HiFiPi – Part 4 – Pins ‘n stuff (1)

Posted: October 24, 2016 in Audio, Hacking and playing, Hardware
Tags: ,

So now the real learning starts. I need to decide what I want the box to do, how to do it and then build it. Easy huh? The code that I reference here is also in my GitHub repo, it’s likely that these snippets get superseded as I figure out how the stuff works. I’m also using the excellent Fritzing to figure stuff out with the wiring, so it may pay to look at that as well if you want to design your own parts.

The other thing to note is that my switch circuits run from the GPIO Pin to GND – making the Pins use the PULL UP resistor in the Python code. If I had run the switches from Pin to 3.3V, PULL DOWN resistors would need to be declared.

As we are access the GPIO, all the python code will need to be run under sudo to gain access.

Use Case 1 – We must be able to turn the player On and Off via a switch

OK so after a lot of Googling it becomes apparent that all the normal method to deal with this – add on boards with switches – are inappropriate because of the X400. As the X400 provides power to the Pi, none of the aftermarket solutions will work.

Use Case 1 (Amended) – We must be able to turn the player On and Off

The answer (in theory) is to use the ‘reset’ switch header as an ON and to add a switch to the GPIO with some code to handle the OFF part.

There is a P6 header on the RPi boards that allows you to reset the device when it is shorted. While absolutely not suitable for a clean shutdown, it will restart the box if it is in a powered but shutdown state. The issue is that this header isn’t populated, so we will need to add some pins prior to connecting a switch of some kind. This means soldering the pin header to the board itself (gulp) and then connecting the new header pins to a switch. So that’s the ON part sorted.

For OFF, connecting a momentary switch to one of the GPIO Pins and then detecting it with some Python to trigger the appropriate shutdown command appears to be the easiest and safest method. This protects the SD card and ensures that everything is clean before shutting down. As power is still applied, we can use the new ON switch to restart when required.

usecase1
Simple shutdown switch
#!/usr/bin/env python
# -*- coding: utf-8 -*-
# Assumes switch is connected to GND - so uses PULL UP

import subprocess
import time
import os
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)

button = 20

GPIO.setup(button, GPIO.IN, GPIO.PUD_UP)

while True:
 button_state = GPIO.input(button)
 if button_state == GPIO.HIGH:
  pass
 else:
  print ("Shutdown")
 #os.system("sudo shutdown -h now") # Send shutdown command to os
 time.sleep(0.5)

Use Case 2 – Play, Pause, Next, Previous & Stop

So after some more Googling it seems that I can use the MPC client (the Music Player Daemon command line bit) to control playback from the command line. As Volumio includes this part, simply issuing commands allows us to control things via the terminal.

 mpc play

mpc pause

mpc stop

Issuing just

mpc

Will return a list of commands you can use.

So we’re back to the GPIO with an additional 5 4 momentary switches, each of which then uses Python to issue the appropriate mpc command.

** UPDATE **

Oops. While working on building the device modes (see Part 5), I realised that GPIO Pin 17 (Board Pin11) is already claimed by the IR receiver on the X400. This means that I have now lost my ‘play’ button. The solution is to remove GPIO 17 from the code and change the GPIO 27 (Board Pin13) from

mpc pause

To

mpc toggle

Which, unsurprisingly, toggles play/pause. The other advantage is 1 less button to fit somewhere on the final case!

usecase2

Play/Pause, Stop, Previous & Next

Python Code:

#!/usr/bin/python
# -*- coding: utf-8 -*-
# Assumes switch is connected to GND - so uses PULL UP

import os
import subprocess
import time
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)

#Define GPIO Pins that have buttons
play_pause = 27 #Play/pause mpc toggle
stop = 22 #mpc stop
prev_track = 18 #mpc prev
next_track = 23 #mpc next

power_off = 20 # sudo halt now

#Configure PULL Up and pin connections
GPIO.setup(play_pause, GPIO.IN, GPIO.PUD_UP)
GPIO.setup(stop, GPIO.IN, GPIO.PUD_UP)
GPIO.setup(prev_track, GPIO.IN, GPIO.PUD_UP)
GPIO.setup(next_track, GPIO.IN, GPIO.PUD_UP)
GPIO.setup(power_off, GPIO.IN, GPIO.PUD_UP)

#Begin loop to wait for button press
while True:
 if GPIO.input(play_pause) == GPIO.LOW:
  print ("Play/Pause")
  subprocess.call(['mpc', 'toggle' ])
 elif GPIO.input(stop) == GPIO.LOW:
  print ("Stop")
  subprocess.call(['mpc', 'stop' ])
 elif GPIO.input(prev_track) == GPIO.LOW:
  print ("Previous")
  subprocess.call(['mpc', 'prev' ])
 elif GPIO.input(next_track) == GPIO.LOW:
  print ("Next")
  subprocess.call(['mpc', 'next' ])
 elif GPIO.input(power_off) == GPIO.LOW:
  print ("Shutdown")
  #os.system("sudo shutdown -h now") # Send shutdown command to os
 else:
  #print ("Nothing")
  pass

 time.sleep(0.25)
Parts List (so far):

Wire – whatever is appropriate to you setting!

2x 1 row pin headers

Momentary contact switch x1 – for ON

Momentary contact switch x1 – for OFF

Momentary contact switch x54 – Media shuttle commands

Links & Refs:

GitHub: https://github.com/nwootton/HiFiPi

Reset Switch – http://www.raspberry-pi-geek.com/Archive/2013/01/Adding-an-On-Off-switch-to-your-Raspberry-Pi

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