>Hi all!
>
>I need to do some simple interfacing to 220V powerlines.
>A PIC16C84 must monitor eight different lights to determine if they
>are switched on or not. All the switches are situated together, so if
>I could find a way to "tap" the powerlines, and feed this to the PIC,
>I don't hafta lay km's of wire to the lights themselves (this won't
>be practical anywaty). I'm a little nervous about connecting mains to
>the PIC. Also, I hafta keep the component count really low, mainly
>for budget-, and space-related reasons.
>
>Anyone have some simple suggestions that could help me?
>
>Thanx!
>Gaffs.

It is possible to sense the voltage directly. Use a couple of high-value
(several meg) resistors in series to a PIC input. Senses only the positive
half-cycle, of course; the internal diode clips the negative one. However,
there are safety issues here; doubtless you know about the potentially
fatal danger of having your logic circuitry share a common with the power
line. One safe way is to use a special optocoupler that HP makes for
providing a logic output from a power-line input.

Also, monitoring the switched voltage doesn't tell if you if a lamp has
burned out. Can you steal a few milliamperes from the lamp current to feed
the optocoupler? That would be more fail-safe.

If the lamp current is high enough, it might be possible to use a clamp-on
inductive sensor to sense lamp current and provide isolation at the same
time, but that's probably a more expensive approach.

Reg Neale