With 9VAC (not rectified) you could just use a series connected 33K to 100K resistor. One end connected to 9VAC and the other connected to an input pin or interrupt pin. The zero cross would be close to when the pin changes state. The rise time would be a little late and the fall time would be a little early, but this could be determined and corrected for.
So this is interesting, can't wait to try this out. i have a few 330k resistors lying around. Would that be ok, would there be significant loss anywhere ? To begin with, am guessing the the signal would go into the digital port of the arduino, am i rite on that or is it analog. my mains are 220v at 50Hz. Now to detect it, i should check the state of the pin so that would be a 1 or 0. 1 being high that is when the wave reaches zero, rite ?
Nope the relay was just an example, my load would be a MOT (Microwave Oven Transformer)1100A when shorted and 3v when open. Waiting for a 22Ohn 50W resistor which go in series with the primary.
Zero crossing detection using an optoisolated device like the H11AA1 is easy, safe and inexpensive.
Yea i did come across a lot of them. but was not sure which one to go with and the H11AA1, if am not wrong is outdated and these days the MOC3083M are used. the zero is completely new to me so i'am learning these before i start with the circuit. Since i could not find one i came down to having a resistor in series and just using delay() for a 50 - 100ms would do the job, but i would go for the zero crossing always since i understand a little better now.
One of the members here had made a this and that how i learn to start off with the idea. So he has the one of the 9v ac into pin 3 where he detects the zero cross with the arduino sketch(can't seem to find, guess its in his library, Albert.h mentioned in his site) and fires the SCR with an optocoupler.
So i'll try to figure out the code for the zero crossing. So how much volts on the input of any pin would be ok for the arduino to sense the input.