Old AC Smoke Alarms with Electromechanical Horns

Hey everyone!

I have a question about those old AC smoke alarms that had electromechanical horns. I’ve noticed that some of them, like the Firex in this video (I think it’s an FX-1020)


make a 60Hz buzz, while some of them, such as the SmokeGard 907A in this video


make a 120Hz buzz.

As far as I’m concerned, the mains frequency in the US is 60Hz, so why is it that some of them make a 60Hz buzz, and some make a 120Hz buzz?

This actually also applies to commercial AC horns as well. Most of them, like the Simplex 4050, Federal Signal 350 and Faraday type 2 all make a 120Hz buzz, but a Tork horn that 7002t made a vid of awhile ago only made a 60Hz buzz. Any help is greatly appreciated.



it depends on the power source.

If you are using an AC horn, the buzzing caused by rapid switching of the power source is controlled by the AC current. Since standard household current is 60 Hz, that means the hammer in the horn strikes the sounder plate 60 times per second.

That’s not all–the sound the buzzer makes also depends on the mechanism. While they still move at 60 Hz, the size of the sounder, the design of it, the shape, and the mechanism type will change the sound. While it’s still moving at 60 times per second, it just does not sound like it. On the 4040 and the 4050 (and I think also the 4030) the mechanism is two metal plates that clap together quickly. On the Tork Alert you mentioned above, it’s basically the AC version of a 9838 and uses a hammer that strikes a plate.

The 4050, 4040, and 4030 all make a 60 Hz buzz because of what I described above.

If you are using a DC horn, you now need a pair of contacts that are opened and closed by the magnet, wired so when the magnet is powered, the contacts open, cutting power to the magnet, which releases the contacts, completing the circuit again. The pitch of the alarm depends on the distance the contacts have to move (farther distance means slower) and the higher the time between strikes is, the lower the pitch of the horn.

The 9833, 9838, and 9806 have different pitches because of the position of their switching mechanisms. Oh and it may not look like it but the pitch on the 9833 and the 9838 can be changed easily.

With the smoke detectors, it seems that some of them run purely on DC power, with their AC inputs running through a rectifier. It also seems like others run completely on AC current.

Sources: electricity books, experimentation and observation.

Oh okay I see. I actually just checked the user’s profile who posted the vid of the SmokeGard, and it says he’s from Canada. Do they use 120Hz, or is it 60Hz like it is here in the US?

Thanks for your help!


Actually wait a sec I think they have 60Hz

You are correct, Canada uses 120V, 60 Hz just like the US.