This is a Raspberry Pi based thermostat. The word "hat" as applied to a Raspberry Pi includes an EEPROM with the proper device-tree overlay and driver code on it. Since I don't know how to do that yet, this is just a "cap". As usual for a project that is in development, a lot of the text below is more of a "what we're trying to be" rather than a "what it is already". Futuristic optimism follows.

The printed circuit board and corresponding drivers and application code are designed to work with a Raspberry Pi model A+ and a PiTFT frame-buffer resistive touch-screen LCD display (Adafruit product ID: 1601). I'm sure other Raspberry Pi models will also work, and probably any LCD display that can be driven with the PyGame library at 320 x 240 resolution.

The circuit contains three main elements, wait, 4... four main elements... I'll start again.

The circuit contains four main elements:

• Raspberry Pi power supply (5 volts 1.25 amps) derived from 18 to 24 volts AC, or 9 to 35 volts DC
• six GPIO-controlled mechanical relays configured like a conventional thermostat
• a temperature sensor with good resolution
• a real time clock (RTC), with backup battery (CR1220)

For the full hardware description, see the README.pdf in the hardware folder.

What does it do?

A thermostat is a relatively simple device, it turns on equipment when the temperature exceeds high or low limits. There are generally three controls, the setting of the temperature (dial), a switch for selecting "Heat", "OFF", or "Cool", and a switch for the fan "On" or "Automatic" (whenever the system is running). Adding a computer to this allows scheduling of target temperatures, recording of temperatures and equipment usage, and additional display functions.

This thermostat will drive a standard household furnace (or heat pump) and/or air conditioner with the same functionality of a standard old fashioned mercury bulb thermostat but with the computational power of a Raspberry Pi computer. Three heat stages (or a heat pump with two heat stages) and two cooling stages are provided. Mechanical relays are used which allow nearly universal functionality with any system.

What else needs to be said here? Hmmm.... dependencies...

• WiringPi2 library, available from https://github.com/Gadgetoid/WiringPi2-Python

Please direct inquiries to github-spam-trap@mdve.net

--Roger