Borg a Furnace


Ahoy !

Recently one morning my wife turned on the furnace and we were quickly inindated with a cloud of smoke.  After the smoke cleared, I looked into the details and found the motor completely burnt.  We purchased a new motor, but in the back of my head I'm always wondering, "How did that happen?"

There was no mechanical problem that I could see.  I don't think the motor stalled.  The Fan and motor both seemed to move realitively easily.   Windings can go POOF if the mechanics involved require too much energy.

I replaced the motor, only to find out the main circuit board (brain) of the furnace refused to turn it on.  I then ordered a replacement from Ebay (its pretty old circuit board).

Pulling the old brain out - I verified it was scorched.  In the process of replacing the brain and keeping track of the 30 odd wires, Carp suggested I "Borg it in"

Brilliant !

So how to Borg my furnace.
I'm thinking using one of the ESP8226 for wifi to circuit control.  From wifi I could borg it into an mrl instance. And from that I could use MRL to provide all the wiz bang stuff the new gimicky thremostats provide.  Like, scheduling, sleep mode, economy mode, remote control via phone..

So here is the beginning of this project - with references to my Y W R G C - control lines - which turned out to be 24 AC  :P

Need a way to control signal 
     probably just a couple transistors with  biasing resistors and a 24Vac

Need a way to sense signal 
     saw a few designs of opto-coupler being driven by 24Vac with resistor

References :

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Mats's picture


Looks like something you could drive with an Arduino relay board and a few other components. Relays are not fancy, but can handle high voltage and current. 30V  DC / 10A, no problem.



GroG's picture

Relays are Great

Ya !
I want to drive it with an ESP8226 for wirelesss borg'ing.
If my memory serves - the typical current out of an Arduino digital pin is ~30 mA .
I've worked with I2C boards which have as little as ~15mA which is not enough to consistently drive the standard relay coil :(

A quick google gives me 12 mA for the ESP8226 ... GAH !  .. so I "must" use a transistor, even if I use a relay board, because 12 mA is completely "whimpy"

So if I have to bother with transistors just to switch the coil, might as well use a transistor which can  handle 1/2 of the 24 Vac.   At least that is my current plan .... - so minimal components 2 Tranistors (TIP120) + 2 biasing resistors - for control of 1 line.

Mats's picture

New furnance brain

I was thinking of using a prebuilt relay board, with optocouplers. You could use a PCF8574 board as a driver. It's an i2c io-expander often used to drive LCD screens, but can be used in many other ways. It can sink 100mA per pin. It can also be used for digital input.

And if you use i2c, ADS1115 ( A/D converter ) can be used for analog inputs, and Adafruit16CServoDriver can act as a D/A converter.  

And the esp8266_01 sketch already contans the code to be able to use all MRL i2c devices on the esp8266,

Very similar concept to what I used in the DiyJoystick:



GroG's picture

I like the idea of esp8266 ->

I like the idea of esp8266 -> io expander -> relay modules
I've had issues with i2c io expander in that the current was not enough to switch even an opto-coupler.

This is the relay board I currently have

calamity's picture

most of the relay (like the

most of the relay (like the one in your picture) use a 5v logic to work, esp8266 pins are 3.3v

it will surely help if you use a relay board that can use the 3.3v logic.


There is some esp8266 board with more pins than the -02 version so you don't need an io expander

I use this esp3266 for some time now ( to control a power outlet relay similar to this one ( and it's working really great. 

I have try at first with a 5v logic relay like you show but it have work some time, then after a while, it begin to work when it want. sometime it trigger it, sometime not


Mats's picture

i2c level shifters

It's also possible to use i2c bidirectional level shifters like this one:


GroG's picture

Ah .. thanks for the Link

Ah .. thanks for the Link Mats.

I didn't know they came in little packages like this ... I'm used to doing the old way with transistors.  
Nice compact package.

I'm refactoring my sprinklers right now - they use the same 8-relay module + raspi + arduino.
Way too much hardware - overkill.

This is a picture of the parts previously together.  The square chrome thing is a 3A 5V regulator - it was discarded from some test equipment.   3A !!  ... ya that's overkill too, but sometimes its good to overbuild power supply.

The sprinklers require 24Vac - but I'm currently using an old discarded laptop powersupply which is 20Vdc - it also feeds the regulator.

I'm think esp8226 + the level converter can replace the raspi & arduino...