Sahko

A simple program for controlling electrical devices by turning GPIO pins on and off on a Raspberry Pi based on the electricity spot price.

Now also includes an experimental web interface for editing the schedules.

Prerequisites

One or more devices you want to control, e.g. a boiler, heaters, etc.
Raspberry Pi with relays connected to GPIO pins so that you can turn the devices on and off, or configure them in some other way
Internet connection on the Raspberry Pi
Rust toolchain and cross for easy cross compilation on your development machine

How it works?

The program is meant to be run periodically, for example every minute using cron.

Each time it is run, it will do the following:

Load the schedules for today. If they don't exist yet, it will fetch the spot prices for today using the spot-hinta.fi API and compute a schedule for each configured GPIO pin based on the prices.
If the current time of day is 17:00 or later, it will do the same for tomorrow's schedules.
Check that each controlled GPIO pin is in the correct state according to today's schedules and turn it on or off if necessary.
If email settings are configured, the program will send an email when a new schedule is computed or when a pin's state is changed.

This design aims for robustness and security:

There's no need to keep the program running and restart it if it crashes or the Raspberry Pi reboots.
No unnecessary writing to the filesystem: the schedules are only written once per day. This will prolong the life of the SD card.
No unnecessary network traffic: the spot prices are only fetched once per day. If the network connection is not available, the request will be retried on the next run.
No user interface: updates are pushed to the user via email.

Setup

Create config.json, see below for reference.

Build for Raspberry Pi using cross:

$ cross build --bin sahko --target arm-unknown-linux-gnueabihf --release

Copy config.json and target/arm-unknown-linux-gnueabihf/release/sahko to the Raspberry Pi.

Edit crontab:

$ crontab -e

Assuming you have the binary and the config in Raspberry Pi's home directory, add the following line to run sahko every minute:

* * * * *  cd $HOME; ./sahko

That's it!

Config

Example config:

{
  "schedules": [
    {
      "name": "Heater",
      "pin": 17,
      "low_limit": 2.0,
      "high_limit": 8.0,
      "min_on_hours": 4,
      "max_on_hours": 18
    }
  ],
  "email": {
    "server": "smtp.yourdomain.com",
    "username": "your_username",
    "password": "your_password",
    "from": "sahko <noreply@yourdomain.com>",
    "to": ["me@mydomain.com"]
  }
}

Schedules

The config file contains a list of schedules, one for each pin you want to control. Optional fields can be set to null or omitted.

name: Name of the schedule, used only for display purposes
pin: GPIO pin (Broadcom numbering)
low_limit (optional): Electricity price limit at or below which the pin is always on, up to max_on_hours per day
high_limit (optional): Electricity price limit above which the pin is always off
min_on_hours: Minimum number of hours the pin should be on per day, when the price is between low_limit and high_limit
max_on_hours: Maximum number of hours the device should be on per day, when the price is below low_limit
min_consecutive_hours: Minimum consecutive hours to keep the switch on in the middle of the day

Email

The email section is optional. If it is present, the program will send updates to the specified email addresses.

Fields:

server: SMTP server address
username: SMTP username
password: SMTP password
from: Sender address
to: List of recipient addresses

Development

If the program is build on some other OS than Linux, a mock implementation of the GPIO interface will be used. This allows the program to be built and tested.

Run tests:

$ cargo test

akheron/sahko