The hardware boards of PCB follows the rules that are the sum of the combined knowledge referred by the CAD system. The users through the analysis of accuracy achieve the experience in the system. The area is minimized by considering three boards without using a single board for the overall system. A condition-based program is installed in the microcontroller circuit. The program is recorded in the microcontroller unit of the monitoring device to control the operation. The controller takes some decisions to analyze data. This analysis relies on information received from user’s measured pollution levels and the information, obtained from meteorological parameters that affect weather conditions. The PCB device of AQMS is focused by calculating net weight of 172gm having three boards stacked together. Lightweight of the device makes it more compatible for handling and it is easily portable. The details of board layout design are presented in the following sections.
5.1 Layout of Top Board
In order to achieve a double-sided PCB, following steps have been executed in the design. For satisfactory relationship among three boards, header pins are placed according to the process of the design. All sensors and display device is assembled at the top level of the board. The bottom layer is planned for connection with the next board. Traces and ground planes have been drawn on both sides of the board. Figure 2 illustrates the first board of PCB in 3D stacked design of AQMS. Auto-routing is done as discussed in Section 4 and the design is further miniaturized. Two metal layers are used in routing to avoid short connections in design.
The width of the routing net depends on the flow of current through the copper wire to supply voltage. The width of the supply voltage is set for 5V, 3.3V, and ground net and internal net as per the designer’s expectation. In case of 5V supply, traces are placed with spacing of 1.5 mm. Similarly, for 3.3V spaces are given as 1.3 mm and for ground spacing of 1 mm is placed. A 10K potentiometer is placed to control the brightness of LCD. The number of ViA indicated by round shape, as shown in the Fig. 2, is added for joining the two metal layers and to dissipate heat. Finally, the ground plane has been drawn in a rectangular shape and DRC is performed to remove all the defects. To make the ground plane in Eagle, the first task is to run the polygon command. The ground plane is built for two layers. The ground plane is mandatory covering majority of the board to prevent the crosstalk between adjacent circuit traces. The area of the board is maintained 100 mm × 80 mm.
5.2 Layout of Middle Board
Figure 3 shows the layout of the middle board. This board is also double-sided. All the components are attached on the front side.
ATmega328P 8-bit AVR microcontroller is dedicated in this design of embedded system to execute applications for air quality monitoring. Regulating circuits are used of SMD components LM1117MP-5.0 and LM1117MP-3.3 maintaining supply in the design. The design consists of multiple sensors, IC chips and plated through hole (PTH) materials. 16MHz frequency of crystal oscillator is mounted to provide a clock speed. A reset switch is introduced in the board, it clears all previous errors and initialize a new one. All the resistors, capacitors and other components are integrated as per design rules to make the device optimized. After auto-routing, the region is optimized to 89 mm × 75 mm. The width of the trace associated to the USB port is set by 1.7 mm because of the large current passes through it. The surface current that generates on metal surface is a big intervention that appears in PCBs. It is therefore important to suppress interfered noise, so that it is not propagated to rest of the circuit. The width of other traces is set as previous design. Finally, the ground plane and DRC steps are followed.
5.3 Layout of Bottom Board
Figure 4 presents the PCB layout of bottom board. Here, the size of the board is compressed to 110 mm × 80mm. In the design a USB port has been attached and power is given from the battery source through it. This power flows throughout the design of AQMS. Hence, two ESD suppressors are mounted in this PCB to protect the internal circuitry and are suited for high speed applications. ESD suppressor helps the device being safe from overvoltage of continuous supply.
Strategy of switching technique for low power AQMS is mapped on the bottom board. A demultiplexer IC is used in the bottom board to direct the ON / OFF condition of the power supply of the sensors to reduce the power consumption of the device. Sensor data from the AQMS device is transmitted wirelessly. The zigbee wireless module completes the process via zigbee shield in the PCB to establish the communication system. Successive steps are taken for completing the layout design. The complete flow diagram of PCB design methodology is given in Fig. 5 below:
Fabrication and assembling process of three boards (each board has two layers) has been done, which comprises of six layers with double-sided interconnections. The dimension of the overall board is 110 mm × 80 mm × 24 mm. The volume of the device indicates the compact size of the design. Complete device of AQMS is shown in Fig. 6. The front view and side view of air quality monitoring system is shown in Fig. 6a and Fig. 6b. The side view of PCB device gives a clear idea of 3D structure in stacking form. In the PCB design, surface mount device (SMD) components are assembled for high accuracy, area optimization and easy handling purposes.
The environmental pollutants are measured by PCB based AQMS. To see the correctness of the results, the results of the breadboard design of AQMS are compared with the results of the PCB design. In the design, off the shelf components are placed according to the schematic (Fig. 1) on the Vero board. The breadboard design of AQMS is shown in Fig. 7. It can be seen that the information of the pollutants received from fabricated PCB based AQMS match the real-time data of the pollutants observed in the breadboard design of AQMS. The accuracy is measured as 95±5% and the system has the availability of local storage and display purposes.