(cloudless) battery sensor for logging temperature + humidity

Since I started this topic over a year ago, I just wanted to say a big thank you to you all for the various replies with a special thanks to @1technophile (also for the impressive work he has done on openmqttgateway) Merci !

I ended up getting the excellent 19€ Olimex ESP-32 gateway board and small Xiaomi Mi Temperature and Humidity (LYWSD03MMC version 2) which I could very easily flash OTA with the flawless ATC firmware
These are cheap 4€ devices that I found on Banggood. It was cheaper to get then per pack of 4, also there are shipped with the CR2032 battery wich honestly last a good 9 months.
I took 20 of them and throw one in each room !

Accuracy is not the best (as expected) but I found a more accurate T/H source s reference and it was possible to calibrate them by manipulating the offset. At the end this solution is good enough for general household T/H monitoring.

MQTTGateway is configured to listen to BLE and in no time I have added them all in OH3 with the MQTT binding + mosquito server.
Config was super easy, when you have one it is just a of copy paste effort.

It turned out that coverage for a very large house is OK with just one gateway. I placed it in a central location and it i can pull from 4 floors including basement. I took the ESP32 with external antenna and the matching metal case (8€) . To my surprise its able to get signal all the way from the garage and even outside (just behind a windows though)

MQTTGateway has also helped me to integrate more Mqtt devices (Tasmota/Sonoff such as TH16 and POWR2 for the hot tub heating control) as well as a plant monitoring sensor (Mii Flora)

The fact that it comes with a LCD screen, allow a quick look at values when passing-by but most importantly data is collected in the background and kept into OpenHAB persistance.
Next year I am changing my central heating system and I wanted to have historical values in order to compare them when the new heating is installed.

20 sensors for less than 110€. I found that this is the most “cost effective off-the-shelf solution”, looks good, high WAF, no soldering, cloudless and easy to setup.

Thanks again

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Just to add another option: $25 for the sensor and $25 for 433Mhz receiver. Howto is here. Configuration is pretty easy. With Ubuntu no need to compile anything… Everything is available through apt.