I've inherited a project involving... umm... six? yeah, six hardwired smoke detectors.

Wouldn't you know it - only four can be found. We have no idea where the other two went. All the hardware exists - brackets, connectors, etc., but the detectors themselves? GONE. Vanished. Got sucked into a spontaneously-generated wormhole and transported to an alternate dimension.

I told him to go buy six new, matching units.

He came back several days later with two completely different ones. It is his belief that I should be able to use them.

Y'know what? Whatever. I don't care, there's hot and neutral there, I can just leave the interconnect wire disconnected and forget it. His alternate plan is to just use 9V battery powered ones anyway.

and yet I wonder to myself (but don't dare try it without someone else's input), is the 'interconnect' standard? Do different brands/models use the same signalling? Can I mix brands? Will it work? Will the world end? Will it create another wormhole and teleport more matter? I mean, the latter wouldn't necessarily be a bad thing if I could coordinate it with the presence of his very irritating ten-year-old son...

Shouldn't be a big deal unless manufacturer's have completely different connection guidelines, or wattages, voltages, etc.

Def. verify before.

The other method I have to verify is to simply clip my meter on to the interconnect wire and press the test button. Frankly if the signal voltages are the same I'm going to wire them up and see if they work. I'm just not at the project house for the weekend (taking a much-needed break) and won't be able to do this until Monday.

The funny thing is that the prblems never seem to show up until you have been gone from that job for a month or so!

No, and that is the problem.

There is no standard.

There's a 4-wire standard, for 24V-powered smoke detectors with a set of contacts to report to a central controller that smoke is detected.

There is a 3-wire variant, which shares one of the contact terminals with one of the power terminals.

There are newer digital serial standards.

If I want to turn this guy's house into an office building, school, or factory, then there are definitely standards available. Unfortunately, they require him to buy stuff, something he seems very... uhh... well, did I mention I told him to buy a set of matching smoke detectors and he came back with only two, which don't match either the existing units or themselves???

It appears from my searching the internet that Firex (existing) simply connects the 9VDC supply to the interconnect wire to signal, and both of the new ones are Kidde, which is apparently the same company - whose website seems to indicate they are compatible... At this point, I'll believe it when I see it.

Work resumes Monday.