Robin wrote:
The link was for battery powered alarms not interconnected alarms.
That is what I assumed Anderw had fitted.

Sorry, my mistake. I had galloped through from the OP and failed to
spot Andrew's reference to standard battery detectors.

Now the module at the top of the page may work with interconnected
smoke alarms. When I tested the output on an interconnecting wire it
went from 0V to 9V DC when the alarm was activated. The link below
seems to confirm this.

Thanks again. I think I might nevertheless try i.d.c. one of the S300
you originally pointed to as the Kidde alarms work on battery as well
as mains and it would be a neat way of making the connection. And
thanks too to you and Andrew for pointing me to things I never knew
existed .

I suspect that you were correct when you said that a mains powered smoke
alarm with battery backup will not cause the voltage drop needed to activate
the S300 device.

Also you would need one S300 for every smoke alarm in your house (the
interconnect cable is just a trigger not a supply). However the S320 would
only need to be connected to one detector (unless you want the alarm to know
which detector was triggered).

Cheers

--
Adam