JSON format based on Aaron's idea? {"currentType":"DC","unit":"V","value":"-24.120","absValue":"24.129","negative":true}
Check out Homie JSON format. SCPI message format.
The schematic for the RS232 adapter is wrong! See here:
http://www.designelectronics.com.au/DesignElectronics_UT61E-APO.htm
http://gushh.net/blog/ut61e-protocol/
http://kiedontaa.blogspot.com/2016/02/bluetooth-adapter-for-uni-t-ut61e.html
https://github.com/stv0g/dmm_ut61e
ESP8266 software serial with 7-bit support:
https://github.com/plerup/espsoftwareserial
https://www.eevblog.com/forum/chat/ut61e-auto-power-off-modification/
Battery voltage monitoring: clever trick is to put the bottom of a voltage divider onto a digital I/O pin, which is high impedance while the MCU is turned off so no power bleeds. The MCU needs to set the pin to LOW to activate the voltage divider. Idea from:
https://tinker.yeoman.com.au/2015/12/04/esp8266-smart-shelf-part-1/
Comparator to detect rising and falling edge:
https://www.digikey.com/en/products/detail/texas-instruments/TLV7011DCKT/8604643