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The 2nd op-amp (feeding the BJT) isn't a comparator - it's a voltage follower - whatever voltage is on the +Vin input gets seen on the emitter of the BJT - notice the negative feedback from the emitter - it's a gain of unity buffer amp. The first op-amp is a camparator as your question states. Page 1 of the data sheet says it all. $V_{OUT} = F_{IN}\times ... 0 I first decided to directly compare voltage levels of any two different phases with the simple circuit above. In order to see what would happen, I simulated the scenario to see the voltage levels. Red: Waveform of R with respect to GND. Magenta: Waveform of S with respect to GND. It looks OK. But there is a moment at which both the phase voltages ... 1 Why don't you just attenuate the voltage on one pair of wires and feed it into an opto-isolator like this: - This picture was taken from here and the major benefit of using an opto isolator/coupler is that you don't have to have your electronics directly connected to potentially lethal AC voltages. This makes them safe to work on and easier to get working ... 1 As the generator gives balanced 3-phase, you can use three large same resistors witch give you the neutral point and then find the frequency from neutral point and one of phases. simulate this circuit – Schematic created using CircuitLab In the previous schematic we divided the$100M\Omega$resistor into$3M\Omega$and$97M\Omega\\$ to ...