Shutting Down A Raspberry Pi

You can probably tell from the other posts in this blog that I'm a big fan of the Raspberry Pi microcomputer and have used it for several of my projects, usually without such niceties as a keyboard, mouse or monitor.  Using it in this way raises the issue of how to shut it down nicely so as not to avoid corrupting its boot disc (i.e. the SD card) by switching off the power suddenly.  My standard approach to this is to use a gpio pin to look for a signal to shut down the system.  I also find it useful, or at least comforting, to use another gpio pin to make an LED flash to indicate that the Pi is running and another one to light another LED to indicate that shutting-down is in progress.

When the Pi starts up, it runs a Python program called 'ShutdownCheck.py'.

ShutdownCheck.py assigns two gpio pins to be outputs, one for a red LED, one for a green LED, plus a 3rd gpio pin as an input.  The input pin is pulled high by the SoC's internal pull-up resistor, and can be pulled low by connecting it to ground through a simple momentary action switch, such as the type found in obsolete mice an on the front panels of outdated PCs that you found in a skip.  The LEDs' anodes are connected to output pins, their cathodes through a common resistor to ground  ShutdownCheck.py toggles the state of the output pin with the green LED and checks the input pin.  If the input pin is still high, it goes to sleep for a few seconds before looping back to toggle the green LED and check the input pin again.  If the input pin has gone low, i.e. the switch has been actuated, then the program sets the red LED's output pin high, sets the green LED's pin low and issues the command 'sudo shutdown -h now' - which will shut down the system.

A small variation on this, which I used recently because I had some 3-colour common-cathode LEDs, was to have the green & blue LEDs light up alternately until the shutdown request is received.  The python code for that version is shown below. 

# Python Script to check for shutdown request, alternate blue
# and green LEDs when running and light a red LED when shutting 
# down.  When red LED goes out again, Pi can be powered-off.
# Works well with a common-cathode RGB LRD.

import RPi.GPIO as GPIO
from os import system
from time import sleep

# set up GPIO pins to check for shutdown request
# and to light up LEDs.  Alternating Green/blue LEDs when active
# red LED during shutdown.  Note sdcheck is active
# low, i.e. grounded through switch to shut down.

sdcheck = 26
redled = 19
grnled = 13
bluled = 6

GPIO.setmode(GPIO.BCM)
GPIO.setup(redled,GPIO.OUT)
GPIO.setup(grnled,GPIO.OUT)
GPIO.setup(bluled,GPIO.OUT)
GPIO.setup(sdcheck,GPIO.IN, pull_up_down=GPIO.PUD_UP)

green_on = 1
blue_on = 0

while True:
	green_on = (green_on+1) % 2
	blue_on = (blue_on+1) % 2
	GPIO.output (grnled,green_on)
	GPIO.output (bluled,blue_on)
	if not (GPIO.input(sdcheck)):
	  print("Shutdown request received")
	  GPIO.output(grnled,GPIO.LOW)
	  GPIO.output(bluled,GPIO.LOW)
	  GPIO.output(redled,GPIO.HIGH)
	  system("sudo shutdown -h now")
	  break
	sleep(5)

I inverted the logic of the shutdown request (i.e. low input, not high, is interpreted as the command to shut down) as it does not need to use any of the powered pins that would be necessary for the 'normal' protocol, just the gpio pins and ground.

I've settled on using the four (or five for the r,g,b version) pins at the bottom left corner of the gpio header, as seen with the USB ports at the bottom and the gpio header on the right, as shown in the illustration right. Below is a photo of a switch plus red & green LEDs built onto a small header plug.