This repository contains a hardware power controller that enforces safe, predictable power-on and power-off behavior for a Raspberry Pi used in real-world automation systems.
It grew out of the work described in Raspberry Pi Internet of Things – Part 029, where the problem wasn’t software, but power: a Pi running irrigation, valves, radios, or sensors cannot simply lose power without consequences. Filesystems corrupt, tasks stop mid-cycle, and batteries get drained dry.
The design documented here takes those ideas further by moving power control out of the Pi and into a small, dedicated controller. The Pi decides when it is safe to shut down; the hardware enforces when power is actually applied or removed. A latching relay does the physical switching, while simple signals coordinate startup, shutdown, and recovery when utility power fails or returns.
The result is a Raspberry Pi that behaves like part of a system, not a hobby board dangling from a power supply.
.
├── Power_Controller_Design_Document_v1.1.md
├── README.md
├── code/
│ └── powercontrol/
│ ├── powercontrol.c
│ ├── makefile
│ └── Doxyfile
└── hw/
└── powercontrol_2.1/
├── powercontrol_2.1.kicad_sch
├── powercontrol_2.1.kicad_pcb
├── powercontrol_2.1.kicad_pro
├── powercontrol_2.1.kicad_prl
└── build.zip
This is the authoritative design specification for this project.
It defines:
- Hardware intent
- Firmware behavior
- I2C command contract
- Extended readback interface (
0xF0) - Design philosophy and explicit non-goals
The firmware runs on an AVR (ATmega8 / ATmega88 class) and provides:
- Latching relay control (pulse-only, never held)
- Manual wake via INT0
- AC_OK wake via INT1
- Red / Green status LEDs
- I2C slave interface (address
0x08) - Watchdog-style host supervision
- Low-power IDLE sleep when inactive
The firmware is single-loop, flag-driven, and avoids performing real work in ISRs.
-
powercontrol.c
Main firmware implementation. This is the source of truth. -
makefile
Minimal AVR-GCC build file. No build system gymnastics. -
Doxyfile
Optional Doxygen configuration for generating internal documentation.
Typical workflow:
cd code/powercontrol
make
make flash
Target MCU, programmer, and fuses are defined in the makefile and are expected to be adjusted per environment.
This directory contains the KiCad revision 2.1 of the Power Controller board.
- +12 V input
- On-board 5 V regulator
- AVR MCU running internal 8 MHz RC oscillator
- ULN2003A relay driver
- Dual-coil latching relay
- Manual wake button
- AC_OK external wake input
- I2C interface with series protection resistors
- Explicit power and ground wiring (no hidden pins)
The MCU remains powered at all times so it can observe wake events even when the downstream system is off.
build.zip contains fabrication outputs (Gerbers, drill files, etc.) suitable for board manufacturing.
- Address:
0x08 - Write-only commands for relay and LED control
- Extended readback via command
0xF0
Example:
i2ctransfer -y 1 w1@0x08 0xF0 r4
Expected response:
0x13 status_byte timer_MSB timer_LSB
| Command | Function |
|---|---|
| 0x00 | NO-OP watchdog ping |
| 0x01 | Relay OFF |
| 0x02 | Relay ON |
| 0x03 | Red LED OFF |
| 0x04 | Red LED ON |
| 0x05 | Green LED OFF |
| 0x06 | Green LED ON |
| 0x10 | Set ping timeout (uint16 ms) |
This project explicitly does not attempt to provide:
- RTC or scheduling
- Network connectivity
- Filesystems
- Soft power control
- UPS functionality
Those responsibilities belong on the host system.
If the host behaves, it stays powered.
If it locks up, it gets cut off and restarted.
If a human presses the button, power comes on.
If AC disappears, the system reacts predictably.
- Firmware / Software: MIT License
- Hardware designs: CERN-OHL-P v2 (Permissive Open Hardware Licence)
Copyright © 2026 Vinnie Moscaritolo