How does it work?
The circuit is the main part of the sensor system. In the circuit, the sensors each collect an air sample and provide electrical information based on the amount of pollutants present. The Arduino converts this electrical information to numerical data, and also powers the circuit. More details about the circuit are explained below..
The code tells the Ardino how to interpret the electrical information from the sensors and convert it into numerical data. The code also teaches the Arduino how to interact with the App. (The code has already been prepared, it just needs to be copied into the Arduino software).
The app allows the sensor to be controlled from a smartphone. The sensor collects data in response to a command from the app, and then displays the data on the app screen. Data from the app can be shared in an email, and then graphed.
A few basics of electronics
In an electrical circuit, electrons flow from a power source, around a path, and then end up at a ground.
The current is the rate of electrical flow in the circuit, and the voltage is the force of the electrical flow.
Wires conduct the electrical flow through the circuit. Resistors decrease the voltage of the electrical flow through the circuit,
causing the electrical flow to have the appropriate voltage at each place in the circuit.
How does the circuit work?
1. The electrical flow starts from the “power source” in the Arduino.
2. The flow is then carried from a wire to the sensors
3. The sensors interact directly with the pollutants and translate the level of physical pollutants to electrical information.
4. A wire carries the electrical information from the sensors back to the ground in the Arduino.
5. The Arduino reads the electrical information, and translates it to numerical data
6. The Bluetooth transfers the numerical data from the Arduino to a cell phone.
How do the sensors work?
Particulate matter - The Shinyei PPD42
The Shinyei PPD42 is considered an “optical sensor” because its measurements are based on how much light is scattered. The Shinyei PPD42 draws air into the chamber by utilizing a convection current created by a resistor. Once a sample of air is inside the chamber, the particles in the air scatter the light from an internal LED. (To elaborate, the LED creates a beam of light that is pointed in a particular direction, and particulate matter in the air then “scatter” (deflect) this beam in another direction.) The amount of light that is scattered is detected by an optical receiver. The optical receiver creates an electric “pulse signal” each time a certain amount of scattered light is detected. The amount of pulse signals created is proportional to the amount of particulate matter in the air.
Nitrogen oxides - The Sensortech MiCS-2710
The MiCS-2710 is considered a "metal oxide sensor" because it uses surface-based redox chemistry to determine the concentration of NOx. Specifically, NOx molecules bind to the sensor and either remove or add free electrons to the sensor. The sensor then measures this change in conductivity (caused by added or removed electrons) and converts it to meaningful data.
*Note: The MiCS-2710 sensor is no longer being produced. We are currently evaluating an alternative NOx sensor. Instructions on the following pages can
be used to build a device that includes both the NOx and PM sensors, or only the PM sensor.