Simple hardware interface for the Raspberry Pi Zero

I spent some hours, last weekend, working to another little step into my Raspberry Zero camera project. After experimenting remote software interface possibilities I started building some minimal bare hardware interface in order to, at least, safely turn on and off the Raspberry Pi.

A bare minimum interface

The bare minimum hardware interface for a Raspberry project is made of a power push-button and a status LED telling if the device is still active or it can be safely removed from power.

Plenty of tutorials are available on the Internet, I mostly followed instructions from here and here.

The power status information is directly provided from the UART pin without the need of writing additional software. UART must be enabled in ‘/boot/config.txt’ configuration file

enable_uart=1

Then I just connected a LED diode between UART pin and ground (trough a current limiting resistor of course).

Also the wake-up functionality is implemented by briefly connecting GPIO pin 6 to ground trough a normally-open push button.

The power-off functionality, at last, need writing a simple Python script listening for power button press then calling the shutdown command.

	#!/usr/bin/env python
	import RPi.GPIO as GPIO
	import subprocess
	GPIO.setmode(GPIO.BCM)
	GPIO.setup(3, GPIO.IN, pull_up_down=GPIO.PUD_UP)
	GPIO.wait_for_edge(3, GPIO.FALLING)
	subprocess.call(['shutdown', '-h', 'now'], shell=False)

view raw listen-4-shutdown.py hosted with ❤ by GitHub

I then copied the script in ‘usrl/local/bin’ folder and made it executable

sudo mv listen-4-shutdown.py /usr/local/bin/sudo chmod +x /usr/local/bin/listen-4-shutdown.py

The script must run as root, in order to have shutdown privilege, and must start as the Raspberry is powered on. This can be achieved by writing a start-up shell script that allow the python program to be executed as a service.

	#! /bin/sh
	### BEGIN INIT INFO
	# Provides: listen-for-shutdown.py
	# Required-Start: $remote_fs $syslog
	# Required-Stop: $remote_fs $syslog
	# Default-Start: 2 3 4 5
	# Default-Stop: 0 1 6
	### END INIT INFO
	# If you want a command to always run, put it here
	# Carry out specific functions when asked to by the system
	case "$1" in
	start)
	echo "Starting listen-4-shutdown.py"
	/usr/local/bin/listen-4-shutdown.py &
	;;
	stop)
	echo "Stopping listen-4-shutdown.py"
	pkill -f /usr/local/bin/listen-4-shutdown.py
	;;
	*)
	echo "Usage: /etc/init.d/listen-4-shutdown {start|stop}"
	exit 1
	;;
	esac
	exit 0

view raw listen-4-shutdown.sh hosted with ❤ by GitHub

This shell script can be then installed by moving it in “/etc/unit.d/” folder and activated with the following command

sudo mv listen-4-shutdown.sh /etc/init.d/
sudo chmod +x /etc/init.d/listen-4-shutdown.sh
sudo update-rc.d listen-for-shutdown.sh defaults

What’s next ...

A simple point-and-click camera needs at least one more push-button to take shots and one more, software controlled, status LED to inform user camera is working. So next step will be a more complex Python script to handle multiple buttons and LED.