An improved SDS011 connector cable for Airrohr devices

At the Clean Air Eastbourne project we build Airrohr Luftdaten devices.

These use the SDS011 sensor. The official instructions suggest plugging dupont cables directly into the sensor, however we have found these tend to become loose and can be unreliable.

Our solution is to build our own cables.

We buy 7pin 2.54m 6S1P JST-XH cables from AliExpress. These come with wires already attached. We take out the red, black, and yellow wire by pressing down on the connector and gently pulling the wire out the back. This leaves us with the 4 wires we need in the correct slots.

Next we attach female dupont connectors with double housing to the other ends of the wires. One double connector goes on the blue / green pair, and another on the white / orange pair of wires. I used a PA-09 crimping tool from (affiliate link) to make the attachments.

When plugging into the NodeMCU the connections are
White – VU
Orange – Ground
Green – D2
Blue – D1

Debugging a Mendix widget

I had to debug a custom Mendix widget that was failing. All that was shown on screen in red was “Could not create widget BHCCDropZone.widget.BHCCDropZone”.

We’d recently upgraded to Mendix 7.18.1, so something looked like it happened during the upgrade.

The first step was to make sure the appropriate access rights to the entity the widget uses were right. In this case they were.

Next was to up the log levels from INFO to DEBUG in the Mendix Modeller to see if that gave me any clues. I could see the widget’s constructor and postCreate being run as I had logger.debug calls in those functions being run. However, one of my debug calls was not being reached, so I had narrowed down the location of problem.

After this I added JavaScript breakpoints in the failing method using Chrome’s developer tools. The widget is only loaded the first time the page is reached. When I reached this page I singled stepped through, and found an exception being thrown by the JavaScript but being caught by Mendix. My error was…

"TypeError: Converting circular structure to JSON
    at JSON.stringify ()
    at http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:164930
    at ()
    at t.log (http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:164840)
    at http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:196250
    at http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:166406
    at Array.forEach ()
    at t.log (http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:166377)
    at t.debug (http://localhost:8080/mxclientsystem/mxui/mxui.js?636758982080189581:74:165718)
    at Object.postCreate (http://localhost:8080/widgets/BHCCDropZone/widget/BHCCDropZone.js?636758982080189581:80:11)"

I could now see I had a circular data structure.

Working backwards through the stack trace, I could see the problem was in my widget’s postCreate function, and it was a debug function causing the problem. This was the faulty code causing the widget to fail.

My postCreate function looked like this…

postCreate: function () {
  console.log("BHCCDropZone session", mx.session);
  logger.debug( + ".postCreate");
  logger.debug("this", this);

The faulty line was this…

logger.debug("this", this);

Trying to send the contents of this to the debug logs was causing problems as it was a circular data structure. Removing this line of JavaScript fixed the problem, and the widget started to work again as expected.

I hope this helps others in future when debugging “Could not create widget” errors in Mendix.

Using a http proxy from a Mendix Java action

As part of some work I have been undertaking to integrate the UK Government Notifications service into Mendix, I needed to be able to make API calls from behind a firewall using a proxy in a Java action.

Due to the lower level Java actions in Mendix run at, proxy settings are not automatically applied, and must be added manually. I wanted to explain how to get the proxy settings from Mendix, and use them a Java action.

I’ve previously explained how to add proxy settings to Mendix, so I assume this step has been completed.

In a Java action, we need to get these from the HttpConfiguration singleton.

import com.mendix.http.HttpConfiguration;
import com.mendix.http.IHttpConfiguration;
import com.mendix.http.IProxyConfiguration;

IHttpConfiguration httpconf = com.mendix.http.HttpConfiguration.getInstance();
IProxyConfiguration proxyconf = httpconf.getProxyConfiguration().orElse(null);

We can now check if we have a proxy configuration set, if we don’t proxyconf will be null.

The username and password for the proxy can be retrieved using the getUser() and getPassword() methods.

String username = proxyconf.getUser().orElse(null);
String password = proxyconf.getPassword().orElse(null);

If they are present we can build a Java Authenticator object and set it as the default authenticator.


if (username != null && password != null) {
    Authenticator authenticator = new Authenticator() {
        public PasswordAuthentication getPasswordAuthentication() {
           return (new PasswordAuthentication(username, password.toCharArray()));


Next we need to create the Proxy object. We need to get the host and port of our proxy server from Mendix using the getHost() and getPort() methods.


InetSocketAddress proxyLocation = new InetSocketAddress(proxyconf.getHost(), proxyconf.getPort());
Proxy proxy = new Proxy(Proxy.Type.HTTP, proxyLocation);

The proxy can be used for Java network actions.

An example of using this would be the UK Government Notifications client. It has a second optional paramater in it’s constructor for a Proxy.

client = new NotificationClient('APIKey', proxy);

Using a proxy server from the Mendix Modeller

There are times when building online services you find yourself behind a firewall and need to use a proxy. Sometimes these are transparent, but other times you need to add settings by hand.

In a Mendix app, an example may be when you need to consume a REST service from outside you home network.

To configure proxy settings in Mendix, you need to go to our Project’s “Settings”. Open “Configurations”, select your working configuration, and click “Edit”. Select the “Custom” tab and add the following “Names” and “Values”.

http.proxyHost The name your proxy
http.proxyPort The port your proxy is running off of.

If my proxy was running on, my settings would be

http.proxyPort 8080

Sometimes the proxy will also need a username and password. You can set these using http.proxyUser and http.proxyPassword. For example

http.proxyUser RobertPrice
http.proxyPassword SecretPassword

You should now be able to access external services through the proxy from Mendix.

Example proxy settings for the Mendix Modeller

More information on using a proxy in Mendix is available at Using a proxy to call a REST service.

Extracting text in Mendix using RegexReplaceAll

It’s a fairly common requirement to be able to extract text from a larger string.

Using Mendix, the easiest way I’ve found to do this is using the RegexReplaceAll Java action from the Community Commons module.

We use a regular expression extract the text, then return this selected text in the action.

For example, take the following string returned from the Nexmo SMS module.

--------- part [ 1 ] ------------Status [ 9 ] ...SUBMISSION FAILED!Message-Id [ null ] ...Error-Text [ Quota Exceeded - rejected ] ...Message-Price [ 0.03330000 ] ...Remaining-Balance [ 0.03200000 ]

If we want to extract the Error-Text we can use the following regular expression.

^.*Error-Text \[ (.*?) \].*$

Here we’re saying look for the text between the square brackets after the string Error-Text. We use round brackets to say we want to remember this matched text. We can then use the regular expression match position to return the matched text – in this case $1.

If we run this over our string we get the following

Quota Exceeded - rejected

To use this in a Mendix microflow, assume we have our status message in a String $StatusMessage that we pass into the Java action. This is our Haystack.

Next, we use the regular expression as a String for our Needle regex.

Finally, we say we want $1 as a String to be our Replacement.

We return the String as $Details.

This is what the microflow and action should look like.

View Robert Price’s Mendix Profile.

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.

Running a SDS011 particulate sensor on a Mac using PHP

The Novafit SDS011 particulate sensor is an a very affordable sensor for detecting particulate pollution. It is capable of detecting both PM2.5 and PM10 with a relative error margin of +/- 10µg/m3.

It can output data via it’s serial port. The one I bought came with a serial to USB adaptor, allowing it to be plugged into my Mac. It did need a driver, and I used ch340g-ch34g-ch34x-mac-os-x-driver.

Data is sent at 9600 baud, with 8 data bits, no parity bit, and 1 stop bit. 10 bytes are sent at a time.

Byte Name Content
0 Message Header AA
1 Commander No C0
2 DATA 1 PM2.5 Low byte
3 DATA 2 PM2.5 High byte
4 DATA 3 PM10 Low byte
5 DATA 4 PM10 High byte
6 DATA 5 ID byte 1
7 DATA 6 ID byte 2
8 Check-sum DATA 1+DATA 2+..+DATA 6
9 Message tail AB

PM2.5 (μg /m3) = ((PM2.5 High byte *256) + PM2.5 low byte)/10
PM10 (μg /m3) = ((PM10 high byte*256) + PM10 low byte)/10

We can read this using PHP. The following PHP script can be run on the command line and outputs the PM2.5 and PM10 levels every 2 seconds to a terminal window.

exec('stty -f /dev/cu.wchusbserial1420 9600 raw');
while (true) {
    $handle = @fopen( '/dev/cu.wchusbserial1420', 'r' ); # Open device for Read access
    if ($handle) {
        $binarydata = fread( $handle, 10 ); # Read data from device
        $data = unpack('H2header/H2commander/vpm25/vpm10/Sid/H2checksum/H2tail', $binarydata);
        echo sprintf("PM2.5: %dµg/m³\nPM10:  %dµg/m³\n", $data['pm25']/10, $data['pm10']/10);
        fclose ($handle); # Close device file
    } else {
        echo "Unable to connect to pollution sensor\n";

The device is used by reading the 10 bytes it returns each message from the USB device. We can use PHP’s unpack function to break the binary data returned into an associative array for ease of use. We do have to do divide the PM2.5 and PM10 results by 10 to get their real values though. This can then be echo’d to the terminal.

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.