Raspberry Pi single board computers are wonderful devices. They can do so many things, including interact with sensors, inputs, and outputs with their GPIO (general purpose Input/Output) pins -- however, analog input and output are not available on the stock Raspberry Pi.

What happens when you want to turn some knobs or slide some faders? No fear, it's simple to add an Analog-to-Digital Converter (ADC) and this guide is going to show you how!

You'll use the retro-tastic CYBERDECK Hat to add some STEMMA QT / Qwiic compatible I2C connectors to the Raspberry Pi 400, and then plug in the PCF8591 Quad 8-Bit ADC + DAC breakout board. The PCF8591 makes it simple to add four potentiometers to your Pi project.

Add a PiTFT to the mix and you'll have a compact and stylish computing setup that you can use for live music coding jams including four knobs to twiddle!

Parts

Optional

Rapsberry Pi OS

If you're using the Raspberry Pi 400 Desktop - Full Computer Kit you'll already have an SD card with Rapsberry Pi OS on it. If you are using the Pi 400 Computer Only, you'll need to get an SD card and install Rapsberry Pi OS on it. 

Once your Pi is set up and running proceed with the directions below.

Software Setup

Now, make sure you have Python 3 setup, as Python 2 is no longer used or supported.

 pip3, is the software package installer you'll use. Upgrade it to the latest version with this command from a terminal:

sudo pip3 install --upgrade setuptools

If above doesn't work try

sudo apt-get install python3-pip

Once that has finished you'll be returned to the prompt.

Make sure you're using Python 3!

The default Python on your computer may not be Python 3. Python 2 is officially discontinued and all our libraries are Python 3 only.

We'll be using python3 and pip3 in our commands, use those versions of Python and pip to make sure you're using 3 and not 2.

Python Installation of PCF8591 Library

 Since each platform is a little different, and Linux changes often, please visit the CircuitPython on Linux guide to get your computer ready!

Once that's done, from your command line run the following command:

sudo pip3 install adafruit-circuitpython-pcf8591

If your default Python is version 3 you may need to run 'pip' instead. Just make sure you aren't trying to use CircuitPython on Python 2.x, it isn't supported!

Sonic-Pi

You can use the knobs and ADC on your Pi any way you like, but for the purposes of this guide, you should try it out with Sonic-Pi, the live coding music software.

Head to this link and then follow the terminal installation instructions. They look something like this (the version may update after this guide was written, so follow the instructions on that linked page):

sudo apt update
sudo apt install ./sonic-pi_3.3.1_1_armhf.deb

Python OSC

In order for the knobs to send messages to Sonic-Pi (or other synth software) you'll use OSC (Open Sound Control) which is a modern alternative to MIDI for inter-instrument communications.

One great feature of OSC is its ability to send and receive messages among different programs on the same computer via UDP.

Install the python-osc library by running the following commands from the terminal:

pip3 install python-osc

Sending and Receiving OSC

Here's a terrific example of sending OSC messages from a Python script and listening/receiving those messages in Sonic-Pi. I've excerpted sections of it here:

In this example we described an OSC path "/osc*/trigger/prophet" which we're syncing on. This can be any valid OSC path (all letters and numbers are supported and the / is used like in a URL to break up the path to multiple words). The /osc prefix is added by Sonic Pi to all incoming OSC messages, so we need to send an OSC message with the path /trigger/prophet for our sync to stop blocking and the prophet synth to be triggered.

Sending from Python

We can send OSC to Sonic Pi from any programming language that has an OSC library. For example, if we're sending OSC from Python we might do something like this:

Later you'll create a Python script that reads the knob inputs, and then converts the values and sends OSC messages to be read by Sonic-Pi.

PiTFT Install

Follow these steps to install the PiTFT in FBCP mode. This mode mirrors the HDMI output onto the PiTFT, and can be used stand-alone, no HDMI monitor required!

Analog Signals

Analog signals are different from digital signals in that they can be any voltage and can vary continuously and smoothly between voltages. An analog signal is like a dimmer switch on a light, whereas a digital signal is like a simple on/off switch. 

Digital signals only can ever have two states, they are either are on (high logic level voltage like 3.3V) or off (low logic level voltage like 0V / ground).

By contrast, analog signals can be any voltage in-between on and off, such as 1.8V or 0.001V or 2.98V and so on.

Analog signals are continuous values which means they can be an infinite number of different voltages. Think of analog signals like a floating point or fractional number, they can smoothly transiting to any in-between value like 1.8V, 1.81V, 1.801V, 1.8001V, 1.80001V and so forth to infinity.

Many devices use analog signals, in particular sensors typically output an analog signal or voltage that varies based on something being sensed like light, heat, humidity, etc. Some examples of sensors with analog outputs:

• Microphones
• Photocells (light sensitive resistors)
• Temperature sensors
• Force-sensitive resistors
• Flex sensors
• Thermistor (temperature sensitive resistor)
• Ultraviolet light sensor
• Light sensors
• Distance sensor
• Anemometer (wind speed sensor)
• Resistive touch screen
• Ultrasonic distance sensor
• Liquid level sensor
• Potentiometer (variable resistor)

Wire the Pots

The four potentiometers will act as voltage dividers, sending anywhere from 0V to 3.3V to their respective ADC pins on the PCF8591.

Voltage dividers work by connecting one of the outer pot legs to ground, the other outer leg to positive voltage (in this case 3.3V), and then the center (wiper) leg to an analog input pin on the ADC.

On a breadboard, you can connect your pots as shown here.

Perf Board

For this finished project, you'll connect your ADC board and pots on a perf board. The perf board has lots of solder points, but none of them are connected by default as is the case with a breadboard. So, you'll wire everything directly.

This diagram shows the connections necessary, however you'll do the wiring underneath the board, so this image is a bit like an x-ray view.

Solid core hook-up wire works well for this type of hand-wired trace circuit.

Once you've built the circuit, you should use a multimeter in continuity mode to test that you don't have any shorts.

CYBERDECK Hat

The CYBERDECK Hat is perfect for adding accessories to the RPi 400. It breaks out all of the GPIO pins at a useful angle, plus it adds convenient STEMMA QT/Qwiic plugs for easy I2C use.

The CYBERDECK also gives us a great place to mount additional boards. This mounting plate allows us to add the knob board to the CYBERDECK.

Download the model from the link below and print it in PLA at 0.2mm layer height, 60% infill.

With the Pi powered off, insert the CYBERDECK Hat into the GPIO pin slot, being careful to align the pins properly.

Read Knobs in Python

Time to try things out! Fire up your Pi 400 and then launch a Python editor such as Thonny or Mu.

Copy the code below, paste it into your Python editor, and then run the code by pressing the "play" button.

Turn the knobs and you'll see the four values changing, from 0 to 65,535.

Python Knobs OSC Code

Copy the code shown below and paste it into a new empty document in Thonny or Mu. You can save it as code.py and then run it.

This will cause the knob readings to send out OSC messages. Next, we'll create Sonic-Pi code to read the messages and play music!

Sonic-Pi Script

Here's a Sonic-Pi script that will listen for the OSC values sent from the knob readings, and use those values to adjust the notes, filter cutoff, sustain, and gain.

Copy this and then paste it into a new buffer in Sonic-Pi.