220v AC Voltage analyzer.

i would like to build a circuit that will monitor house voltage, and trip off a relay if voltage drops below ~200VAC, if possible (but not critical) to also drop if voltage is ABOVE 250VAC.

response time should be somewhat fast,
(for comparison the reaction speed of an offline UPS switching to battery generated power is enough.)

i do have some experience in building circuits, but i am really black about this....
any ideas from what should i even start???...

thanks in advance to all helpers!!!
Ron.

You would start with a small transformer to isolate your circuit from the power line. Full-wave rectify the AC from the transformer and average the rectified voltage with an RC filter (this voltage is separate from the peak rectified voltage that would typically power the circuit). Run this average voltage to a window comparator whose trip levels are selected to activate the relay between 200V and 250V line voltage.