GPIO Power Switch

Cut power to a single board computer (SBC) and attached components with a MOSFET, power on and off with a button, indicate power and booted/ready status with an LED.

GPIO Power Switch

• Rationale
• Overview
• Hardware
  • Assembled
  • Schematic
  • Layout
• Setup
  • GPIO shutdown
  • GPIO power key
  • Ready LED

There's two different options, with the difference being whether the ready status LED is switched on with a high or low signal from the GPIO pin.

Rationale

Not all single board computers actually power off when they're shutdown, and many will continue to power attached devices through the 3.3 and 5V pins on the GPIO header.

The easiest way to deal with this is to cut power with a switch of our own, making use of various signals to and from the GPIO supported by the linux kernel to only cut power when the device has shutdown.

Overview

This circuit carries out multiple functions:

• MOSFET SR latch

  When 'reset' the power MOSFET is switched on, connecting the load to ground. When 'set' the power MOSFET is switched off, disconnecting the load from ground.

• Push button

  If the MOSFET is off, when pressed 'resets' the MOSFET, connecting the SBC to ground and causing the SBC to boot. If the MOSFET is on, when pressed signals the SBC to shutdown by sending a KEY_POWER event code via the GPIO.

• Status LED

  Red when the MOSFET is switched on by default, can be changed to green with a GPIO signal to indicate the devices is booted, a particular system service has started, or whatever other criteria is set in software to toggle the relevant GPIO pin.

The basic connection between this circuit and the SBC involves three GPIO pins and looks like this:

Wiring overview

The key thing to note is that the SBC runs off GND_switched.

This switch should work with any device that has a GPIO and supports gpio-poweroff (to send a signal when shutdown is complete) and gpio-keys (to accept a KEY_POWER signal from the push button).

Hardware

Assembled

Assembled (ready high)

Schematic

There's three main sections of the circuit:

• An SR latch handles cutting the power and retaining the state
• A board for the button and LED, to mount on a panel
• An interface between the GPIO and the other components

The latch is common to both the ready high and ready low circuits, but the other sections differ.

SR latch schematic

panel (ready low) schematic

interface (ready low) schematic

panel (ready high) schematic

interface (ready high) schematic

Layout

ready low front layout

ready low back layout

ready high front layout

ready high back layout

In this layout the push button and LED are mounted on a separate board from the rest of the circuit, which allows easy mounting on a panel in an enclosure. Obviously the layout can be changed as needed for any intended application.

Note

The connecting cable between the two boards is not straight-through in this layout. Routing tracks on the prototype board is easier if the two boards are independent of each other in that regard, leaving the cable to handle routing between them.

Setup

GPIO shutdown

The gpio-shutdown signal should be active high. Sending a 3.3V signal to indicate we're shutdown (as opposed to pulling the pin low to ground) makes sense because once the MOSFET cuts power to the SBC this high signal will fall back to low.

This is important because the behaviour of the SR latch is undefined when both set and reset are true at the same time, so we need the gpio-shutdown signal from the GPIO to be low/false when it comes time to restore power, to ensure the push button can reliably toggle the latch.

Note

The GPIO_SHUTDOWN signal requires CONFIG_POWER_RESET_GPIO and CONFIG_POWER_RESET_GPIO_RESTART to be enabled in the kernel, which may not be the case for a default OS install.

An example device tree overlay (from the moonbuggy/Orange-Pi-Zero-GPS-NTP project) is available at overlays/sun8i-h3-gpio-poweroff.dts. In this example we're using pin 21 (PA16) on the Orange Pi Zero.

Reference: gpio-poweroff.txt

GPIO power key

When the push button is pressed it will pull a GPIO pin to ground. Using gpio-keys we can create a device tree overlay to configure this pin as an input key and assign a KEY_POWER event code. (All available event codes are listed in include/uapi/linux/input-event-codes.h.)

An example overlay (also from the moonbuggy/Orange-Pi-Zero-GPS-NTP project) is available at overlays/gpio-key-power.dts. In this example we're using pin 12 (PA07) on the Orange Pi Zero, with 116 being the numeric code for KEY_POWER.

Reference: gpio-keys.txt

Ready LED

Using a dual-colour LED, we have a default 'powered' indicator (red) that is on whenever the power MOSFET is on. A high or low signal (depending on which board is built) from a GPIO pin will switch this colour off and turn the second colour (green) on, giving us a 'ready' signal.

The intention is that the change in LED colour should be triggered by software and indicate that the OS is booted, a service has started or some other specific condition has been met and the device is usable.

The simplest way to manage this is with a systemd service executing gpioset commands at appropriate times. Once again, moonbuggy/Orange-Pi-Zero-GPS-NTP provides an example in the Status LED section. In this application, the device is considered 'ready' once chrony and gpsd are running and we're using pin 16 (PA19) for the signal.

The current limiting resistors will need to be adjusted for the specific LED in use and the desired brightness.