Neopixel LED temperature gauge with Raspberry Pi




In this tutorial we’ll be creating a circular temperature gauge using Adafruit’s Neopixel LEDs, a Raspberry Pi and components from Monk Makes Starter Kit! No Arduino required! And to make it even more better, we’ll be using Plot.ly to plot the data in real time, turning this little project into an Internet of Things (IoT) sensor! Read on below to get started!

I recently got my hands on some LEDs from Adafruit’s excellent Neopixel range. These brilliant LEDs come in all sorts of shapes, ranging from strips, circular rings to matrices. What makes these LEDs especially great, however, is that they are really, really bright! Unlike ordinary LEDs which have a diffusing cylindrical like cap on the top, these simply have a flat transparent plastic covering. To give you an idea, a brightness level of ~80 out of 255 is the maximum at which you can look at them with your naked eye. The only slight caveat to these LEDs, is that they can only be driven by a dedicated microcontroller like an Arduino board. This meant a Raspberry Pi was out of the question. But thanks to a great library written by Jeremy Garff, it’s now possible to drive them with a Raspberry Pi (and a 5V power supply)!

So what’s this project all about?

Before we start, let’s explain exactly what we’re trying to do! We’re going to hook up Adafruit’s Neopixel 12 LED ring to a Raspberry Pi, coupled with a temperature sensor. This will allow us to use the LED ring as a circular gauge for the temperature readings. And for those feeling extra ambitious, an extra section will show you how to transmit the readings over the Internet to a real time plotting service (Plot.ly) and allow you to watch the temperature readings from anywhere, so long as you have an internet connection.Neopixel LED temperature gauge with Raspberry Pi

So what do we need?

As you probably know, there are many ways to set up a temperature sensor with a Raspberry Pi. In this case, I’ll be using Monk Make’s fantastic Raspberry Pi Starter Kit, which comes with all the components to set up a capacitor-based temperature sensor, including the code that translates the resistance readings to actual temperature values. We’ll obviously also need Adafruit’s Neopixel LED ring, and a 5V power supply to power the LEDs. Although the RPi has a 5V pin, it’s output voltage values do not in fact meet the requirements for the Neopixel LEDs. In my case, I already have an Arduino-based board, so I simply borrowed the pins from that as a power source.

  • Raspberry Pi (currently, the Neopixel LEDs are not compatible with the Pi 2)
  • Adafruit Neopixel Ring (12 LEDs)
  • 5V power supply (can borrow from an Arduino board)
  • Temperature sensor (Monk Makes RPi Starter Kit)

 Wiring it all up

Neopixel LED ring wiring

Wiring up the Neopixel ring to the Raspberry Pi is really simple! Simply connect one jumper wire from the ‘IN’ hole on the ring to pin #18 on the RPi GPIO header. This pin is chosen because its the default pin in the RPi Neopixel library. If you change the pin on the RPi, you’ll need to change this in the library you download as well as your Python code. If you’re using the Monk Makes Starter Kit jumper wire (or perhaps any other generic ones), you may find the jumper wires are not as thick as the holes in the Neopixel ring, and hence fall out easily. To circumvent this issue, use the holes on the breadboard to hold the wires in place. The breadboard’s circuit is not actually required for the Neopixel ring, we’re using it simply as a support.

You’ll need another two wires to connect to the ‘PWR’ and ‘GND’ holes on the Neopixel ring, for the power supply. The wire from the ‘PWR’  pin should connect to a +5V power source (positive terminal), and the wire from the ‘GND’ to either a negative terminal or ground pin. If you’re borrowing an Arduino board’s supply, like me, connect to the 5V and GND pins, next to the ADC pins.

Thermistor wiring

If you’re using the Monk Makes Starter Kit, there’s a project card that shows you how to set up the thermistor for temperature readings. We’re using that exact same set up, also shown in the diagram above. If you’re using any other temperature sensor, set it up according to its relevant guides.

 

 Installing the library

Before we can run any Python code, we need to install the necessary libraries. First, download, as a .ZIP, Jeremmy Garff’s rpi_ws281x library from here. You could use a git command to download the repository directly onto the RPi directly, but I found in practice for some reason, the files downloaded this way seem to be corrupted. Before we install the library, you’ll need to install some additional packages on your Raspberry Pi. If your Pi has Internet access, type the following into the terminal:

sudo apt-get update
sudo apt-get install build-essential python-dev scons swig

Neopixel LED temperature gauge with Raspberry Pi schematicIf you don’t have Internet access, you can search for these packages online and manually dump the packages onto your Pi yourself. You’ll need to make sure the packages for ARMHF architecture. If you’re running Raspbian OS, simply download the Debian packages. Install these packages by typing sudo dpkg -i package.deb at the terminal. The packages listed above have other dependencies (required packages) which are automatically downloaded by the sudo apt-get install command, hence you’ll need to manually install those as well. These dependencies will be listed as errors after the installation fails for one of the packages above.

Important changes to make to the library

The library was written to drive an 8×8 Neopixel matrix, but in our case, we’re working with a 12 LED circular ring. To correct for this, once you’ve downloaded and unzipped the library on your RPi, edit the main.c file. You may use the command sudo nano main.c to edit the file. Around line 56, change the code so it reads as follows:

#define ARRAY_SIZE(stuff) (sizeof(stuff) / sizeof(stuff[0]))

#define TARGET_FREQ WS2811_TARGET_FREQ
#define GPIO_PIN 18
#define DMA 5

#define WIDTH 12
#define HEIGHT 1
#define LED_COUNT (WIDTH * HEIGHT)

The required change involves changing the height to ‘1’ and the width to ’12’, as we’re using a 12 LED strip and not a matrix. The GPIO pin is also defined as 18 by default. Only change this if you’ve wired your Neopixel ring to the RPi differently. Press Ctrl-X to save the file once done. For more information on how this library works, check out Adafruit’s tutorial here.

Compiling the library

Once you’ve installed all the required packages, you’ll need to compile it. From the terminal, cd into the directory containing the unzipped files for the library. Once inside the directory, type scons to compile the library. Next,  type cd python to enter the ‘python’ directory, followed by typing sudo python setup.py install. This will install the Python library for you.

 

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