Building a simple air quality indicator

I demonstrated Using a bicolour LED with an NodeMCU in an earlier blog.

We now extend that code and instead of having a single LED, we have two. One LED will indicate the current levels of PM2.5, and the other LED will indicate the current levels of PM10. To do this we need to connect to the internet and download these levels from the Clean Air Eastbourne API. The API returns a single float as a string with the current value in µg/m3.

The wiring is very similar to the previous blog post, except we include an extra LED connected to D3 and D4 on the NodeMCU.

The code

The NodeMCU has support for WiFi built in thanks to it’s ESP8266 chip, so it’s very easy to go online in our in setup()loop() function we poll for values every 30 seconds. We turn these values into floats and compare our snapshot against the WHO annual guidelines limits, and UK annual legal limits. If we’re below WHO and UK limits, we show green. If we’re above WHO limits, but below UK limits, we show amber. If we’re above both WHO and UK limits, we show red.

#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>

// Wifi connection details
const char* ssid = "Wifi Network";
const char* password = "Wifi Password";

// Poll URLs
const char* PM10_URL = "";
const char* PM25_URL = "";

// How often to poll the website for updates (in microseconds)
const int POLL_DELAY = 30000;

// Limits for particulates from the WHO and UK.
// These are annual limits.
const float WHO_PM10 = 20.0;
const float WHO_PM25 = 10.0;
const float UK_PM10 = 40.0;
const float UK_PM25 = 25.0;

// The TriColour LEDs are wired to the following pins
int redPinPM10 = D2, greenPinPM10 = D1;
int redPinPM25 = D3, greenPinPM25 = D4;

void setup () {
  pinMode(redPinPM10, OUTPUT);
  pinMode(redPinPM25, OUTPUT);
  pinMode(greenPinPM10, OUTPUT);
  pinMode(greenPinPM25, OUTPUT);

  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    Serial.println("Connecting to Wifi...");


void loop() {

  if (WiFi.status() == WL_CONNECTED) { //Check WiFi connection status

    HTTPClient http;  //Declare an object of class HTTPClient

    Serial.println("Getting PM10 data...");
    http.begin(PM10_URL);  //Specify request destination
    int httpCode = http.GET();                                        //Send the request
    if (httpCode > 0) { //Check the returning code

      String payload = http.getString();   //Get the request response payload
      Serial.println(payload);             //Print the response payload

      int reading = payload.toFloat();

      digitalWrite(redPinPM10, LOW);
      digitalWrite(greenPinPM10, LOW);
      if (reading >= WHO_PM10 && reading < UK_PM10) {  // turn the LED if over WHO limits, but under UK
        Serial.println("Over WHO guidelines ");
        digitalWrite(redPinPM10, HIGH);
        digitalWrite(greenPinPM10, HIGH);
      } else if (reading >= UK_PM10) {                 // turn the LED on if over UK limits
        Serial.println("Over UK limits");
        digitalWrite(redPinPM10, HIGH);
      } else {
        Serial.println("under limit");
        digitalWrite(greenPinPM10, HIGH);
    } else {
      Serial.println("Unable to get data from API");
    http.end();   //Close connection

    Serial.println("Getting PM2.5 data...");
    http.begin(PM25_URL);  //Specify request destination
    httpCode = http.GET();                                            //Send the request
    if (httpCode > 0) { //Check the returning code

      String payload = http.getString();   //Get the request response payload
      Serial.println(payload);             //Print the response payload

      int reading = payload.toFloat();

      digitalWrite(redPinPM25, LOW);
      digitalWrite(greenPinPM25, LOW);
      if (reading >= WHO_PM25 && reading < UK_PM25) {  // turn the LED if over WHO limits, but under UK
        Serial.println("Over WHO guidelines ");
        digitalWrite(redPinPM25, HIGH);
        digitalWrite(greenPinPM25, HIGH);
      } else if (reading >= UK_PM25) {                 // turn the LED on if over UK limts
        Serial.println("Over UK limits");
        digitalWrite(redPinPM25, HIGH);
      } else {
        Serial.println("under limit");
        digitalWrite(greenPinPM25, HIGH);
    } else {
      Serial.println("Unable to get data from API");
    http.end();   //Close connection

  } else {
    Serial.println("Not connected to Wifi");

  delay(POLL_DELAY);    // wait before we poll again.

Using a bicolour LED with a NodeMCU

This is a quick example on how to use a bicolour LED with a NodeMCU.

The bicolour LED has both red and green LEDs in a single package. There is a common cathode for both the red and green anodes. Applying a current to the red anode turns the red LED on. A current to green anode turns the green LED on. A current to both anodes creates yellow.

The circuit is very simple. We wire the red anode to D1 on the NodeMCU, the green anode to D2, protecting them both with a 220ohm resistor. The cathode is wired to ground.

Linking a bicolour LED to a NodeMCU

We program the NodeMCU using an Arduino sketch.

int redPin = D1, greenPin = D2;

void setup() {
  // set the pins to output mode
  pinMode(redPin, OUTPUT); 
  pinMode(greenPin, OUTPUT);

void loop() {
  // red
  digitalWrite(redPin, HIGH); delay(500);
  // yellow
  digitalWrite(greenPin, HIGH); delay(500);
  // green
  digitalWrite(redPin, LOW); delay(500);
  // off
  digitalWrite(greenPin, LOW); delay(500);

In the setup, we set D1 and D2 to both be outputs. We then turn D1 and D2 on and off in sequence every half a second to achieve a sequence of red, yellow, green, and off.

NodeMCU WiFi setup

NodeMCU devices have WiFi built in, and it’s easy to configure using Lua.

One of the easiest examples of setting this up was on Limpkin’s blog. I took this and tweaked it slightly so it works with the more recent Lua wifi module.

station_cfg.ssid="Rob's iPhone"
station_cfg.pwd="secret password"
tmr.alarm(1, 1000, 1, function()
    if wifi.sta.getip() == nil then
        print("IP unavailable, Waiting...")
        print("ESP8266 mode is: " .. wifi.getmode())
        print("The module MAC address is: " .. wifi.ap.getmac())
        print("Config done, IP is "..wifi.sta.getip())

Upload this using luatool or similar as the init.lua file that runs when the NodeMCU powers up.

In this example, the script tries to connect to my iPhone hotspot. We use tmr.alarm to check once a second if we have an IP address. If we don’t, we let the user know. If we do, we stop the timer from continually executing, and print out some of our wifi configuration details.

It’s at this point we could start using the connection to talk to the internet if we needed to.

Using an i2c 128×64 OLED display with Lua on a NodeMCU

Small 128×64 OLED i2c displays can picked up for just over £3 on Amazon, and they are great for interfacing with a NodeMCU IoT device.

Wiring is simple…

  • NodeMCU 3.3v – OLED VCC
  • NodeMCU D1 – OLED SDA
  • NodeMCU D2 – OLED SCL or SCK

Wiring an i2c OLED display to a NodeMCU

Your NodeMCU firmware must have been compiled with the i2c and u8g modules included. If you need to add these, see my previous post – NodeMCU Lua setup using a Mac. It also explains how to upload the example Lua code below.

The u8g module comes with some great examples. I’ve taken some of the relevant parts for a simple two page Lua script that prints out two alternating screens of text.

sda = 1 -- SDA Pin
scl = 2 -- SCL Pin

function init_display(sda,scl) --Set up the u8glib lib
     sla = 0x3C
     i2c.setup(0, sda, scl, i2c.SLOW)
     disp = u8g.ssd1306_128x64_i2c(sla)

function updateDisplay(func)
  -- Draws one page and schedules the next page, if there is one
  local function drawPages()
    if (disp:nextPage() == true) then
  -- Restart the draw loop and start drawing pages

function drawScreen1()
     disp:drawStr(5, 20, "")
     disp:drawStr(5, 35, "    @robertprice")

function drawScreen2()
     disp:drawStr(5, 20, "  NodeMCU OLED Test")
     disp:drawStr(5, 35, "   12th July 2017")

local drawDemo = { drawScreen1, drawScreen2 }

function demoLoop()
  -- Start the draw loop with one of the demo functions
  local f = table.remove(drawDemo,1)

tmr.alarm(4, 5000, 1, demoLoop)

This may look a bit complex for just displaying text, but it aims to be reusable. We use the tmr.alarm to avoid bogging the CPU down in a loop.

NodeMCU driving an i2c OLED display

I’ve tried this with two different i2c OLED displays, and they both work.

NodeMCU Lua setup using a Mac

I recently bought a NodeMCU. This is a small ESP8266 based card with built in WiFi, MicroUSB, and a Lua interpreter that can be used for developing IoT (Internet of Things) devices.

I had trouble getting it working initially, so I wanted to share how I fixed this on a Mac.

Install Mac drivers

If you’re not using a Mac, you can skip this part.

You plug the NodeMCU into the Mac via USB. The Mac won’t support this by default, and you need to install a driver. The driver you need to install is the Silcon Labs CP210x USB to UART Bridge.

Once installed, plug in the NodeMCU and check that the device /dev/tty.SLAB_USBtoUART exists.

To connect to the NodeMCU, you’ll need some tools. To test, download CoolTerm. In Options, for Port select SLAB_USBtoUART, and for Baudrate select 115200.

If you are lucky you’ll get a prompt, if not you may need to build and install some new firmware.

If you do get a prompt type…

print "Hello";

… and Lua should echo “Hello” back to you.

Build and install the NodeMCU firmware

If you need to build new firmware, there is a very useful online site called that I used to build the firmware with the right modules I wanted for my project.

Once you’ve build the firmware, you’ll need to install the Python to flash the firmware to the NodeMCU.

git clone
cd esptool
sudo python ./ install

Check the flash size of your NodeMCU.

To flash the firmware hold down “FLASH” and press “RST” on the NodeMCU, then use the following command (remembering to disconnect CoolTerm first if connected)… --port /dev/tty.SLAB_USBtoUART write_flash -fm dio 0x00000 ~/Downloads/nodemcu-master-12-modules-2017-07-07-21-24-03-float.bin

As runs, you should see something like this… v2.0.1
Detecting chip type... ESP8266
Chip is ESP8266
Uploading stub...
Running stub...
Stub running...
Configuring flash size...
Auto-detected Flash size: 4MB
Flash params set to 0x0240
Compressed 754992 bytes to 505060...
Wrote 754992 bytes (505060 compressed) at 0x00000000 in 44.5 seconds (effective 135.6 kbit/s)...
Hash of data verified.

Hard resetting...

Connect to it using CoolTerm, then press the “RST” button. You should see some messy characters, then something like following (depending on what you built into your firmware)…

NodeMCU custom build by
.branch: master
.commit: c8ac5cfb912ff206b03dd7c60ffbb2dafb83fe5e
.SSL: true
.modules: adc,bit,cron,crypto,encoder,file,gpio,http,i2c,net,node,ow,pcm,sjson,sntp,spi,struct,tmr,u8g,uart,websocket,wifi,tls
 build .built on: 2017-07-12 09:30
 powered by Lua 5.1.4 on SDK 2.1.0(116b762)
lua: cannot open init.lua

Upload Lua code to the NodeMCU

The best way to get Lua code onto the NodeMCU is to use the Python luatool.

git clone
cd luatool

To test we’ll upload a simple Lua script that will blink the NodeMCU’s onboard LED, save this as “blink.lua”.


gpio.mode(LED_PIN, gpio.OUTPUT)
value = true

tmr.alarm(0, 500, 1, function ()
    gpio.write(LED_PIN, value and gpio.HIGH or gpio.LOW)
    value = not value

Upload it to the NodeMCU using the following command (making sure Coolterm is disconnected first)…

python luatool/ --port /dev/tty.SLAB_USBtoUART --src blink.lua --dest init.lua --dofile

You should see the following…

->"init.lua", "w") -> ok
->file.close() -> ok
->file.remove("init.lua") -> ok
->"init.lua", "w+") -> ok
->file.writeline([==[LED_PIN = 0]==]) -> ok
->file.writeline([==[]==]) -> ok
->file.writeline([==[gpio.mode(LED_PIN, gpio.OUTPUT)]==]) -> ok
->file.writeline([==[value = true]==]) -> ok
->file.writeline([==[]==]) -> ok
->file.writeline([==[tmr.alarm(0, 500, 1, function ()]==]) -> ok
->file.writeline([==[gpio.write(LED_PIN, value and gpio.HIGH or gpio.LOW)]==]) -> ok
->file.writeline([==[value = not value]==]) -> ok
->file.writeline([==[end)]==]) -> ok
->file.flush() -> ok
->file.close() -> ok
->dofile("init.lua") -> send without check
--->>> All done <<<--

The onboard LED on the NodeMCU should now be blinking.

The NodeMCU executes the init.lua script by default, so when we upload we tell luatool to upload our blink.lua script as init.lua.