DIY Air Quality Monitor: Cost‑Effective EPA‑Compliant Detector

This compact detector offers a budget‑friendly solution for assessing ambient air quality. It targets the five key pollutants outlined by the EPA’s Air Quality Index—ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrous oxide—providing accurate readings for all except sulfur dioxide, which remains unmonitored by the current sensor suite.

In addition, a town gas sensor alerts users to flammable leaks, while a temperature‑and‑humidity module accounts for environmental factors that influence sensor performance.

Although the device has not yet undergone full calibration, we have extracted preliminary data from manufacturer datasheets to estimate pollutant concentrations. The low‑cost sensors vary significantly in precision, so calibration against known gas concentrations is essential for reliable results.

Step 1: Materials

Control and Power

• Arduino Uno
• 5 V power supply
• RGB 16×2 LCD shield

Sensors

• Shinyei PPD42 Particulate Matter Detector
• MQ‑2 Gas Sensor
• MQ‑9 Gas Sensor
• MiCS‑2714 Gas Sensor (NO₂)
• MiCS‑2614 Gas Sensor (O₃)
• Keyes DHT11 Temperature and Humidity Sensor

Box and Assembly

• Access to a 3D printer
• Solder board
• 5 V fan
• 10–15 × 24‑gauge wires

Step 2: Overall Circuit Diagram

The diagram above shows the complete circuitry required for the pollution detector. Most connections are flexible—if you need to re‑assign a digital or analog pin, simply adjust the accompanying code. A detailed schematic for the solder board follows later.

Step 3: Particulate Matter Sensor

We employ two Shinyei PPD42 dust sensors to capture fine‑ and coarse‑particle data. Each sensor outputs two digital signals: one for particles <2.5 µm and another for larger particles. The sensors receive 5 V power and connect to the Arduino’s digital inputs. The photodetector’s output is converted into a low‑pulse occupancy percentage that correlates directly with particulate concentration.

A comprehensive reverse‑engineering analysis by Tracy Allen can be found at https://takingspace.org/wp-content/uploads/ShinyeiP.pdf.

Step 4: Gas Sensor Circuit Board

The diagram above illustrates the board that hosts the gas and temperature/humidity sensors. While your board may differ physically, the pin‑out and power relationships remain the same. We recommend using a PCB for surface‑mount devices, though careful hand‑soldering on the back of the module is also viable.

Step 5: Ozone and NO₂ Sensors

MiCS‑2614 and MiCS‑2714 surface‑mount sensors detect ozone and nitrogen dioxide, respectively. Their sensing element is a resistor (≈10–20 kΩ) between pins G and K. A separate heating element (≈50–60 Ω) connects pins A and H to maintain optimal operating temperature.

When soldering without a PCB, use a low‑temperature solder and apply great care. In the schematic, we place an 82 Ω resistor in series with the MiCS‑2614 heater and a 131 Ω resistor (or 120 Ω + 12 Ω in series) for the MiCS‑2714 heater. The sensing resistors form a voltage divider with a 22 kΩ resistor: R_sen = 22 kΩ × (5 V / V_out – 1).

Step 6: MQ Toxic Gas Sensors

The MQ‑2 and MQ‑9 units measure propane, butane, LPG, and carbon monoxide. Like the MiCS sensors, they use a SnO₂‑based gas‑sensitive resistor and an internal heater. The MQ‑9 employs a transistor to regulate heater power, whereas the MQ‑2 uses a fixed resistor.

Refer to the solder‑board schematic for exact pin connections: for MQ‑2, connect pin A to 5 V, pin G to ground, and pin S to ground via a 47 kΩ resistor; for MQ‑9, connect pin A to the transistor, pin B to 5 V, pin G to ground, and pin S to ground through a 10 kΩ resistor.

Step 7: Temperature and Humidity Sensor

The DHT11 sensor measures both temperature and humidity, factors that significantly influence gas‑sensor readings. Mount the sensor so that the left pin connects to 5 V, the middle pin to a digital Arduino pin (default port 2 in our code), and the right pin to ground. Adjust the code if you use a different pin.

Step 8: Power and Fan

The entire assembly requires only a single 5 V supply, which can be supplied by a common USB or wall adapter. A 5 V fan ensures adequate airflow through the enclosure, preventing sensor overheating.

Step 9: Container

We fabricated the enclosure using a UP 3D printer. The STL file used for the final print is available for download.

Step 10: Coding

The provided Arduino sketch reads raw data from all sensors, outputs sensor resistance values, Shinyei low‑pulse occupancy percentages, and temperature/humidity readings to the serial monitor, and cycles these values on the LCD screen.

Before compiling, download the required libraries: LCD shield library and DHT sensor library.

Step 11: Interpreting the Data

We are currently refining algorithms to convert raw sensor outputs into actionable pollutant concentrations. Calibration against known reference sources will be essential for precision.

For particulate matter, we used research by David Holstius to correlate Shinyei PPD42 output with EPA measurements:
PM₂.₅ = 5 + 5 × (low‑pulse occupancy %).

Gas‑sensor resistances are interpreted using the curves provided in their datasheets. For MQ units, a resistance drop to <50 % of the baseline typically signals the presence of target gases; a drop to <10 % indicates concentrations approaching 1 000 ppm, near regulatory limits.

Once we derive approximate concentrations, we reference EPA technical documents and CDC guidelines to contextualize health risks.

Full interpretation code will be released once calibration is complete.

Source: Air Pollution Detector