I made a second RuneAudio player this weekend.. The main difference, apart from the physical box, is power management.

On my first setup, toggling the switch abruptly cuts power to the Raspberry, which can cause filesystem corruptions on the MicroSD card (and experience shows, in just ten days, that the MPD database sometimes ends up corrupted).

So I wanted a system that would allow me to turn the Raspberry on by switching the switch, and turn it off cleanly by re-switching the same switch, and consume zero watt when off.

The simplest solution I have found is a switch capable of controlling two separate circuits, such as this one or that one (look for DPST, “Double pole single throw”).

One of the circuits of this switch will cut power to the power supply and will be setup in parallel with a Raspberry-controlled relay, while the state of the second circuit, connected to one of the Raspberry’s GPIO pins, will be monitored in software, and trigger a clean shutdown.

In the diagram above, the main power is on the blue and brown wires; the 5V on red and black; the relay control wire is the orange one, and the second switch’s circuit is in violet.

We’re going to write a small Python script that will start by setting GPIO 5 to output mode, then pull it high, which will close the relay. Then, we’ll configure GPIO 27 as input, then read its state in an infinite loop. Depending on the state, we’ll either continue to wait or start a proper shut down.

# Cat /root/power-control.py
import RPi.GPIO as GPIO 
import os 
import time 
import sys 

GPIO.setmode (GPIO.BCM) 

#setup the relay control pin to high, so the relay closes
GPIO.setup (5, GPIO. OUT) 
GPIO.output (5, GPIO.HIGH) 

#Setup the switch monitoring pin
GPIO.setup (27, GPIO.IN, pull_up_down = GPIO.PUD_UP) 

#Monitoring loop 
while true: 
    state = GPIO.input (27) 
    if state == True: 
        #Switch is still ON 
        time.sleep (0.5) 
    else: 
        #Switch OFF, trigger shutdown
        os.system ("/var/www/command/rune_shutdown poweroff")
        os.system ("shutdown -h now") 
        sys.exit (0)

Two remarks:

• Depending on your cabling and the type of relay, the GPIO.HIGH may be LOW. Adapt to your setup.
• We never re-open the relay. This is done automatically, late in the process of the Pi shutdown, and this has the effect of cutting power immediately, so we don’t want to do it earlier.

We’ll now register this script as a Systemd service so that it starts automatically on boot:

# cat /usr/lib/systemd/system/power-control.service
[Unit] 
Description=Power control 
After=multi-user.target 

[Service] 
Type=idle 
ExecStart=/usr/bin/python /root/power-control.py 

[Install] 
WantedBy=multi-user.target

And finally, we activate and start it:

# systemctl enable power-control 
# systemctl start power-control 
# systemctl status power-control

The remaining software configuration is basically the same as in this first article: configuring RuneAudio, a network music player. There are also numerous good english-written howtos on the subject on Internet.

A few pictures of the physical setup, just because I like it: