Robin wrote:
We have a burglar alarm with a couple of spare "zones".
Unfortunately, when we had our fire alarms fitted (mains powered
and all interlinked)

snip

(It detects this by the increased
current draw of the smoke alarm siren.)

snip

You can still get them. Bottom of this page

http://www.hoyles.com/acatalog/inter...ys-timers.html

I assume this requires the main supply to the alarm to be removed else
the unit won't (or might not) see the current draw. But does it work
with just one module in one of the interconnected alarms? I can't see
why it would not but there's a lot I can't see.

(Not a hypothetical question as I've got interconnected mains powered
Kidde smoke/heat alarms and plan to fit a burglar alarm next year.)

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

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.

http://www.edcheung.com/automa/smoke_det.htm

It should be possible to use the 12V module to link interconnected alarms to
the burgular alarm.

--
Adam