I've been reading the HIGH and LOW voltage detector circuit mentioned here . I have a similar idea to sense the voltage of a load where the open voltage is 32vac and need to detect a drop in that. The circuit i am trying to achieve. The Load here is a MOT (Microwave Oven Transformer) rewound with 3 turns of 25mm2 copper cable to form a spot welder. The opamp circuit detect the voltage at the primary which is 33vac and when the secondary are shorted the voltage drops to 26vac. I need to detect this drop/change in voltage.