IoT Design
SMART air pollution monitoring System combines the IoT technology with pollution monitoring in real-time. It provides its users a real-time access of weather and pollution data from various locations spatially over the sensor network. Weather information such as temperature, humidity and pollutant concentration like Carbon Monoxide, Ozone, Carbon Dioxide, Sulphur Dioxide and fine particulates (PM2.5) and Ntrous oxides are gathered simultaneously from this system. All the data is collected and sent to the Server using the GPRS network using M2M Sim and GSM Module. In this system we are using a stable and non-combustible Lithium Iron Phosphate (LiFePO4) battery which is connected with a solar panel for daily recharging. The processing is performed by Atmega 328 which has a modified Harvard architecture 8-bit RISC processor core. It provides extensive compatibility of Sensors with built-in GPRS mobile network connectivity. The sensor used in the system are MQ-135, MQ-131, SO2 Sensor, MQ-7, DHT-11, PM2.5 Sensor.
There are different types of Gas sensors but the Mĭngăn Qǐ lai(MQ) series are Metal Oxide Semiconductor Type Gas Sensors which are widely used in air quality monitoring systems. According to Singh and Bonne (2017) “Gas sensors are Devices that can detect the presence and concentration of various hazardous gases”. The change in gas concentration creates a corresponding potential difference due to changing resistance of the sensor material. This change in Potential difference is measured as the output voltage. Based on this potential difference, the concentration and type of gas can be estimated. The sensivity of the sensor to a particular gas depends on the sensing material present inside the sensor. These sensors can only operate within the operating limit of gas concentration mentioned in the datasheets. When the concentration of the gas goes beyond the fixed value the digital pin provides saturated voltage. Analog pin can be used to measure concentration of the gas.
Gas sensors are of various types depending on the sensing element it is built with. Various types of gas sensors that are used in various applications are as follows:
- Metal Oxide based gas Sensor- Metal Oxide Semiconductor (MOS) sensors detect concentration of a gas by measuring the change in resistance of the metal oxide due to adsorption of gases.
- Optical gas Sensor - Optical Phenonmenon such as Scattering, Absorption, reflection of light, Refraction and fluorescence etc. are utilized for detecting a gas by the optical gas sensors.
- Electrochemical gas Sensor - Electrochemical sensors work by reacting with the pollutant gas which thereby creates an electrical signal which is directly proportional to the gas concentration.
- Capacitance-based gas Sensor- It works based on changes in relative permittivity with change in gas cencentration.
Metal oxide based gas sensors commonly know as MQ series are used to develop IoT devices in this system as its longevity is higher and cost is less.
Gas Sensor Working Principle
The Metal oxide Gas sensor detects gases based on the change in resistance of the sensing element. An example of most commonly used sensing element is Tin Dioxide (SnO2); which is an n-type semiconductor having free electrons. Generally, the atmosphere contains more oxygen than pollutant gases. The oxygen present in the atmosphere attracts the free electrons present in Tin Oxide (SnO2) and attaches them to its surface. At this juncture, the output current is zero as there are no free electrons to carry the current. The oxygen molecules (blue color) are attached to the free electrons (black colour) which are present in Tin Oxide (SnO2) preventing it from having free electrons to conduct current (Fig.1).
When the sensor is placed in polluted environment, the polluted gas which is reducing in nature (Fig. 1) reacts with the attached oxygen particles which breaks the bond between oxygen molecule and free electrons whih consequently releases the free electrons. These free electrons can now conduct current. The magnitude of current is proportional to the number of free electrons available in the metal oxide. If the gas is present in higher concentrations, more free electrons will be available.
Device Execution Scheduler
The Pollution monitoring device is developed for sensing Polluted gas concentration of Carbon Monoxide, Ozone, Carbon Dioxide, Sulphur Dioxide and PM2.5 which is gathered simultaneously with weather information like temperature, humidity from this system. All this data is collected and sent to the Server using the GPRS network using M2M Sim and GSM Module. In this system, we are using a stable and non-combustible (LiFePO4) battery which is connected with a solar panel for daily recharging. The processing is per-formed by Atmega 328P Microcontroller which has a modified Harvard archi-tecture 8-bit RISC processor core. It provides extensive compatibility of Sensors with built-in GPRS mobile network connectivity.
The system is powered using a Rechargeable battery which works along with Solar panels. The system is generally in Sleep mode and every 20 minutes, device gets activated and waits for 5 min to heat the Sensors. After reaching the operating temperature, it starts taking a reading from the sensors. The data acquisition and timing from the sensor is as follows DHT 11 - 5 to 10 Seconds, SO2 Sensor – 30 Seconds, MQ 131 – 10 Seconds, MQ 135 – 10 Seconds, PM2.5 - 10 Seconds and MQ7- 2.5 Minutes (Table 1). Generally, the data acquisition time from the sensors is 5 to 7 minutes as every sensor has its own delay to measure and send the data. The data received from the sensor is the average average of 10 to 100 readings that are obtained in consecutive readings. Only in case of DHT sensor, directly the value of temperature and humidity is obtained whereas in other sensors, it gets converted into the desired unit. After receiving the data from all the sensors, the GSM module gets initialized. In this device, a GSM Module is connected along with an M2M SIM card for providing internet connection. GPRS is also responsible for sending data to the server (Fig. 2).
Table 1. Responses, Operating Ranges and resolution of various components
Air Temperature
|
Operating Range
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0°C to 50°C
|
Resolution
|
1°C
|
Relative Humidity
|
Operating Range
|
20% to 90%RH
|
Resolution
|
1%
|
Sulfur Dioxide
|
Response Time
|
<30 sec
|
Range
|
0 to 20 ppm
|
Resolution
|
0.15 ppm
|
Carbon Monoxide
|
Output
|
2.5V-4.3V in 150ppm
|
Range
|
10-500ppm
|
Particulate matter (PM2.5)
|
Detection Range
|
10 - 500 ppm
|
Response Time
|
5 Sec
|
Carbon Dioxide
|
Speed
|
0 to 20m/S
|
Range
|
10ppm-1000ppm
|
Ozone Sensor
|
Range
|
10-1000ppb
|
Output
|
≥1.0Vin 200ppb
|
Power Supply
|
Battery
|
Battery: 7.3V/12AH
|
Solar Panel
|
12v 20 watt
|
Software Development Stage
The IoT device is made by integrating various gas sensors like MQ7, MQ135, MQ9, SO2 sensor, DHT11 etc. along with other components with the Atmega 328P Microcontroller. Figure 3 shows the Circuit diagram of the developed assembly.
The sequence of operations of the device are specified for the IoT device as follows:
- The system is powered using a Rechargeable battery along with Solar Panels. The system by default remains in power saving mode. After 20 minutes of hibernation, it gets activated. All the components in the assembly are initialized and kept running for 5 minutes which heats the sensors to the operational temperature. Consequently, the device is ready to take readings from the sensors.
- Generally, the data acquisition time from the sensors is programmed to be 5 to 7 minutes, since every sensor has its own delay to send data. Thus, if the data is not received from the sensor, the device retries to read the data from the sensors. The data received from a sensor is the average of the 10 to 100 readings. In the case of DHT sensor it will directly read the value of temperature and humidity but in other sensors, there is a conversion factor programmed in the instrument with reference to the Central Pollution Contol Board (CPCB) Instruments.
- After receiving the data from all the sensors, the GSM module gets initialized. A GSM Module with an M2M SIM is used for providing GPRS connection in the device. GPRS is also responsible for sending data to the server. GPRS, which stands for General Packet Radio Services is a packet-based wireless communication service which provides internet connection at 56-114kbps of data rate.
- The GSM finds the network and connects to its operator after the connection has been built. It then tries to connect with a server, and wait for connection to get established between the device and the server. After that it starts sending the data to the server.
- If the connection has not been built it again tries to communicate and get a response from the server. it will repeat the process six times, if the server is still not connection then it will store the information and proceed.
- It also saves the data in its flash memory and in the next cycle it repeats the same process. This process takes places three times in an hour and after that, if the server id is still not responding or getting connected, it will erase the last hour data and will save new data.
- This cycle continuously runs and the system sends the data. Meanwhile, in the day time, the system is charged by Solar Energy and in the night the stored power is used
Table 2 shows the development environment of the IoT. The entire device execution program is developed over ATMEGA 328P Microcontroller using Embedded C++.
Table 2 Development environment
Microcontroller
|
Atmega 328P
|
Processor
|
8-bit RISC processor core
|
Programming Language
|
Embedded C++
|
GSM Module
GSM/GPRS module is used to establish internet connection to the developed device. Global System for Mobile communication (GSM) is an architechture which is globally used for mobile communication. An extension of this GSM is the Global Packet Radio Service (GPRS) which provides higher data transmission rate. GSM/GPRS module consists of a GSM/GPRS modem connected along with a power supply circuit and communication interfaces (like RS-232, USB, etc) of the microcontroller. In this device ATMEGA 328P has a direct connection port 0(RX), 1(TX) i.e., Receiver and Transmitter Pins for the GPRS module (Fig. 4). GSM/GPRS has a wireless MODEM which is designed for communication of a computer/microcontroller with the GSM/GPRS network. This module can work along with a SIM (Subscriber Identity Module) card (in this device an M2M SIM) just like mobile phones to activate communication with the network. It also has an IMEI (International Mobile Equipment Identity) number which can be used for identification. With the help of jumper wires the GSM Module is tested for its conceptual data sensing to server workflow.
Instrumentation and Packaging
The bread board testing is followed by integration, soldering and proper encasing with mesh structures to develop the IoT Device for measuring pollution. Figure 5 shows the final equipement assembling various components.