind dust CO CO COf NO NOf NOs NOf O Osf C m s mg m3 ppm ppm ppm ppm ppm ppm ppm ppm O 51 08 25 7 1 083 0 74 0 745 1842 1780 1606 53 97 51 96 50 09 51 12 1719 50 48 50 92 25 7 25 5 0 72 0 725 1748 N 43 7 CONCLUSION The objective was to create a prototype of an air quality measurement device The project included a server which handles incoming data and a website which helps researchers in data analyzation The device should measure air molecules particles humidity temperature and wind speed and send the data into the webserver every hour The main focus was to study and test compatibility of the sensors and to study and test the measurement accuracy and lifespan The sensors were different and their measurement methods were different Some sensors needed airflow but the particle sensor gave faulty readings if there was airflow The burning reaction of the Alphasense sensors and the heating of the particle sensor affect the measurement values of the humidity and temperature and the CO sensors These problems can be solved with the design explained in Chapter 4 3 The lifespan of so
37. ols are added so the user does not need to create graphs by hand The additional tools are overview and zoom which allows the user to retrieve only small parts of the graph Figure 40 shows the web solution and zooming FIGURE 40 Website s automatic graphs 41 6 TESTING Testing was done several times over the production time of the device All the values except temperature and humidity were compared with default air pollution values from the European Environment Agency s website There were not any existing sensors to test values except humidity and temperature so both values were tested with normal electronic household sensors At the beginning all sensors were assembled into a breadboard with MBED Sensors were programmed and they were tested for the first time Measurement values were compared to corresponding normal air pollution values The next test was conducted after all sensors were assembled into the PCB s The purpose of this test was to try getting sensors to work all together and to give reasonable air pollution values The last test was run after both PCB s were assembled into the casing This test was the most important one because it showed what went wrong in the design and if there was something affecting the test results The main goal was to get the code as ready as possible Calibrations of the the three sensors were only a test Other calibrations were not done because the device was only a prototype Th
38. ounter electrode cannot get enough of oxygen then the whole sensor stops sending valid data until there is enough oxygen again Alphasense Ltd 2012 c The reference electrode ensures that the working electrode is always in the correct region of the current voltage curve This affects the constant sensitivity linearity and the minimum sensitivity of the sensors to interfering gases There is also a stabilation time until sensors start to send valid data These times are shown in Table 3 Because of the way that the sensors work their lifespan is short They need to be replaced after the original signal drops under 50 percent The measurement accuracy also deteriorates over time Deteriorating and lifespan information is shown in Table 4 Alphasense Ltd 2012 c TABLE 3 Stabilation time of Alphasense sensors Alphasense Ltd 2012 b New sensor or after long After brief removal e g Sensor period of removal for replacement Hours Minutes CO 2 10 NO 12 12 Hours NO 2 10 O 2 10 20 TABLE 4 Deteriorating and lifespan of the Alphasense sensors Alphasense Ltd 2012 b ppb change in year months until 50 of the Sensor lab environments original signal CO 100 36 NO 0 50 24 NO 0 20 18 O3 0 50 18 21 4 HARDWARE DESIGN AND MANUFACTURING The prototype was designed with a freeware version of Eagle The freeware version limits the useable bo
39. peed Every hour data is sent into the server from where the researchers can receive the data as files or in linegraphic format and examine it The main focus is to study and test the compability measurement accuracy and lifespan of the sensors This thesis addresses with only the design and production of the prototype The first research problem was to create a prototype device from prechosen sensors microcontroller and casing The main task was to create two printed circuit boards PCB to control all the sensors and the mobile networking microchip The second research problem was collecting the data from the devices which can be located anywhere in the campus area or nearby locations like Herning city The device sends data once an hour and there has to be a server which manages the data The researchers can collect data into a website from where it can be retrieved The third research problem was the airflow and sensor compability How the airflow affects the sensors and whether the other sensors affect the measurements In the first section of this thesis you are presented with the sensors used in this project All the theoretical background of sensors is explained The manufacturers have provided information in datasheets and in accurate explanations the functionality of the sensors The manufacturer s datasheets are the main source of information for this thesis The thesis continues into circuit board design and explaining devic
40. s problem can be solved by creating an underpreasure with fans The airflow affects the measurement of the Alphasense and the particle sensors and it must be considered in the design The solution for the casing problem is to split the device into two sections The new casing layout is presented in Figure 24 27 Airflow Airflow FIGURE 24 New casing layout The first part I has all the sensors except the particle sensor coming out from the holes of a wall which splits the casing into two parts The sensors measure air which is just barely changing by the help of two fans The Alphasense sensors must be assembled to the end of the airflow because they will affect the measurement values of other sensors The second part II has both PCB boards the GSM GPRS shield and the particle sensor Both of the fans stop blowing air when the particle sensor is measuring When the measurement is completed the fans change air slowly from the casing The antenna of the GSM GPRS sensor should come out from the casing There also should be a hole for the two buttons which control the GSM GPRS shield The anemometer needs to be placed outside the casing because it is too big to be assembled inside The sensor can be attached easily to the cover with a magnet or 28 screws The cord of the sensor is attached to sensor PCB s screw terminal and it needs a small hole on the side of the casing Figure 21 shows that the screw terminals of ane
41. series of instructions must be executed both to set up the function call including pushing arguments onto the stack and to return from the function In some cases many CPU cycles are used to perform these actions However when a function is expanded in line no such overhead exists and the overall speed of your program will increase Schildt 2003 264 31 5 2 Sensor functions 5 2 1 Humidity and temperature functions The temperature and humidity sensor is connected through an PC bus The device returns four bytes of data which contains both humidity and temperature The process can be seen in Figure 26 After receiving both values they must be converted into float values and calculated through eguations provided by the manufacturer These calculations are presented in Figure 27 for humidity and Figure 28 for temperature Honeywell International 2011 Data Byte 1 Data Byte 2 Data Byte 3 Data Byte 4 r r EPS Bop eof eee e eE L 10 G 1 EC I ru r Slave Address 6 0 Read Status Humidity Data 13 6 Humidity Data 7 0 Temperature Data 13 6 Temperature Data 5 0 Bits generated by Master o Bits generated by Slave sensor do not care FIGURE 26 Humidity and temperature data communication through PC Honeywell International 2011 nue HumidityOutputCount Humidity RH 0 1 x 100 FIGURE 27 Equation for the percentage of relative humidity Honeywell International 2011 TemperatureO
42. th tick is an acknowledge ACK bit or a no acknowledge NACK bit After the byte is received and more data needs to be sent ACK must be returned ACK is created by the receiving device which sets SDA line to LOW When the last byte is received the receiving device needs to send back the NACK bit The receiving device sets SDA line to HIGH and after that the connection can be stopped or started again by the master In stop condition the master changes the SCL line to HIGH if no new start condition is given then SDA line will also be changed to HIGH The signal timing can be seen in Figure 3 NXP Semiconductor 201 2a Pt 9 s IOS a TE Master drives the line HIGH on Sth clock lr Slave never drives the USDA line USCL Noo 1 2 8 9 1 2 8 9 I srorp k 3107 NACK NACK z START or STOP or repeated START byte complete repeated START condition interrupt within slave condition 002239657 FIGURE 3 FC timing chart NXP Semiconductor 2012a 2 1 2 RS 232 and UART RS 232 is a full duplex serial communication standard which allows communication in both directions The maximum line length should be kept relatively short because a longer cable would have alternating voltages on each side of the ends which would increase noise margin of the signal Noise mostly affects the data signal and might corrupt the transferred data Campbell 1989 50 The simplest RS 232 needs only two data transmission lines the transmit exchange data
43. utputCount Temperature C an x 165 40 FIGURE 28 Equation for temperature Honeywell International 2011 32 The humidity and temperature measurement values are read from the same sensor The master device MBED needs to wake up its slave device humidity and temperature sensor so the device understands that the MBED is listening to the PC line and the humidity and temperature sensor can start to send measurement values This waking up process is done by starting the PC line and sending one byte to the line The byte s first seven bits contain the slave s address 0x27 and the last bit will be a write bit 0 which tells the slave to be on writing mode Lastly the PC line needs to be closed When the sensor is woken the program requests humidity and temperature readings This is done by opening the PC line and sending the address byte again This time the first seven bits are the address of the slave but the last bit needs to be changed into the read bit 1 This is done by changing the address with a bitwise OR operator After the request is sent the MBED starts to listen There are a total of four bytes to receive The first two bytes are the humidity value s high and low bytes which must be acknowledged with an acknowledge bit 1 The third byte sent by the slave is the temperature value s high byte which also needs to be acknowledged The last byte is the temperature value s low byte which must be no ac
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