Thermostat

2005 Winter

My HVAC service technician advised me to get a new thermostat. The one he recommended cost ~$300. I had been hoping to find an excuse to build a PIC based system with an Ethernet interface, so this seemed like a good opportunity to build a better mousetrap thermostat, at a lower cost than that, and with the features that I wanted.

Here are the specifications that I set out to meet:

• Controls multiple zones
• Supports multiple heating/cooling sources (heat pump, water pump, fan, electric, etc.)
• LCD and simple push button for local user interface
• Ethernet and HTML interface
• HTML/web interface for configuration and user time-based programs
• Time source from NTP (Internet) or onboard RTC clock c/w battery

I had been searching for over a year for a PIC based board with an Ethernet interface. I had almost settled on a board from edtp, when I found the SBC65EC from Modtronix. I ordered two of those, along with some prototyping bits and pieces and very quickly had proven to myself that it would handle all I wanted.

I planned to use the SBC65 as a daughterboard, with the interface as the main card snapped into snap track. The relay outputs were relegated to a separate card; also designed to fit in snap track. For the LCD, I found the LCD025 at BG Micro. They were only US$5 each (subsequently the price went up to US$6), so I bought 4. Imagine my delight when they arrived with another card that had the exact arrangement of momentary switches and LEDs that I was planning to implement! It took a while to reverse engineer the second card, but I didn't have to design nor build that subsystem. I also got some inexpensive reed relays at BG Micro. For the temperature, the plan was to have one temperature sensor on board using a Dallas/Maxim DS18S20, with the other zones and outside temperature fed via a 1 Wire bus. I already had a humidity sensor based on 1 Wire technology, so that would fit in well. Since the SBC65EC already had an I2C interface, I choose the DS2482 chip to interface from I2C to 1 Wire. For the clock, I had initially planned to just use NTP, but I included the Dallas DS1307 on the PCB as a backup.

2005-May-30

Here is a picture of the early stage prototype hardware. From left to right: LCD with switches and LEDS, PIC programmer (mounted on snap track), SBC65EC (mostly obscured by proto-board), and a breadboard. On the breadboard is one relay; you can also see a single DS18S20 rising like a serpents head, supported only by green and red wires. The breadboard also has 2 RF receivers (un-related to this project).. Also visible at the top of the picture is a two-port USB-to-asynchronous interface.

2005-Sep-28

I've built all the custom PCBs and debugged the hardware. Looks a lot neater now! All I have to do is the software and the web pages.