We're indirectly measuring the ac line voltage through a 12 volt ac wall transformer, a resistive voltage divider, half-wave rectifier and rc filter with time constant of about 1 second. (more)

Peak line voltage as a perecent of that expected from rms average voltage. Switching loads "flat top" the ac line.

This unit is placed outside the basement window at the end of a steel tube that holds the sensor several feet away from the house.