@bjacobse
reason behind 1mhz mcu is to run it on lower voltage but switching between speed still need higher voltage. if i m not wrong!!!

Life is never easy and definitely not straight, there is always something wrong perhaps back to educating myself...

it would be interesting to see what sleep function does, may be need to read library code to understand this in more detailed.

your work gave me starting point but there is lot to learn/read about this.... thank you for sharing,

further update:
while playing with 1mhz bootloader and feeding arduino to raw pin from 18650 battery, sensors is successfully sending battery voltage and battery percentage but no temperature sensor reading. to prove that something isnt right with 1mhz bootloader i burned 8mhz default bootloader i get battery voltage, battery percentage and temperature reading. can not think of something obvious

Link to 1mhz bootloader

this might help
MQTT Basics – 14:34
— Kevin

@skywatch great video and thank you for sharing,
@bjacobse i was thinking something similar to preheat pcb and solder with very low airflow from hotairgun

@Fanfan 20mA during transmission is higher then mine but then i have cheap chines version of multimeter may be its not very accurate as yours. are you using any low clock bootloader?

Power consumption with onboard regulator and onboard LEDs removed without any further modification:
18.11 mA startup
66.9 uA during sleeping
3.01 mA during Transmission

Pro mini with 1mhz bootloader and on board regulator/LEDs removed:
14.42 mA startup
7.50 uA during sleeping
1.40 mA during Transmission

i think Weller WS81 is soldering iron but i m looking for hot air version for smd components.

@yveaux any specific model?