On Tue, 13 Oct 2020 17:08:56 +0100, "Brian Gaff \(Sofa\)"
wrote:

The issue often is those who just hold their finger on the bell. The ting
noise would only sound when you took your finger off. I get this with my
door intercom.

It's how ours works as well but most people just seem to press it
'normally'.


Many years ago in Radio Constructor, somebody modified an ordinary door
knocker to operate a chime. It used a read switch and a magnet as I recall
and was probably far more complicated than you would want involving two
power supplies and a relay.

I was thinking there could be room for something driving a sounder,
powered by a cap that's charged by the main unit?

6V from the main bell / battery charges a cap via a resistor (and
diode) that keeps the current down so that it doesn't do the first
'ding' when you first hook it up.

Cap get's to 5.4V and has enough capacity to drive a 5V buzzer or
sounder for a second or so.

As you press the doorbell the supply line is shorted and rings the
bell (the 'ding' part) and at the same time something (op amp,
LMC555?) sees the supply voltage go lower than the cap voltage and
toggles it's output for half a second or so (monostable) driving the
buzzer?

Or to remove the 100uA parasitic load of the LMC555, a micro DIL
relay, triggered by the pushbutton from the cap supply that connects
the timer that is also latched on whilst the sounder sounds and then
releases itself?

Or something ... ? ;-)

Cheers, T i m