DIY Digital Thermostat

[rdagger]

My girlfriend has been complaining for me to stop working in the garage and replace our faulty home thermostat which controls a typical gas heating and air conditioning system. I’m handy with electronics and figured I could easily impress my girlfriend by building a digital thermostat using components I have laying around.

I don’t know much about HVAC wiring. Can someone give me a quick explanation of the typical wiring?
I think our unit has 5 wires. Am I correct to assume that they are 24V DC+, ground, A/C relay, heat relay and fan relay?
Does anyone know what a typical digital thermostat uses to perform switching (relay, triac, optocoupler)?

[W-Cummins]

Yeah, forget it. You can buy a new programable one for $33 here is a link (http://www.thermostatshop.com/lux.shtml) to a cheap one. I wouldn't even turn on the soldering station for $33 let alone fab a pcb, design the circuit, program the microcontroler, find a nice case, a display, etc....

William....

[rdagger]

Yeah, forget it. You can buy a new programable one for $33 here is a link (http://www.thermostatshop.com/lux.shtml) to a cheap one. I wouldn't even turn on the soldering station for $33 let alone fab a pcb, design the circuit, program the microcontroler, find a nice case, a display, etc....

William....

It's a hobby. I enjoy soldering and working with microcontrollers. This will give me an excuse to learn more about 1-wire protocols, digital temperature sensors, LCD displays and few other possible sensors. Plus the thermostat can be customized to tell me the temperature and humidity in every room and outside. I could also add Internet access to control the HVAC from my cell phone.

[Stuart in MN]

Check out http://home.howstuffworks.com/home-thermostat.htm It mainly explains how a mechanical T-stat works (including the anticipator circuit, which you'll probably want to include in your digital version) but it does have the standard wiring color code. It talks briefly about digital thermostats but doesn't say much.

I'm not sure what digital thermostats use for switching, but if I were building one I'd use relays - those little PC board mountable ones are pretty cheap and reliable.

[rdagger]

Check out http://home.howstuffworks.com/home-thermostat.htm It mainly explains how a mechanical T-stat works (including the anticipator circuit, which you'll probably want to include in your digital version) but it does have the standard wiring color code. It talks briefly about digital thermostats but doesn't say much.

I'm not sure what digital thermostats use for switching, but if I were building one I'd use relays - those little PC board mountable ones are pretty cheap and reliable.
I found some good info on the Robert Shaw (InvenSys) website (http://www.icca.invensys.com/).
It looks like there are three 24V 1A relay wires for the A/C, heater and fan. Then there are 2 power wires from the A/C and heater that are usually combined. Shorting a power wire to a relay wire will activate the corresponding system. Unfortunately, it doesn't look like there is a common wire. Without a ground, I guess a digital thermostat must require a battery or an external power source?

[tedbick]

yes they usually have 2 aa batteries or hooked up to a transformer.

[iiibdsiil]

How about you build em and offer a group purchase? :)

I DEFINITELY wouldn't mind turning my air on an hour before I got home from my vacation or whatever. I know that Smarthome.com offers that X-40 or whatever stuff, but when you add it all up it gets pricy fast.

[rdagger]

How about you build em and offer a group purchase? :)

I DEFINITELY wouldn't mind turning my air on an hour before I got home from my vacation or whatever. I know that Smarthome.com offers that X-40 or whatever stuff, but when you add it all up it gets pricy fast.


The problem with DIY thermostats in the U.S. is that they would not be UL approved and it would probably be illegal to sell. Also it could void your home ownders insurance.

[iiibdsiil]

Dang :(