Mains High and Low Voltage Protector Circuit, Using IC LM 324

The inclusion of op amps makes the circuit operations much accurate and tripping points adjustable to any levels as desired by the user.
As shown in the figure, we can see two OP amps from the IC LM 324 are used for the required detection.

Refer circuit diagram

 The upper op amp has its non inverting input rigged to a preset and is terminated to the supply DC voltage, pin #2 here is provided with a reference level, so that as soon as the potential at pin #3 goes above the set threshold (by P1), the output of the op amp goes high.

Quite similarly the lower op amp is also configured for some voltage threshold detection, however here the pins are just reversed, making the op amp output go high with low voltage input detection.

Therefore, the upper op amp responds to high voltage threshold and lower op amp to low voltage threshold.
For both the detections, the output of the respective op amp becomes high.

Diodes D5 and D7 make sure that their junction produces a common output from the op amp output pin outs. Thus whenever any one of the op amp output goes high, it is produced at the junction of  D5, D7 cathodes.