Switch for 12v device that is triggered by another low voltage input

Hi all. Firstly I am fairly new to the electronics world, but I am trying to come up with a solution to this problem and I believe there would be a way to do it.

I have an off-the-self alarm system (Eufy) that has a speaker in the main unit. The speaker is tiny and there aren’t any options to add additional speakers / sirens.

What I am wanting to do is have an external 12v siren powered by a 12v power supply that is switched on when the Eufy speaker is activated. Eg splice into the Eufy speaker wiring and when voltage runs to the speaker it instead runs to some sort of electronic switch that detects the voltage and closes the 12v siren + psu circuit.

Does anyone know any off-the-self solutions for this, or a place that could make this, or an idea on how I could make it?

Any help at all would be so appreciated. Many thanks!!

Hey Mark,

Welcome to the forum :slight_smile:

This is the perfect project for a relay! (or a MOSFET if isolation is not needed but a relay will work just fine for this).

This little guy would do the job fantastically.
Did up a bit of a mud map to give you an idea of how I would wire it;

The ground for the siren would be grounded separately to the power supply of choice.

With 12V power to VDD and GND going to earth, When the speaker from the Eufy activates, it will power the EN pad (this pad needs 1V-20V to activate). This in turn will activate the relay. With a 12V power source on the COM pad, it will power the NO (Normally Open) pad when the relay is activated and your external siren should come to life.

Hope this helps :slight_smile:


Firstly, you’re amazing! Thank you so much and for the mud map too. I will totally be getting that to do it, it sounds perfect!

Can I ask 2 possibly silly questions please.

  1. The 12v + going to the VDD, could / should this be the 12v+ going to the COM?
  2. The GND would just be the negative of the 12v supply? Does the negative of the Eufy speaker need to connect to anything?

No such thing as silly questions mark!!

You sure can! You can use the same 12v source for both the VDD and the COM if you wish.While i wouldn’t go loading the COM pad up with a varity of different 12V accessories, one siren should be fine.

You are correct with the GND being the negative for the 12V supply. Assuming that you do in fact use the same 12V supply for both sides of the relay, you will need to hook up the negative of the Eufy speaker to the negative of the power supply.

The negative of the Eufy speaker will ultimately need to be connected to the negative terminal of whatever 12V supply is connected to the COM pad and the positive side will go to the NO pad.

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Perfect :slight_smile:

So EN= Eufy Speaker +, VDD and COM = 12v+, GND = Eufy speaker - and 12v -

When you say not too much connected, this would be base on the amp rating of the 12v supply right?

And I assume if the Eufy speaker turns off this relay also turns off?

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Hi All
I don’t think you could just connect this sort of thing straight to the speaker. The relay would not respond for a start. You would have to rectify the speaker audio and then have enough volts to switch the mosfet on the board which has a Gate threshold switch on of 1.3V (data sheet) so would have to be somewhat above this to switch fully on. I did not go into it that far.

This idea may work but don’t be disappointed if it turns out to be a bit more complicated.
Cheers Bob


Hi Robert,

I think you’re right with needing extra hardware. Since this seems very similar to the “auto-on” feature you see in home theatre subwoofers, I took a look to see whether anyone had created an external circuit that did the same job, and it looks like they have:

I don’t know enough about discrete analog circuits yet to comment on how applicable this project will be, but I thought I’d add it to the discussion.



Hi James and All
Yes that circuit in that project is basically what is needed. The Mosfet ON time constant of the 33µF cap and 10MΩ resistor would need to be looked at though. That will take 330 seconds to decay to about 38% of gate voltage which will probably be a mosfet still ON condition. That would mean in this case the siren would continue for about 6 minutes after the chime stops. A bit long methinks. But all this is adjustable. Depending on the mosfet used the gate capacitor might be enough with 10MΩ to do the job. Anyway with a similar circuit this project is doable.
Cheers Bob