I want to build a circuit that automatically switch off the power when the AC voltage is at its maximum value. It's to power a coil.
The circuit need to achieve the lowest power factor (to use reactive power only). The goal of this is because I'm studying electronics and I see how this circuit works.
So you are trying to cut off the voltage to an inductor at the peak of the AC voltage waveform? If there's no other components in the circuit then the current at that point is zero due to the 90° phase-shift between voltage and current in an inductor.
If you want to just see how that works, simulate it with a SPICE analog simulator such as the free LTspice program from Linear Technology. That's a lot easier then trying to built it.