James Cameron wrote:

> I've never done it, but I've seen described a simple high voltage
> capacitor, then a diode forward biased, then into a 5V2 zener with an
> electrolytic capacitor across it.

  Oh dear, dear.  Let«s see if I can fix this:
 
                                       +5V          0V
                                        O           O
                                        |    +      |
                                        +----] [----+  Electrolytic
    240VAC             | |              |           |
      O-----VVVVV------| |---+--|>|-----+----]<|----+-----O
                       | |   |                      |
                             +---|<|----------------+
  active  resistor  high vcap  diodes       zener       neutral

  The resistor is to limit the inrush current and burn up if the
capacitor fails.
 -- 
  Cheers,
        Paul B.