Zero crossing detection is possible using a voltage comparator whith low offset characteristics loaded in the output whith a C-R series with a time constant lower than 1 microsecond (1 nF + 470 ohms, for instance)
You can get a reliable and simple 90 degree's crossig detection by mean of an integrating device previous to a zero detector. Integrator can be performed through the voltage of a capacitor connected in series to a resistor, an both to the 5 volts source (100 nF + 4.7 kohms, f.i.) Julian