What is it?

ESP32-S2 board with a 3xAA battery holder. Ultra low power consumption components. Has onboard temperature and humidity sensor, but just about anything can be connected to it. Most GPIOs are exposed through headers. There are three ADC inputs as screw terminals, these can also be used as CT channels to connect Current Transformer clamps to measure power.

Why did you make it?

I need to control for risk of mold in areas that are hard to access and with no power, like crawl space, attic, storage building, etc. I wanted a board to measure temperature and humidity, but also be able to hook up other sensors and functions. I only need to measure once an hour, and sleep the rest of the time, which means this board will run for more than 3 years (tested with Energizer Ultimate Lithium and Amazon Basic Alkaline High Performance, will include test result and calculations in a few days)

What makes it special?

There are several boards powered by batteries out there, but who knows if you can buy them years from now. This board runs on 3xAA batteries that you can buy in any store. The battery holder makes it solid and no enclosure is needed. This board has "awake power", which means you can hook up other sensors and equipment that will only be powered during awake state and will not consume any power during sleep state.

How to use it

The board comes installed with code for ESPHome API and ready to use with e.g. Home Assistant. There is also code for ESPHome MQTT available (will make it run for 5+ years!) and also Arduino code on my Github. More example codes will be added. You can also upload your own code. Using OTA is possible, but since it will be sleeping most of the time, you will first disable sleep, wait for the next awake or press reset to keep it awake.

Hackster.io wrote about it, note that they misunderstood OTA, OTA is possible but as described will need some tweak.