emulate air conditioner's thermistor to control it

Hi there,

I am working on a project to control my air conditioner; It is a Daikin and doing it through the IR is impossible (200 byte undocumented frame that is check-summed), so i decided to do so through removing the air temperature sensor from the inside unit and having a digipot emulate the resistance. By setting higher or lower values i would tell it to either "run harder" or come to a stop.

I bought a DS1803 digipot and have it working nicely on my breadboard.

I must admit I am weak on the hardware side.

I tried to connect the two wires that remained after I removed the built-in air-conditioner thermistor to the H0 and W0 (wiper) of the DS1803. That results in my arduino crashing..

Please help me with the correct wiring, I suspect there is more to it than just linking the H0 and W0 to the thermistor wires.


You need to first of all connect the ground on your AC (air conditioning) unit to the ground on your arduino.

Then before you connect anything else up you need to measure the voltage between ground and those two pins. If it is 5V or less then it is safe to connect them. If it is not then you have a lot of work to do.

Thanks Mike,

As I mentioned, I am "hardware challenged", please bear with me.

My DMM (on DC scale) shows milivolts when I touch the Arduino GND and one leg of a LED on the air-conditioner (this is the only GND I have access to without further disassembling the unit), so it is within the range 0-5V as you mention.

I have the 0-100K version, but it actually reads 0 to 84 with the DMM.

I picked this one as it suits the Daikin thermistor range of 7 to 67 Kohms for 50 to 0 degrees C (I have the datasheet for it)

I plan to run calibration tests at a later stage and eventually come up with a lookup value that corresponds to desired temperature <–> DS1803 setting

so it is within the range 0-5V as you mention.

I said connect the ground on you unit to the ground on the arduino. Then measure between the ground and the two points where you disconnected your original thermistor.


ground line on air-conditioner and Arduino GND give me a fluctuating 0-0.15 V (looks like a sine)

ground line on air-conditioner and the points where original thermistor was give me same values, again fluctuating in a wave manner.

Voltage between the thermistor wires is steady 1.7V

Thanks :slight_smile:

Fine that should be good to go if you connect the grounds together. :slight_smile:

and that's all?

how do I wire the thermistor wire ends?


Sorry you then wire the wiper and one end of your digital pot to where the thermistor was. The reason it crashed before was probably the lack of a common ground.


I just burned a digital I2C temperature sensor I had in place for calibration purposes..

an investigation showed that my previous measurements were wrong.. it is 10V DC flowing between the GNDs :frowning:

I had a series switch to flip the AC thermistor input between the original thermistor and the DS1803 and when I fliped it to DS1803 the 10V showed up.. burning my temperature sensor but luckily all other survived.

I am afraid I reached the point where you say "you have a lot more to do" :slight_smile:

please help me with sorting this out

Well you have two choices, isolate your controller or get in some buffering.
Possibly the simplest is some buffering. Get a logic level FET and feed the gate with a smoothed PWM signal. Keep the common grounds, pur the source to ground and the drain to the hot end (the higher voltage end) of your thermistor. You are now running the FET in a linear mode where it is acting as a voltage controlled variable resistor.

Mike, thanks for all your support

your last post put me into a two day educational reading. here is what I came up with, please comment if that would work:

higher resolution here:

any suggestion is welcome, I am very weak at electronics. Am I missing any components like resistors, capacitors? I plan to put this up exactly as sketched.


wow, I am just blindly following Mike's instructions :-[

I will check that before i damage anything else, especially in the cold December ...

I had hard time figuring out if the thermistor has one leg to the ground :cry:

Checking the small PCB3 that has the connection to the thermistor I am interested in emulating reveals nothing but a number of cable connecting to the main PCB.

So I used my DMM to measure the resistance from one end of the thermistor cable to the board's ground and it is low - 2KOhm; the other end reads something like 20KOhm.

I suppose the lower resistance end connects to the ground.

Another thought I had: What if I power the arduino from the 12V on the AC's electronics? Will that resolve my troubles and will I be able to use my DS1803 instead?

Thanks guys, I am mostly software person and this is really interesting and new to me (read: please bear with me)

Richard you said:-

Have you already established that one side of the A/C’s thermistor is grounded?

I thought we had by the measurements taken between the two grounds, martin reported:-

ground line on air-conditioner and Arduino GND give me a fluctuating 0-0.15 V (looks like a sine)

mind you he also reported:-

so it is within the range 0-5V as you mention

but was later to retract this.
I suggest this is checked again.
Martin - 2K is not a low value to ground is is comparatively high resistance and suggests that you don’t have a single ended thermistor input. Unfortunately that picture doesn’t give us the information we need. Running the arduino from 12V if available from the AC unit will give a common ground automatacally so maybe that is the best way to proceed. You must disconnect this when you plug in your USB lead however.

This has been very educational for me.

Here is some voltage information.

I think I am slowly understanding how i budned the other temperature sensor on the I2C bus, I am sure it has to do with the 12V floating between thermistor ends and GND. The DS1803 has resistance of around 85Kohm so this is why I was reading 10V after i wired my stuff (the wrong way)

Feedback and how to resolve this is welcome :slight_smile:

i think this is getting too complicated for me to resolve on my own;

I will probably go for a servo motor turning a normal potentiometer solution

I am sure for experts like you this is piece of cake but for me is a real challenge.

I am afraid not to damage my AC..

I also dislike the motorized pot solution but it seems that I am only capable of such low-tech solution (learned by burning some components )

I will gladly take advice on how to make my DS1803 work with the layout I have.

Mike, Richard help please :slight_smile:

I just need a hint on how to do the wiring of DS1803

I am an AC engineer who works mainly on Daikin equipment. I am affraid to tell you that you are way off the mark. Controlling the thermistor the way you plan will not do what you think it will. The Daikin equipment (especially inverters) are a lot more complicated than a simple on/off system and the thermistor is only has a small part to play in the operation of the unit.

If you mess with the value of the thermistor the unit will most likely go in to fault. It compares the thermistor value with other sensors throughout the system to maximise performance and efficiency. If you change the value of the thermistor then it will not be in the range expected according to the other sensors, so it will stop the machine and bring up a fault (probably room thermistor, but could be another fault depending on conditions).

The only real way to do what you want to is either modify the data on therminal 3, which is risky or make or buy the interface board for remote control. There is a plug on the indoor PCB that wont have anything plugged in, that is for the interface board.

I can try and get you a datasheet for the interface board if you want, but you would need to be pretty good at reverse engineering software to make one.

EDIT: If you send me the complete model number, i´ll send you the service manual which has better schematics and thermistor values in.

Thanks for your reply.

I have the FTXS71FV1B. Actually my latest idea is to use the existing thermistor and connect one digi pot in parallel and one digi pot in series to it.

This way I can add/substract small resistance from the current reading thus forcing the unit to work harder or ease depending on the mode selected (heat/cool)

I am also interested in the remote control board that you mention, would you please recommend me one? where to read more about it/buy?

Great AC's by the way :slight_smile: