Dual level sauna thermometer with wireless connectivity

I have always wondered what the actual temperature difference is between the head and feet level of my sauna. I’ve also been considering to alter the ventilation to make this difference lower, but how would I know if I actually improve it? The answer is a dual level thermometer!

It is of course not enough to just show the temperature on two different levels, I also need to be able to follow it from my sofa while I wait for the sauna to heat up! To be able to do this, I added 2.4ghz connectivity through serial so I can build a second unit which is able to show the same information remotely.

For the logic I used Arduino Nanos as I had those lying around, and for the sensors I used the waterproof version of DS18B20 as that seemed to be the best fit for this. It is able to measure temperatures up to 125c so heat is not a problem either. For wireless connectivity I used a NRF24L01 Transceiver due to the massive range it has. I tried a simpler module I already had first, but the range was horrendous so I opted to order two new NRF24L01s to get the project done.

I still haven’t made the slave unit which will live next to the sofa in the living room, but for now I just log the temperatures every two seconds through a wireless serial connection on my computer. This way I can plot the heat curves later on, and also ensure that the thing works before investing more time on it.

For the enclosure I designed a simple model I can 3D print. It has a print in place transparent window on front of the screen so I don’t get any steam on it when the sauna door is open. The backplate also has brackets for the Arduino where you just click it in place.

The final result.
The starting point: barebones electronics which can show and transmit the information I need.
Here you can see the transparent window being printed on the enclosure.
I decided to not make a wiring harness for this as it is unlikely that I will take it apart ever again.
The electronics are crammed into the enclosure.
I fixed the antenna in place with this special hot glue from Bosch which is designed for plastics. The circuit board was not straight, so I didn’t even try to model a bracket for it on the enclosure.
A piece of aspen will become the frame for the top temperature sensor.
I charred the wood slightly, and made sure that the sensor doesn’t move by applying a little bit of epoxy on the backside of it.
The top sensor is attached with two stainless screws.
It is on the head level where you usually would have a sauna thermometer.
The bottom sensor is on feet level.
It doesn’t need a frame as it is hidden behind the light shade.
6mm holes are drilled to the wall and the wires for the sensors are fed through. The backing plate for the enclosure is attached first so I can cut the wires to the correct length.
The front of the enclosure is glued in place to ensure that there is no steam getting to the inside. The unit is located to the dressing room so the temperatures will not be high, but there is still steam coming through the door when it is opened.
Success! It’s weirdly exciting to follow the temperature while in the sauna.
The wireless connectivity works as well. Now I need to get the slave unit made!

Note from future me: Now the living room module is also ready – https://silfer.works/sauna-thermometer-living-room-module/

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