question about interconnected smoke detectors

> Hi,
> I have noticed in newer homes that they have smoke detectors that take
> both a battery and are plugged in to the AC. I think the wires running
> between them seem to have an additional conductor and if one smoke
> detector goes off and starts beeping, shortly thereafter, the other
> ones start beeping as well. How does this work? If you were to mix
> and match between different smoke detector vendors (all having the
> interconnect feature, of course), would they all work the same? Is
> this an industry standard connection? I assume it is an additional
> conductor that either gets shorted to ground or hot by the tripped
> smoke detector and when the other ones detect that short to ground/hot,
> they start beeping as well.
> Does any company make something that could tap in to that extra wire so
> that, say, if I wanted to turn on emergency lighting or something like
> that, it could trip a relay to do that? I'm thinking something that
> wires in like a door bell transformer where the line voltage AC is all
> in the box and the relay contact screws are outside where they belong,
> not sharing a box with the line voltage at all.
> Of course, if company XYZcorp makes an "interconnect smoke detector
> relay module" that is UL etc approved and can be legally connected to
> the smoke detector wiring harness in a standard single gang box and
> will trip a NO/NC relay when the detectors are beeping, I'd like to
> know about that.
> Since IMO, smoke detectors are life safety, I'm not thinking about
> modifying anything or experimenting on my own. It is more just a
> curiosity on how the thing works and if there is an industry standard
> or not. I am sure some of you have modified smoke detectors to do
> something like this, but I would prefer if these mods were kept out of
> this thread since I think there are many that feel this is no problem
> and there are many that feel that modifying a smoke detector is a
> dangerous thing to do. I, personally, would rather not be lead away in
> handcuffs after explaining to the fire investigator the cool mod I did
> to my smoke detectors that caused them not to work when I needed them
> most and, golly, I don't see why it didn't work right, should I get a
> lawyer now? the circuit really should have worked....
> Thanks

> autonut843 wrote:
>> Hi,
>> I have noticed in newer homes that they have smoke detectors that take
>> both a battery and are plugged in to the AC. I think the wires running
>> between them seem to have an additional conductor and if one smoke
>> detector goes off and starts beeping, shortly thereafter, the other
>> ones start beeping as well. How does this work? If you were to mix
>> and match between different smoke detector vendors (all having the
>> interconnect feature, of course), would they all work the same? Is
>> this an industry standard connection? I assume it is an additional
>> conductor that either gets shorted to ground or hot by the tripped
>> smoke detector and when the other ones detect that short to ground/hot,
>> they start beeping as well.
>> Does any company make something that could tap in to that extra wire so
>> that, say, if I wanted to turn on emergency lighting or something like
>> that, it could trip a relay to do that? I'm thinking something that
>> wires in like a door bell transformer where the line voltage AC is all
>> in the box and the relay contact screws are outside where they belong,
>> not sharing a box with the line voltage at all.
>> Of course, if company XYZcorp makes an "interconnect smoke detector
>> relay module" that is UL etc approved and can be legally connected to
>> the smoke detector wiring harness in a standard single gang box and
>> will trip a NO/NC relay when the detectors are beeping, I'd like to
>> know about that.
>> Since IMO, smoke detectors are life safety, I'm not thinking about
>> modifying anything or experimenting on my own. It is more just a
>> curiosity on how the thing works and if there is an industry standard
>> or not. I am sure some of you have modified smoke detectors to do
>> something like this, but I would prefer if these mods were kept out of
>> this thread since I think there are many that feel this is no problem
>> and there are many that feel that modifying a smoke detector is a
>> dangerous thing to do. I, personally, would rather not be lead away in
>> handcuffs after explaining to the fire investigator the cool mod I did
>> to my smoke detectors that caused them not to work when I needed them
>> most and, golly, I don't see why it didn't work right, should I get a
>> lawyer now? the circuit really should have worked....
>> Thanks
> You would think that they would make an industry standard for that sort
> of thing, but unfortunately, not only is there no industry standard,
> the interconnect specs are different between different smoke detector
> models from the same manufacturer. There are low voltage
> interconnects, and line voltage interconnects. And ther eare different
> standards for the low voltage interconnects. I wired up 9 smoke alarms
> with interconnects in our house, went and bought 1 hardwired detector
> w/ battery backup for each floor of the house, and the remaining ones
> were hardwired models without battery backup. (I figured that one
> battery backup per floor was sufficient.) Well, after installing
> everything and testing it, I discovered that the battery backup ones
> will only remotely sound the other battery backup ones, and the
> no-battery-backup ones will sound other no-battery-backup ones. After
> reading the fine print buried in the instructions, it does in fact say
> that you have to have the exact same model for the interconnect to
> work. Oh well. I assume that there is some modulated signal that is
> sent over the interconnect line as opposed to a simple high/low voltage
> to signal the other units, although this is just speculation on my
> part, and I've never tested it out. (I'd have to borrow an
> oscilloscope from work if I wanted to try out that theory.)
> Ken
>

> Batterymarch Park, Quincy, MA 02269) and any local codes that apply.
> When interconnecting, use #18 AWG minimum solid or stranded wire. Maximum wire

> feet for #18 AWG or 4,000 feet for #14 AWG (20 OHMS loop resistance).
> DO NOT connect to any other type or model of heat or smoke alarm. Connect