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Arduino Thermostat

There doesn't seem to be much information on the net regarding how to design your own thermostat, so I'm publishing my design notes here.

Old Thermostat

My old thermostat had three relays, model HFD2/003-M-L2-D Datasheet. Note that they do not sell the exact same model on Futurlec. The model in my thermostat is LATCHING, with two coils. You send a pulse to one coil to turn the relay on, and a pulse to the other coil to turn it off. The PCB refers to one as “SET”, and the other as “RESET”.

This thermostat came with what appears to be an NTC Thermistor, that is, as the temperature increases, resistance decreases. At 70F the resistance is about 70k. Hopefully a simple voltage divider circuit will allow me to map a voltage to temperature. I plan on purchasing a different sensor, but I'm going to use this until the other arrives.


Multiple Temperature Sensors

Intelligently Control Fan

Run Fan when heat/ac have not run in a while

Run Fan when there is a difference between temperature sensors

Proximity Sensors


Let's see… can this be done cheaply? I'll estimate the cost:

Quantity Item Price Total
3 DPDT Latching Relay $2 $6
1 Arduino $20 $20
2 DS20B18 Temp Sensors $5 $10
1 LCD (16×2?) $10 $10
1 PCB Mount Connector $1 $1
1 Some sort of case $10 $10
2 Cases for temp sensors $5 $10
1 Misc transistors, res, cap, etc $3 $3
1 Real Time Clock $5 $5

The total comes to about $75. That's cheaper that any nice programmable thermostat

electronics.1242449434.txt.gz · Last modified: 2009/05/15 21:50 by justinhomi
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