Hey Glen,

I've just picked up on this thread so forgive me if I've missed something.

What you are doing here is dangerous!  I gather someone recommended
optoisolators for the safety factor but if you think they're too expensive
use a small power transformer with a low secondary winding (say 6.3vac) and
a bridge rectifier (no filter capacitor) and resistive divider or zener to
get your zero crossing points - if the application is mains powered you
could even use your existing supply/plugpack if you insert an isolating
diode in there to isolate the filter cap.

Or you could make your own transformer using some turns around a current
carrying wire - the result in either case being good isolation from the
mains.  Try and stay away from the single big resistor method as only
special types are rated at high voltages (you may fry yourself as well as
your micro...).

In any application your safety and future persons' safety (who may access
the unit) is paramount - with mains there is usually no second chance.  Do
it right the first time - safety first, price next.

>ac---------|>-----|        res
> 240v             |------/\/\/\-----|rb3    (values for 240v)
>ac---------|>-----|        10m
>

Please don't take this as a flame.
Regards,
Bill

W J Sherwood
Senior Electronics Technician
WNI Science & Engineering
31 Bishop St, Jolimont
Perth, West Australia 6014
tel +61 8 9387 7955
fax +61 8 9387 6686