Take a look at the beginning of page 2 of this pdf. Looks like it's a 9vdc
constant signal for smoke.

http://www.detectorsandalarms.com/images/pdf/kidde_bulletin1.pdf


On Wed, Apr 23, 2014 at 7:56 PM, Bob Blick <bobblick@ftml.net> wrote:

> Does anyone know what the red wire on interconnected smoke detectors
> does when the alarm sounds? I know I could buy an interface box but that
> would be just plain wrong :)
>
> These are USA standard home smoke detectors with black and white wires
> for 120 VAC and a red wire for interconnection.
>
> Thanks, Bob
>
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