A Zigbee Sensor designed to monitor flow and return temperatures. The sensor provides two temperature sensor probes in a single unit. It can be used to monitor a boiler's flow and return temperatures or the difference between the tails of a radiator.
It uses two NTC Thermistors, which can be attached to the flow and return pipes on whatever you wish to measure; radiator tails, boiler flow and return, hot water cylinder.
Battery powered, this sensor should run for over about a year and a half on a coin battery or around ten years on a 2000mAh battery.
Important
This is still a work in progress. I have included a short list of items at the end of this README
- 1 x Seeed XIAO nRF52840
- 2 x 10kΩ 0.1% resistors
- 2 x NTC Thermistors (I use these TT0P-10KC3-T105-1500)
- 1 x push button
In the diagram below, you would replace the two 10kΩ thermistor that are not on the breadboard with the actual probes
This code has been developed using NCS (nRF Connect Software), with pin outs and battery reading chosen for the Seeed XIAO nrf52840. It should run on any nRF52840 with some tweaking. There is an overlay included for the nRF52840DK, which I use for development.
To compile, run the following command in an nRF Connect terminal. The pristine build (-p) flag isn't always necessary.
west build -b xiao_ble -p
The XIAO board can be flashed using two methods. The first is done using the factory installed bootloader. The second is done using a J-Link programmer. I have a J-Link programmer, so that's the method I've been using.
Caution
Whilst this method should work, I haven't have any success! I want to make this project accessible to those without a J-Link, so I will dedicate time to it.
First, connect the board via USB to your computer. Next, double click the Reset button. A new USB drive should appear.
The compilation process will generate a UF2 firmware file and it will be be located here:
zigbee-nrf-flow-and-return-temperature-sensor\build\zigbee-nrf-flow-and-return-temperature-sensor\zephyr\zephyr.uf2
Just copy and paste this file onto the USB drive and the application should start.
If you have a J-Link programmer, connect the board and run this command.
west flash
Once you have flashed the nRF52840 and powered it up, it will enter pairing mode. You can then add it to your Zigbee network using your usual method.
My Zigbee sensor was build with the TT0P-10KC3-T105-1500 probes, as I have already mentioned. These have a nominal resistance of 10kΩ @ 25°C and have a beta value of 3977. The relevant definitions in the code are here.
#define THERMISTORNOMINAL 10000
#define TEMPERATURENOMINAL 25
#define BCOEFFICIENT 3977
#define SERIESRESISTOR 10000
Note
If you want to use a different thermistor, you can change these values. I plan on making these configurable via the project, but for now you will need to change them directly in code.
There several GPIOs configured by this project. These have been picked for the Seeed XIAO nRF52840 board.
| GPIO | Usage |
|---|---|
| P0.03 | ADC (AIN1) pin for Probe 1 |
| P0.29 | ADC (AIN5) pin for Probe 2 |
| P0.28 | Power for the voltage dividers connected to the NTC temperature probes |
| P1.13 | Reset pin. Apply high for 10 seconds to reset the Zigbee configuration |
In order to maintain compatability, this sensor uses standard ZCL clusters rather.
There are three Endpoints 1, 5 & 7
Endpoint 1 contains the Basic cluster, the Identify Cluster and the Power Configuration Cluster. Endpoint 5 contains a Temperature Measurement Cluster Endpoint 7 contains a Temperature Measurement Cluster
I have tested my sensor on HomeAssistant using the ZHA integration and it displays pretty well.
Unfortunately, the probe names are terrible.
sensor.coldbear_fart_sensor_temperature
sensor.coldbear_fart_sensor_temperature_2
I have tried to make a ZHA "Quirk" to display the probes like this:
sensor.coldbear_fart_sensor_flow_temperature
sensor.coldbear_fart_sensor_return_temperature
but I haven't had any success yet making it work how I want. I have included my work in progress in the project.
I've invested a lot of time to try and ensure long battery life and I think I've gotten it to over a year on a coin battery! Average consumption is around 16µA
I've written quite a few blog posts detailing the journey, which are up on my blog. You can start here:
Once powered up, you can add the device to your Zigbee network as you would any other Zigbee device.
Using KiCAD, I have designed a PCB for this code.
A mix of SMD and throughhole mountings, this revision fixes the traces for the reset button (issue in V1) and uses updated footprints for the JST connectors.
Caution
The battery connections on this PCB are just very difficult to make. I've tried on three different boards, but I've not managed to power the XIAO using the battery terminal. If you had a finer soldering iron, you might have better luck.
To work around the battery terminal issue and make it easier to for amateurs like myself to solder, V3 has switched to a socket mount for the XIAO board. I also hope to use pogo pins to make the battery connections.
Note
As of the 15th Feb 2025, I am waiting on the first fabricated versions of this board to verify the socket and pogo pin design will work.
This is incomplete. The goal is to move to SMD mounting compleletely and introduce a CR2032 battery holder to make the unit complete. It's present in the PCB-V4 branch.
Caution
V4 is a work in progress and not ready for use!
All the external connections for NTC probes and battery are via JST connectors.
If you are interested in buying a PCB, do let me know. If there is enough interest, I can place a bulk order.
If you want to support this project, please consider buying me a coffee!
-
Fix reset button PCB traces -
Use reset button to reverse probes i.e. swap readings - Indicate when probes are swapped
- Confirm long-term reliability
- Ensure battery life
