There are 4 distinct sections, each supervising a parameter pertinent to the quality of the supply line. The noise section consists of a 50Hz filter and a speaker where you will hear the noise present on the line.
The bicolor LED should be adjusted for the least light with the 5k pot and gives a visual indication of noise or asymmetry in the wave. The second section will detect any spike which is overimposed on the mains voltage: adjust the pot so that it will not trip if you just switch on the light, sensitivity is high enough to detect a switching operation from a close neighbor.
The buzzer will beep for about 1sec anytime there is a spike. The actual voltage is detected with section 3: the yellow led will blink at a rate of 6 Hz but will visibly double to 12 Hz for a 10% increase or will come to a halt for a 10% decrease of the voltage. The last section will show the flutter or slow variations of the mains voltage. The circuit will work for a 220V mains: for 230V operation, change the 27V zener to 39V and for 60Hz operation change the 3.9k resistor to 3.3k and the 47k resistor to 39k. Operation at 110V will call for a major redesign of the component values and has not been attempted