use info from https://openenergymonitor.org/emon/buildingblocks/measuring-voltage-with-an-acac-power-adapter
basic schematic:
my article in english: https://arduinotehniq.blogspot.com/2016/10/under-overvoltage-relay-for-ac-main_3.html
articol in romana: http://nicuflorica.blogspot.com/2016/09/releu-de-protectie-la-minima-si-maxima_26.html
