Making a wireless temperature sensor for heat pump

Projects General Guidance
1

Hi, I’m new to Arduino and could use some help with a project.

The problem I’m trying to solve:

The heat pump in my new home is rather stupidly placed midway up the stairs to my second floor, the colder air from my upstairs bedroom seeps down and causes the heat pump to think it is colder in my living room than it is and causes a temperature variance of +/-4 degrees C. In my previous apartment I put the thermistor on a cable and moved it to a better location with great success, this is more difficult to do here, and the possible placements are too close to a heat source, PC, TV etc. to work. The only prefabricated solution I could find was Sensibo Air Pro, but since this works via IR the placement would not be ideal.

My technical background:

I am a licensed Automation technician, but I worked mostly as a panel builder (unsure about the title translation). I have not worked in the field for 9 years due to health issues. I built automated safety solutions for the energy sector and rarely worked on equipment with a power draw of less than 24VDC 1A, the full system was usually 8-10A at 230VAC. So, while I have a decent understanding of how electronics work, I have little knowledge when it comes to these low power devices. I have only programmed PLC’s via GUI, I have never actually coded anything.

My solution:

I want to make a battery powered sensor that transmits wirelessly to a receiver connected to my heatpump. The transmitter will consist of an Arduino chip, wireless transmitter and a thermistor. The receiver will consist of an Arduino chip, wireless receiver and a digipot. The digipot will be connected in place of the original thermistor via relay (NO) and will emulate the resistance based on information from the transmitter, the original thermistor will be connected to the same relay (NC) to have a backup function in case the relay should lose power for some reason. I have enlisted the help of AI to guide me in a general direction, these are the components I have found that I think will do what I want.
All the links to Mouser.com(supplier) below shows up as "Access to this page has been denied" in the preview. I'm not sure what to do about that.

Reciever and transmitter: ESP32-C3-WROOM-02-N4 Access to this page has been denied.

Transmitter LDO: MCP1700-3302E/TO Access to this page has been denied.

Reciever LDO: MCP1826S-5002E/DBVAO Access to this page has been denied.

Reciever Digipot: MCP4151-503E/MS Access to this page has been denied.

Transmitter Thermistor: NTCLE203E3103SB0A Access to this page has been denied.

Transmitter capacitor: GRM155D70G106ME18D Access to this page has been denied.

Transmitter capacitor: GRM02YR60G104ME01L Access to this page has been denied.

Transmitter charger: https://www.aliexpress.com/item/1005005751609703.html#nav-specification

Transmitter: 1200mAh LiPo battery.

Edit: Based on feedback the thermistor will be replaced with DS18B20, and i will use Seeed studio XIAO ESP32 C3, as this combines ESP, charger and LDO.

A rudimentary block diagram/wiring schematic and original thermistor curve is included in the post. If you need the KICad file(s) instead of just a screenshot just let me know what files you actually need as it created a folder with several files, and i don't know which is which :sweat_smile:

Skjermbilde 2025-10-17 170729
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Thermistor characteristics
Thermistor characteristics442×495 35.5 KB

Heat pump manual: Download Panasonic CS-NZ9SKE Service Manual | ManualsLib

Questions:

  1. Will the proposed components work?
  2. Is there anything I should change?
  3. How difficult is it to program something like this?
  4. When programming the resistance/temperature curve, is it possible to plot in a few points, then the program estimates the rest of the curve? Or do I have to program in every single point?
  5. The capacitors on the transmitter were suggested by AI before and after LDO to smooth out power delivery and reduce power consumption, smoother power delivery I’m aware of, but will it actually reduce power consumption? Is it necessary to have them?
  6. It would probably be easier to code if I had a thermistor in the transmitter with the same resistance curve, but I have been unable to find a 3rd party one that matches. Panasonic sells a set for intake and pipe sensor for a ridiculous price. If anyone knows of a thermistor with these characteristics that would be great, but don’t waste too much time on it.

If you have an entirely different solution just let me know, I am open to suggestions.

2

looks a possible project
I use a CYD_2.8_LCD_TFT_Touch_Screen Module with a DS18B20 temperature sensor to monitor heat pump room temperature, e.g. over 3 days

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the data was saved to SD card for later analysis

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using ESP32C3 microcontrollers you could use local WiFi or ESP-NOW for communication

3

Yes. Look into the Steinhart-Hart equation or the beta equation. Or enter a few points into an online curve-fitting tool, or put them into Excel, graph them, and use its curve-fitting tool.

4

For example, this one:

3 Likes
5

I do not know the area where the thermostat is going but the duty cycle of it will be very low. It might be possible to use a small solar cell to charge the battery.

6

I would use a digital temperature sensor (such as the DS18B20) at the transmitter in lieu of a thermistor. Also, since you are not super-familiar with low power devices, it may be helpful for you to show detailed schematics for forum members to review.

4 Likes
7

Forget about thermistors.
Digital temp sensors are much easier to use and don't require user calibration.
The DS18B20 is one example.

Connect a temp/humidity sensor to your ESP32 and you also have relative humidity available.
I am running HomeAssistant in my home with a range of tiny ESP32-C3 boards and SHT3x or HTU21D sensors. HA displays it on any smart phone/PC and does the automations. HA also does my home lighting and more. It could also directly control/log a WiFi enabled heatpump.
Leo..

3 Likes
8

Thanks for your input. I think i'll be going for ESP-NOW as this doesn't need a network.

9

It's not going to get any sun where it will be placed sadly.

10

Since you will have an ESP board to control the heat pump, that board can be the access point for the sensor, so you can still use WiFi.

11

Yeah, i think i'll go with the DS18B20, it seems easier.
I'll see if i can't put together a schematic within a few days. Usually i just figure it out while soldering :sweat_smile:
Since none seem to object to the rest of the component list i think i'll start ordering.

12

I would recommend Seeed studio XIAO ESP32 C3. Easier to prototype with than the non-breadboard-compatible module you linked to. Also has built-in LDO regulator and Li-ion battery management, for both charging & discharging. I have tested these modules and they consume only around 30~40uA in deep sleep.

13

Is there any difference in complexity, coding or wiring? I don't need any other functionality, i just need the heat pump to get the temperature where i sit so it can heat/cool based on that and not the temperature where the heat pump is.

14

Thanks, this seems like a much better choice.

15

This seems an over-complex solution and I'm not sure if it's needed.

I'm pretty sure the heatpump can only be on or off, there are no low or high power modes that get used in different temperature ranges?

If so, the ESP just needs to turn the heat pump on or off with a high or low resistance. You could use another relay to switch between 2 fixed value resistors which would cause the heatpump to sense a very low or very high temperature, switching it on or off.

16

From what i understand it is correct that it is either on or off. I think your solution would work, but then i would need to interface with the ESP to change the temperature? With my solution the original remote would still controll the temperature, which is preferable.

1 Like
17

No difference in wiring. I'm not sure about code complexity, but I would guess about the same. But WiFi gives you more options if you change your mind later and want to add more features. You could connect to it with your smartphone, for example, to see current and recent temperatures, set daily/weekly on/off or desired temperature in different time periods, add another sensor to monitor when the heatpump is running, estimate total energy consumption....

18

Does the original thermistor circuit use 5V?
If not you may have problems with that pot

19

Yes it does

20

Not sure i'm going to need it, but i do like to keep my options open.
According to the specs, wifi consumes more power, but that would probably only apply to the one acting as a router? It has not been decided where to place it yet, so i don't know how important battery life will be.