w. v. ooijen / f. hanneman wrote:

> - include a small relay (powered from the wall wart) which switches
> from battery to mains
> - use a switch: one side to the battery, the other side to the mains:
> doubles as on/off when you pull out the mains.

Michael Rigby-Jones wrote:

> Or you could even use a power socket with an NC switch built into one
> of the contacts, as most fo them have.  Much easier, it's what the
> vast majority of mains/battery appliances use.  (Although their long
> term reliability is not terrific)

  Both these (or all 3) have a problem.  I'm just wondering whether he
really wanted a "no-break" switching to retain its RAM contents?

  My feeling is that you use a (Schottky) diode from the battery and
a bridge (or half-bridge) rectifier from the power transformer which
presumably will produce a somewhat higher voltage than the battery.
It doesn't have to be much higher though, as having the reservoir
capacitor *after* the diodes guarantees that any supply from the mains
keeps the battery diode biassed off.

  This presumes either that the PIC can tolerate the higher supply
voltage or you use a low-leakage regulator and that in either case, low
leakage reservoir capacitor after the diodes.  This is where things
actually become complicated.

  You don't generally need a switch in the mains supply; it almost
certainly uses so little power (or shuts itself down) that you can
afford to leave it running.  If it has no shut-down function, you will
however want a battery on/ off switch, but this can be a simple single
pole.  OTOH, you could place the switch between all the diodes and the
reservoir capacitor.
--
  Cheers,
        Paul B.