Using a rotary encoder as a volume control for the Raspberry Pi

monitor-volume

#!/usr/bin/env python3

"""
The daemon responsible for changing the volume in response to a turn or press
of the volume knob.

The volume knob is a rotary encoder. It turns infinitely in either direction.
Turning it to the right will increase the volume; turning it to the left will
decrease the volume. The knob can also be pressed like a button in order to
turn muting on or off.

The knob uses two GPIO pins and we need some extra logic to decode it. The
button we can just treat like an ordinary button. Rather than poll
constantly, we use threads and interrupts to listen on all three pins in one
script.
"""

import os
import signal
import subprocess
import sys
import threading
import time

from RPi import GPIO
from queue import Queue

DEBUG = False

# SETTINGS
# ========

# The two pins that the encoder uses (BCM numbering).
GPIO_A = 26   
GPIO_B = 19

# The pin that the knob's button is hooked up to. If you have no button, set
# this to None.
GPIO_BUTTON = 13 
GPIO_LED_BUTTON = 21 

# The minimum and maximum volumes, as percentages.
#
# The default max is less than 100 to prevent distortion. The default min is
# greater than zero because if your system is like mine, sound gets
# completely inaudible _long_ before 0%. If you've got a hardware amp or
# serious speakers or something, your results will vary.
VOLUME_MIN = 0
VOLUME_MAX = 100

# The amount you want one click of the knob to increase or decrease the
# volume. I don't think that non-integer values work here, but you're welcome
# to try.
VOLUME_INCREMENT = 1

# (END SETTINGS)
# 


# When the knob is turned, the callback happens in a separate thread. If
# those turn callbacks fire erratically or out of order, we'll get confused
# about which direction the knob is being turned, so we'll use a queue to
# enforce FIFO. The callback will push onto a queue, and all the actual
# volume-changing will happen in the main thread.
QUEUE = Queue()

# When we put something in the queue, we'll use an event to signal to the
# main thread that there's something in there. Then the main thread will
# process the queue and reset the event. If the knob is turned very quickly,
# this event loop will fall behind, but that's OK because it consumes the
# queue completely each time through the loop, so it's guaranteed to catch up.
EVENT = threading.Event()

def debug(str):
  if not DEBUG:
    return
  print(str)

class RotaryEncoder:
  """
  A class to decode mechanical rotary encoder pulses.

  Ported to RPi.GPIO from the pigpio sample here: 
  http://abyz.co.uk/rpi/pigpio/examples.html
  """
  
  def __init__(self, gpioA, gpioB, callback=None, buttonPin=None, buttonCallback=None):
    """
    Instantiate the class. Takes three arguments: the two pin numbers to
    which the rotary encoder is connected, plus a callback to run when the
    switch is turned.
    
    The callback receives one argument: a `delta` that will be either 1 or -1.
    One of them means that the dial is being turned to the right; the other
    means that the dial is being turned to the left. I'll be damned if I know
    yet which one is which.
    """
    
    self.lastGpio = None
    self.gpioA    = gpioA
    self.gpioB    = gpioB
    self.callback = callback
    
    self.gpioButton     = buttonPin
    self.buttonCallback = buttonCallback
    
    self.levA = 0
    self.levB = 0
    
    GPIO.setmode(GPIO.BCM)
    GPIO.setup(self.gpioA, GPIO.IN, pull_up_down=GPIO.PUD_UP)
    GPIO.setup(self.gpioB, GPIO.IN, pull_up_down=GPIO.PUD_UP)
    
    GPIO.add_event_detect(self.gpioA, GPIO.BOTH, self._callback)
    GPIO.add_event_detect(self.gpioB, GPIO.BOTH, self._callback)
    
    if self.gpioButton:
      GPIO.setup(self.gpioButton, GPIO.IN, pull_up_down=GPIO.PUD_UP)
      GPIO.add_event_detect(self.gpioButton, GPIO.FALLING, self._buttonCallback, bouncetime=500)
      GPIO.setup(GPIO_BUTTON_LED, GPIO.OUT)
    
  def destroy(self):
    GPIO.remove_event_detect(self.gpioA)
    GPIO.remove_event_detect(self.gpioB)
    GPIO.cleanup()
    
  def _buttonCallback(self, channel):
    GPIO.output(21, True)
    self.buttonCallback(GPIO.input(channel))
    time.sleep(0.5)
    GPIO.output(21, False)

  def _callback(self, channel):
    level = GPIO.input(channel)
    if channel == self.gpioA:
      self.levA = level
    else:
      self.levB = level
      
    # Debounce.
    if channel == self.lastGpio:
      return
    
    # When both inputs are at 1, we'll fire a callback. If A was the most
    # recent pin set high, it'll be forward, and if B was the most recent pin
    # set high, it'll be reverse.
    self.lastGpio = channel
    GPIO.output(21, True)
    if channel == self.gpioA and level == 1:
      if self.levB == 1:
        self.callback(1)
    elif channel == self.gpioB and level == 1:
      if self.levA == 1:
        self.callback(-1)
    GPIO.output(21, False)

class VolumeError(Exception):
  pass

class Volume:
  """
  A wrapper API for interacting with the volume settings on the RPi.
  """
  MIN = VOLUME_MIN
  MAX = VOLUME_MAX
  INCREMENT = VOLUME_INCREMENT
  
  def __init__(self):
    # Set an initial value for last_volume in case we're muted when we start.
    self.last_volume = self.MIN
    self._sync()
  
  def up(self):
    """
    Increases the volume by one increment.
    """
    return self.change(self.INCREMENT)
    
  def down(self):
    """
    Decreases the volume by one increment.
    """
    return self.change(-self.INCREMENT)
    
  def change(self, delta):
    v = self.volume + delta
    v = self._constrain(v)
    return self.set_volume(v)
  
  def set_volume(self, v):
    """
    Sets volume to a specific value.
    """
    self.volume = self._constrain(v)
    output = self.amixer("set 'PCM' unmute {}%".format(v))
    self._sync(output)
    return self.volume
    
  def toggle(self):
    """
    Toggles muting between on and off.
    """
    if self.is_muted:
      output = self.amixer("set 'PCM' unmute")
    else:
      # We're about to mute ourselves, so we should remember the last volume
      # value we had because we'll want to restore it later.
      self.last_volume = self.volume
      output = self.amixer("set 'PCM' mute")
  
    self._sync(output)
    if not self.is_muted:
      # If we just unmuted ourselves, we should restore whatever volume we
      # had previously.
      self.set_volume(self.last_volume)
    return self.is_muted
  
  def status(self):
    if self.is_muted:
      return "{}% (muted)".format(self.volume)
    return "{}%".format(self.volume)
  
  # Read the output of `amixer` to get the system volume and mute state.
  #
  # This is designed not to do much work because it'll get called with every
  # click of the knob in either direction, which is why we're doing simple
  # string scanning and not regular expressions.
  def _sync(self, output=None):
    if output is None:
      output = self.amixer("get 'PCM'")
      
    lines = output.readlines()
    if DEBUG:
      strings = [line.decode('utf8') for line in lines]
      debug("OUTPUT:")
      debug("".join(strings))
    last = lines[-1].decode('utf-8')
    
    # The last line of output will have two values in square brackets. The
    # first will be the volume (e.g., "[95%]") and the second will be the
    # mute state ("[off]" or "[on]").
    i1 = last.rindex('[') + 1
    i2 = last.rindex(']')

    self.is_muted = last[i1:i2] == 'off'
    
    i1 = last.index('[') + 1
    i2 = last.index('%')
    # In between these two will be the percentage value.
    pct = last[i1:i2]

    self.volume = int(pct)
  
  # Ensures the volume value is between our minimum and maximum.
  def _constrain(self, v):
    if v < self.MIN:
      return self.MIN
    if v > self.MAX:
      return self.MAX
    return v
    
  def amixer(self, cmd):
    p = subprocess.Popen("amixer {}".format(cmd), shell=True, stdout=subprocess.PIPE)
    code = p.wait()
    if code != 0:
      raise VolumeError("Unknown error")
      sys.exit(0)
    
    return p.stdout


if __name__ == "__main__":
  
  gpioA = GPIO_A
  gpioB = GPIO_B
  gpioButton = GPIO_BUTTON
  
  v = Volume()
  
  def on_press(value):
    v.toggle()
    print("Toggled mute to: {}".format(v.is_muted))
    EVENT.set()
  
  # This callback runs in the background thread. All it does is put turn
  # events into a queue and flag the main thread to process them. The
  # queueing ensures that we won't miss anything if the knob is turned
  # extremely quickly.
  def on_turn(delta):
    QUEUE.put(delta)
    EVENT.set()
    
  def consume_queue():
    while not QUEUE.empty():
      delta = QUEUE.get()
      handle_delta(delta)
  
  def handle_delta(delta):
    if v.is_muted:
      debug("Unmuting")
      v.toggle()
    if delta == 1:
      vol = v.up()
    else:
      vol = v.down()
    print("Set volume to: {}".format(vol))
    
  def on_exit(a, b):
    print("Exiting...")
    encoder.destroy()
    sys.exit(0)
    
  debug("Volume knob using pins {} and {}".format(gpioA, gpioB))
  
  if gpioButton != None:
    debug("Volume button using pin {}".format(gpioButton))
  
  debug("Initial volume: {}".format(v.volume))

  encoder = RotaryEncoder(GPIO_A, GPIO_B, callback=on_turn, buttonPin=GPIO_BUTTON, buttonCallback=on_press)
  signal.signal(signal.SIGINT, on_exit)
  
  while True:
    # This is the best way I could come up with to ensure that this script
    # runs indefinitely without wasting CPU by polling. The main thread will
    # block quietly while waiting for the event to get flagged. When the knob
    # is turned we're able to respond immediately, but when it's not being
    # turned we're not looping at all.
    # 
    # The 1200-second (20 minute) timeout is a hack; for some reason, if I
    # don't specify a timeout, I'm unable to get the SIGINT handler above to
    # work properly. But if there is a timeout set, even if it's a very long
    # timeout, then Ctrl-C works as intended. No idea why.
    EVENT.wait(1200)
    consume_queue()
    EVENT.clear()

monitor-volume.service

[Unit]
Description=Volume knob monitor

[Service]
User=pi
Group=pi
ExecStart=/home/pi/bin/monitor-volume

[Install]
WantedBy=multi-user.target

rotary encoder with push button

# Using a rotary encoder as a volume control

On my RetroPie machine I wanted a hardware volume knob — the games I play use a handful of emulators, and there's no unified software interface for controlling the volume. The speakers I got for my cabinet are great, but don't have their own hardware volume knob. So with a bunch of googling and trial and error, I figured out what I need to pull this off: a rotary encoder and a daemon that listens for the signals it sends.

## Rotary encoder

A rotary encoder is like the standard potentiometer (i.e., analog volume knob) we all know, except (a) you can keep turning it in either direction for as long as you want, and thus (b) it talks to the RPi differently than a potentiometer would.

I picked up [this one](https://www.adafruit.com/products/377) from Adafruit, but there are plenty others available. This rotary encoder also lets you push the knob in and treats that like a button press, so I figured that would be useful for toggling mute on and off.

So we've got 5 wires to hook up: three for the knob part (A, B, and ground), and two for the button part (common and ground). Here's how I hooked them up ([reference](https://ms-iot.github.io/content/images/PinMappings/RP2_Pinout.png)):

| Description   | BCM #                       | Board # |
|---------------|-----------------------------|---------|
| knob A        | GPIO 26                     | 37      |
| knob B        | GPIO 19                     | 35      |
| knob ground   | ground pin below GPIO 26    | 39      |
| button common | GPIO 13                     | 33      |
| button ground | ground pin opposite GPIO 13 | 34      |

You can use whichever pins you want; just update the script if you change them.

## Volume daemon

Since the GPIO pins are just for arbitrary signals, you need a script on the Pi that knows what to do with them. The builtin GPIO library for the Pi will do nicely for this.
 
You'll see the script below, but here's how it works: it listens on the specified pins, and when the knob is turned one way or another, it uses the states of the A and B pins to figure out whether the knob was turned to the left or to the right. That way it knows whether to increase or decrease the system volume in response, which it does with the command-line program `amixer`.

First, make sure `amixer` is present and install it if it isn’t.

```sh
which amixer || sudo apt-get install alsa-utils
```

(If you’re not using the default analog audio output, consult @thijstriemstra’s comment below for some additional steps that you may or may not need to do.)

Create a `bin` directory in your `pi` folder if it doesn't exist already, then drop the script below into it.

```
mkdir ~/bin
```

If it's not there yet, I'd also put `/home/pi/bin` somewhere in your `PATH`:

```
echo $PATH
# don't see "/home/pi/bin" there? then run...
echo "export PATH=$HOME/bin:$PATH" >> ~/.bashrc
# ...and restart your shell
```

Then drop the script into the `/home/pi/bin` folder:

```
nano ~/bin/monitor-volume # (or however you do it)
chmod +x ~/bin/monitor-volume
```

You can run this script in the foreground just to test it out. Edit the script and temporarily change `DEBUG` to `True` so that you can see what's going on, then simply run it with `monitor-volume`. When you turn or press the knob, the script should report what it's doing. (Make sure that the volume _increases_ rather than _decreases_ when you turn it to the right, and if it doesn't, swap your A and B pins, or just swap their numbers in the script.)

You should also play around with the constants defined at the top of the script. Naturally, if you picked other pins, you'll want to tell the script which GPIO pins to use. If your rotary encoder doesn't act like a button, or if you didn't hook up the button and don't care about it, you can set `GPIO_BUTTON` to `None`. But you may also want to change the minimum and maximum volumes (they're percentages, so they should be between 1 and 100) and the increment (how many percentage points the volume increases/decreases with each "click" of the knob).

If it's working the way you want, you can proceed to the next step: running `monitor-volume` automatically in the background whenever your Pi starts.

## Creating a systemd service

_NOTE: If you're on a version of Raspbian before Jessie, these instructions won't work for you. Hopefully someone can pipe up with a version of this for `init.d` in the comments._

`systemd` is the new way of managing startup daemons in Raspbian Jessie. Because it's new, there's not much RPi-specific documentation on it, and to find out how to use it you have to sift through a bunch of Google results from people who hate `systemd` and wish it didn't exist. After much trial and error, here's what worked for me:

```
nano ~/monitor-volume.service
# paste in the contents of monitor-volume.service, save/exit nano
chmod +x ~/monitor-volume.service
sudo mv ~/monitor-volume.service /etc/systemd/system
sudo systemctl enable monitor-volume
sudo systemctl start monitor-volume
```

If that worked right, then you just told Raspbian to start up that script in the background on every boot (`enable`), and also to start it right now (`start`). At this point, and on every boot after this, your volume knob should Just Work.

##  FAQ

**This didn't work!**

I got this working on an RPi3 running RetroPie 3.x, so all I can say is “works on my machine.” Some things to try:

* You might not have Python 3; if `which python3` turns up nothing, try this:

  ```
  sudo apt-get install python3 python3-rpi.gpio
  ```
* I've heard that in earlier versions of Raspbian, the `pi` user isn't automatically allowed to access the GPIO pins, so you need to run scripts like this as root.  If you're running into permissions errors when you try to run the script from your shell, then that's your problem, most likely. There's no particular reason why you _shouldn't_ run this script as root, except on the general principle that you shouldn't really trust code that you didn't write. Make good choices and have backups.
* I might have made a typo in the gist. Wouldn't be the first time.

If you run into trouble, leave a comment and the internet can help you figure it out.