As stated before the system master is an Arduino Mega [8]. This Mega uses synchronous and asynchronous serial communication to connect to the load cell amplifier, RFID reader, SD read/write board, and GSM/GPS breakout board. Additionally the system communicates with the operator through a series of LEDs and for trouble shooting purposes USB serial communication. Additionally, as the system is intended to run without the aid of a computer, a 9V battery supplies power to the Arduino, this supplies power to all of the auxiliary devices except the GPS/GSM module.
Constructed device inside enclosure
The RFID board chosen for the system is an Adafruit PN532 RFID/NFC board [11]. This board was chosen because it easily interfaces with Arduino. With minimal wiring, and a simple library for operation, this board was a natural choice. The RFID board connects to the system through Inter-Integrated Circuit (I2C) communication. Additionally the Mega prompts a card read on the board through an interrupt request (IRQ) which requires an additional digital pin to implement. The RFID board draws its power directly from the Arduino Mega board through the 5 volt power supply and ground. The board communicates with embedded RFID tags through inductance signals at a frequency of 13.56MHz.
RFID reader/write board wiring diagram
The load cell itself is constructed out of a metallic disk with a Wheatstone bridge of strain gauge resistors mounted to the disk. As the metallic disk experiences forces, it deforms ever so slightly stretching or compressing the strain gauges that are attached to the metallic disk, skewing their resistances. This skewing creates a differential voltage proportional to the amount of weight placed on the load cell. The load cell used in the final implementation of the system is a Sparkfun 50 Kg disk load cell [14]. This load cell was chosen because the average house hold will produce around 35 Kg with an additional 11 Kg for a standard 35 gallon waste bin. This load cell would need to be modified depending on the waste company, their trash truck receptacle are style, and the customers that they cater to.
The load cell amplifier and analog to digital converter used in this project was a HX711 model [15]. This is a very standard model of A/D converter used for load cells. The HX711 uses a 24 bit converter to turn the differential voltage from the load cell into a useable value, and then communicates with the Arduino Mega through synchronous digital communication. Since the HX711 does not use SPI or I2C the implementation of this board only requires the use of two non-specific digital I/O pins. The board comes with a preassembled master header file, so there is little more required to set the board up than to calibrate and tare the initial weight value.
Load cell & A/D amplifier wiring diagram
One of the simpler systems to implement in the Household Waste Tracking System was the SD read/write card [16]. As Arduino already has a built in SD library, the only real challenge was finding the correct pins to use on the board. The SD card communication uses a Serial Peripheral Interface (SPI) Bus interface so the pins for master-in-slave-out, master-out-slave-in, and system-clock were already predetermined. The only pin left to assign was the chip select, which was assigned to be near the other SPI pins. During construction it was found that the Micro SD board required pull up and pull up resistors (not shown in the figure below) on the communication lines, clock line, and the chip select line.
Figure 10 – Micro SD card board wiring diagram (pull-up and pull-down resistors)
One of the largest pieces of the system implementation was the GSM/GPS board [17]. Initially, the system was to be constructed using a Sparkfun MG2639 GSM/GPS board. Ultimately, this board proved to be under documented and ultimately unusable. So, the system was implemented using an Adafruit FONA 808 GSM/GPS board. This system communicates with the Arduino, using the hardware serial UART interface. With the very complete header files and library provided by Adafruit, implementing this system was infinitely easier than the Sparkfun MG2639. Using a prepaid T-mobile cellular plan and Simcard, the system was able to gather GPRS location information and send information using the GSM network and the Http post commands that were documented in the header file. Additionally this board easily gathered location information from GPS satellites. Since the board draws so much energy during use, the system required an additional 2000 mAh, 3.7 to 7.4 V lithium ion battery [18]. In addition, the board did not have internal antennas, so passive antennas were purchased and attached for both the GPS and the GSM connectivity.
Adafruit Fona 808, GSM board Battery, GPS & Cellular Antennas wiring diagram
In addition to the main devices of the system, there is one method of user interaction that is a part of the hardware. As the system progresses through a routine waste reception, there is a series of seven LEDs that notify the operator of the status of the system. Not shown in the figures below are the resistors required to lower the current flowing through the resistors.
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| LED wiring Diagram (LEDs not shown) | LED designation diagram |
