Friday, June 21, 2013

Electric Imp + Arduino + nRF24L01 = Fully Internet Wireless Solution

When we discovered the Electric Imp recently, we almost immediately ordered them directly from US to do research and coding on them. You can read more about the electric imp on their websites or view their videos on Makerfaire, in short, it is a awesome product. 

electric imp

What it does is to simplify the Internet connectivity of electronic projects and the ability to program your device or "things" over the Internet on a web based IDE is a really powerful concept made into reality.

Before the Imp, a few other options existed to have your device "Internet Enabled" and they are either too expensive or requires a lot of other stuff like router / firewall configuration, having a linux box in between like the Raspberry Pi version of Raspbian or plug in an Ethernet Shield on top of an Arduino. I've tried  them all out and could not find any other solution as simple as this.

Firstly, do not be confused by it SD card looking form factor, it is NOT a memory card but a powerful microcontroller plus wifi enabled radio all stuff into a tiny SD card otter shell. When it comes to connecting endpoints like smartphones and laptops to wifi, we are all aware of the trouble of choosing the correct access point and keying in the wifi password to gain access. 

How do you key in the access point name and wifi password to this small little SD card microcontroller ? The answer is light or blinking lights to transmit these wifi authentication information directly to the card itself. They have mobile apps for both iOS and Android platform. View this video demo on how it works.

With the Internet connectivity issue easily solved using an Electric Imp, my next questions is what if I want to connect more than one device ? Should I buy another electric imp and the Imp development board to house it or should I find an alternative (or more cost effective ) method to link up all my other devices/things in the house.

With my other projects all using nRF24L01 radio, the choice is quite obvious for a wireless solution, find a nRF24L01 driver for the electric imp platform. After a few google searches, I manage to find an initial driver for the electric imp on the forum  BUT it was totally not working and unfinished. This is what usually happens to open source and community projects, someone will write the codes when time and interests permits and abandon it when it is either not working or find something else more interesting project to do...

Since I have a little bit of experience forking the nRF24L01's RF24 library and making the RF24 drivers work for Arduino, Raspberry Pi and Atmel attiny85, I took up the challenge of writing the nRF24L01 driver for the electric imp at the same time learning a C like new object oriented programming language called Squirrel that was used on the Imp.

The nRF24L01 I wrote was a combination of RF24 codes with the Mirf codes as the Mirf codes was easier to understand and was originally written in C instead of C++ for the RF24. There were some issues on the SPI but with a help of a Logic Analyzer and Logic software, I was able to fix most of the SPI related issues   

The github repo for the Electric Imp nRf24L01 driver is at You can comment out all the debug output when everything the radio is running ok.

block diagram

My next task is to find a useful application for my wireless solution, since I had a RGB LED strip and always like to make an Internet enabled RGB lighting, I hook up everything like the block diagram above. The web portion is a jquery colour picker from farbtastic with some touchscreen add-ons for touchscreen smartphone/tablets. Do viewsource to see the javacript codes at

colour picker

The Arduino portion is a simple code to read the three RGB codes from the colour picker webpage via nRF24L01 in #RRGGBB in hex and convert them to decimal ( 0 - 255 ) for analogWrite to the PWM pins. I'm driving the 12V RGB LED strip using a ULN2803 IC similar to this page on ambient lighting.

Arduino + nRF24L01 + RGB
The electric Imp part of the hardware is an April development board with the Impee SD card with SPI pins connected to the nRF24L01 radio. Since both the Imp and nRF24L01 runs on 3.3V, I do not need power regulator for this.

Impee + nRF24L01
Those red mini hooks are connected to my logic analyzer when I was troubleshooting the SPI issues I faced earlier. 

In summary, this seems like a complex setup but in actual fact after removing all the nRF24L01 library/drivers, the lines of codes is only the below :-

Imp Server Side :-
  1. ===============================================================================================
  2. Electric Imp agent side ( cloud / server side ) :-
  3. ===============================================================================================
  5. http.onrequest(function(req, resp){
  6.     server.log("Got a HTTP request");
  7.     if (req.method == "POST"){
  8.         local body = http.urldecode(req.body)
  9.         server.log(;
  10.         device.send("agentBuffers",;
  11.         //device.on(recvfromImp,impBuffer);
  12.     }
  13.     resp.send(200"<head><meta http-equiv=\"refresh\" content=\"1;url=\"><html>OK</html></head>");
  14. });

Imp device to send to nRF24L01 ( radio.send() ) :-
  1. ===============================================================================================
  2. Electric Imp device ( the white wifi SDcard in yr home ) :-
  3. ===============================================================================================
  4.   function watchdog2() {
  5.         agent.on("agentBuffers",function(value) {
  6.         server.log("Got a buffer from agent/http");
  7.            if ( value.len() < 33 ) {            // Make sure nRF24L01 payload is only 32bytes or less
  8.            radio.send(value,value.len());          // Send the payload to the radio
  9.            }
  10.         } );
  11.         radio.powerRX();
  12.         imp.wakeup(1, watchdog2 );    
  13.     }

Arduino portion after getting the #rrggbb from nRF24L01 :-

  1.         HEXtoRGB(); 
  2.         analogWrite(RED_PIN,r);
  3.         analogWrite(GREEN_PIN,g);
  4.         analogWrite(BLUE_PIN,b);

Summary Links :-
- Arduino RGB code

Sunday, June 9, 2013

Arduino Serial over nRF24L01

While browsing for nRF24L01 solutions, I came across this UART interface to nRF24L01 that work similar to Xbee, serial data into the module RX/TX and serial data out of the module at the other end.  Since those modules uses Atmel ATmega8 as the MCU, I had an idea to write the code for it from an Arduino IDE and make a few PCBs for it. Making it work like Xbee serial would be fantastic as the cost is only a fraction compared to Xbee. I always joked that the X in Xbee stands for eXpensive.

UART to nRF24L01

For me, the drive to spring into action is usually to solve a problem in hand and during the 50% mega sale, I purchase a Skylab GPS module to mess around with GPS stuff. I've been messing with GPS since 2005 with my purchase of the handheld color Garmin GPSmap60C but these days, GPS is no big deal since any smartphone is usually equipped with one.

Skylab GPS module

After soldering jumper cables to the Skylab Vcc,Gnd, RX & TX pins as they were not the standard breadboard size of 0.1" (2.54mm), I loaded TinyGPS library from Arduiniana and was surprise on the sensitivity of this GPS module. Inside my room, I can receive very strong signals compared to my trusty Garmin GPSmap60C or my Holux M-241 (for geotagging) where I have to stand outside my balcony for 5-10 mins just to get a GPS lock.

I just hook up a 16x2 LCD to the Arduino UNO to display the location and the number of satellite and hdop value for accuracy of the location.

Since the GPS module runs on serial and spit out data continuously, I can use this as my "content" or raw data to feed to my UART to nRF24L01 module and another module just reverse it back to Serial into an Arduino or output to Serial Monitor for a GPS software to interpret them instead of using Arduino GPS library.

Another really important piece that got me started was this blog by JHaskell on Serial communications fundamentals on Arduino that explain about using start and stop delimiters to capture serial input into an array.

With this two pieces of info, I just need to figure out on how to split up the payload to fit the nRF24L01 max payload size and reassemble it back together at the receiving end. 

I initially started with using a $ for the start delimiter and a few other symbols at the end delimiters to denote 1st fragment, 2nd fragment and last fragments on the receiving side. Greg suggested that I use a one byte header to denote fragmentation, and rest of the bits as fragment sequences and the rest as payload. I finally decide to use 2 bytes headers with the 2nd byte reverse for future use and the first byte as follows :-

8 bit    - 1 - fragment, 0 - no fragment
5-7 bit - number of fragments  
4 bit    - unused
1-3 bit - fragment number

I've been reading TCP/IP headers packets for a long time and this is the first time I wrote my own simple protocol to fragment and assembly a wireless packet and even reverse 8 bits for future use...

Here are some pictures before I continue with the rest of the story..

Arduino UNO nRF24L01 GPS module

The first thing you might be wondering is why is there a FTDI USB-Serial breakout board  when the Arduino already have a built-in USB-Serial on pin 0 & 1. This issue started when I was receiving GPS data using Arduino SoftwareSerial library and while this library works with the TinyGPS mentioned above, it was starting to drop bytes ( serial overflow ) and some of the GPS data was lost before I can even sent them over the radio frequency.

Someone at Arduino forum suggested that I use the hardware serial to receive the GPS data and plug in the FTDI USB Serial for debugging purpose/serial monitor. I tried out his suggestions and all the dropped bytes went away immediately. Since the final product do not really need to connect to Serial Monitor, I might as well use the hardware serial instead of the slow SoftwareSerial library.

Arduino UNO nRF24L01
Above is a picture of the receiver, nothing special, just an nRF24L01 with an adapter that plug directly to the Arduino pins or to a breadboard with jumper wires flying all over the place... ( ignore the 16x2 LCD, it was permanently there for other projects )

I will not go though the programming codes on this post as they are still not fully optimize but I will post the links to them on Below are the output from mini GPS software displaying all the satellite signals and my location had been masked to protect my identity.

Mini GPS
Someone on Google+ asked me about using this setup ( nRF24L01+ and GPS module) on a remote control boat and I just discovered that you can use this setup to measure the range of the radio, just mark the initial GPS coordinates and the furthest distance the radio + GPS module can reached and calculate the distance using both the GPS coordinates.

*** Special thanks to Greg for helping me out on ideas, suggestions and coding the serial and nRF24L01 stuff.

Summary Links :-
- My Serial to nRF24L01 code V4
- JHaskell's Arduino Serial communications fundamentals
- Arduiniana TinyGPS library


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