Take the period of a 50Hz sine wave.
Now convert your "nanoseconds" into the change in angle that it
represents for a signal with that period.
Now take that change in phase angle and calculate the maximum change in
voltage that can occur during that change in phase.
Now tell me how you are going to keep noise on that signal well below
that change in voltage..
How then to do it is fairly simple. Connect your signal through a
resistor to base of a transistor connected as common emitter, with a
diode between base and emitter to protect against reverse voltages. Take
the output pulse train from the collector, with a resistor going from
collector to your desired dc supply.
That will give you your pulse train of zero crossings. If the lag,
before the mains voltage rises enough to switch on the transistor, is
too great, include another resistor to make the base a summing point and
add an offset voltage to decrease the lag to "zero".
You can achieve zero lag time with that circuit - although noise and
thermal effects *will* cause errors - including causing the zero
crossing point to wander. So, if you want to improve that, you will need
extra complexity. If you can hold the temperature of the transistor
reasonably constant, it won't wander much.
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