On 21 Nov, 21:28, The Natural Philosopher
wrote:

Depends which shelf. Needs about 50VAC IIRC.

_Intended_ to use 75V AC. Actually _needs_ about 20V AC (personal
experience, but this will vary considerably).

It won't ring with a DC voltage, as the bells needs AC to ring - with
DC they'll just go "ting". Even if an electronic squawker might
respond to DC, as a "ringer" it's wired into a circuit with a series
capacitor. Phones naturally have 50V DC on the line, so a ringer needs
to be insensitive to this!

The old GPO way of arranging local ringing was a relay and mains
transformer to switch AC. You can find these sets on eBay. Another way
would be a hand-cranked generator 26, again easy enough to find on
eBay (tenner-ish, but watch postage as they're heavy).