ZCD Circuit

Can anyone provide me the circuit for Zero Cross Detector. I need to interface it to PIC16F877.

I dont intend to use a amplifier, rather a stepdown transformer, a fullwave bridge and the zener and transistor would be fine. I do know the concept and i think i am getting the circuit right too but i am not able to get the calculation for exact values.

Kindly Help me out with the design for a ZCD


Regards
Sachin Tiwari

Google "zero crossing detector" there are lots of them.

This topic has been covered in the forums on several occasions, no step down transformer is required, all that is needed is a single large value (1meg or more) resistor to limit the current and to chose an I/O line on the PIC that has clamp diodes. The resistor limit the current to the clamp diodes, the I/O line only ever sees the clamping voltage, an isolation transformer can be used if you need electrical isolation.