Keith Dowsett wrote:

> I suggested (by e-mail) using a small capacitor to couple an opto-
> isolator to the mains. (along with a diode or three to protect the
> opto). This dissipates hardly any power.

  Indeed it does.  Unfortunately, it doesn't do one rather important
thing!

  (Hint: Look in the Subject line.)

  Cheers,
        Paul B.