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[on:]

Hello, I need a little help on this topic.

I know you can detect it this way:


But will the internal clamping diodes protect correctly when using 240 volt ac, and there's no common wire to use on ground?

Or will i need to use a rectifier (full / half wave) ?

This is how the 240 household ac looks like:

[Reply #1 on:]

Don't do that at home - or anywhere else.

There's an example of a zero-cross detector as part of a project --
http://arduino.cc/playground/Code/ACPhaseControl

My only quibble is the AC isn't shown fused.  Some would argue two resistors on the input, one on each end.

[Reply #2 on:]

I know the best is to use an optocoupler (I'm currently waiting on my order for a MOC3022 and a H11AA1)

But if i theoretically dropped the optoisolators, would the above circuit need any changes in my case?

[Reply #3 on:]

Let's say the internal clamping diodes are perfect and tough as nails, there would be a HIGH all through one alternation and a LOW all through the other alternation.

[Reply #4 on:]

oh, ok. So i guess it's pretty straight forward then.

Guess i was thrown off by the fact that in every other example of this, they used it with 110 and a common (neutral?) wire.

But won't the alternation causing LOW hurt the circuit in my case, as the polarity change?

[Reply #5 on:]

The point that I was making was that if this scheme was valid, given perfect and tough-as-nails clamping diodes (which is definitely NOT the case), it would still not "zero-detect": Given a 50 Hz cycle, the period of which is 20msec, there'd be a "high" for 10msec and a "low" for another 10msec - it would "positive alternation detect". 

In any event, I do not countenance your proposed circuit.  Just wait for the components you've ordered.