Agreed. My advice would be to stop, disconnect everything to avoid damage and go back to the drawing board. We can work with you to design a much simpler circuit that will work.
Here are some changes I would suggest:
- Save the Mega for another project, just use the Wemos.
- Save the thermocouple for another project, one where temperatures in the hundreds of degrees need to be measured. Use a ds18b20 sensor for this project.
- Get a DC-DC "buck" converter to provide a 3.3V or 5V supply for the Wemos from the 12V battery. This can be quite small, 0.5A output will be enough for the Wemos and sensors. Alternatively, some LN298 modules have a built-in 5V regulator. Perhaps this can provide a suitable supply for the Wemos, I'm not very familiar with such modules. Hopefully another forum member can advise about this (although they may point out that LN298 is a very old design and much better replacements are available).