systemd-gpio-monitor

Systemd-based monitoring of a GPIO pin (for Raspberry Pi).

The basic idea here is to monitor an input pin at the end of the Pi's main GPIO header for a short to the GND pin next to it and initiate an orderly shutdown. This is useful to shut down a headless Raspberry Pi without having to log in through the serial interface or ssh: just touch a paper clip to the pins at the end of the GPIO header. NOT THE 3V3 / 5V END!!

I had originally done this with a small python script but wanted to achieve it with minimal resources. Hence using only systemd and no other shell or script-based polling of the pin's value.

It works by setting up an input pin with internal pullup, assigning a falling edge detection interrupt and monitoring the pin's value through sysfs using a systemd.path unit.

This can easily be adpated to perform other tasks when a GPIO pin change is detected.

Dependencies:

Since I couldn't find a way to set an internal pullup directly through sysfs, I've reverted to using gpio-admin, a program from quick2wire.

Usage:

1. Copy all systemd unit files to /etc/systemd/system.

2. Enable and start monitoring:

• 40 pin header:

  # systemctl enable --now [email protected]

• 26 pin header:

  # systemctl enable --now [email protected]

You may want to first test the action without actually shutting down:

1. Edit [email protected] to echo the poweroff command.
2. Monitor the activation service:

• 40 pin header:

  # journalctl -f -u [email protected]

• 26 pin header:

  # journalctl -f -u [email protected]

Improvements are certainly welcome.