How to Trigger 12V Piezo Horn From 8 Ohm 2W Phone Speaker Ring Signal?

I need to ring a phone that activates a loud ass horn. Phone has no audio ports. I'd be tapping into the 2 wires that connect from phone circuit board to the phone ring speaker.

Inside the handset, the 2 wires show .035-.045 VAC (probed with multimeter while the phone "rings") and connect to an 8 Ohm 2W speaker inside the phone. This 8 Ohm 2W speaker needs to be replaced with a very loud horn or buzzer.

-I purchased a generic 12VDC piezo horn. Need to feed it 12VDC when phone rings. That's all.

I'm stumped. Low signal relay? Some type of inline amp? I can solder a bit, and have only a very basic understanding of this stuff.

Do I look for a low signal relay? like .03VAC to 12VDC?

Thank you

Hi

Sounds (pun intended) to me that you would be better off buying a "sound activated switch". Position it close to the phone and when the phone rings the switch activates. A search will give many options. This has the benefit that you don't need to connect to the target phone.

Regards

The signal level you are getting seems very low, some volts peak to peak in theory is possible at the speaker, was the volume control set to maximum?. Additionally, the signal is likely to be Pulse Width Modulated, class D or any one of a number of variants, just to confuse the issue further. Not knowing what dvm you have, I can't make an accurate comment on its ability to be able to measure your signal. It will have to have adequate frequency response, and you will need to send a continuous tone for some seconds, unless it has a peak hold function, you need to check the specifications for the meter. If the meter is not up to it, you probably need to get an oscilloscope from some where, or at least build a peak detector, which will hold the peak voltage of the ac signal long enough for a dvm set on dc to measure it. Assuming that you can now get a reasonable idea of the signal level, you can make some sensible decisions about what needs to be done to make your idea work. I assume that you have a 12V dc supply for the piezo buzzer? It is possible that with a little bit of signal processing, something like an opto coupler could be used, which would isolate the phone from the system you want to build. A few details about the piezo device would be helpful, what current does it draw at 12V for starters. Once you have discerned what signal level is available from the phone, then you will be in a position to make some decisions on how to process it further.

cheers,
Richard

You can get things called External Ringers which are intended for use in noisy factories etc that would do what you want. I have a couple around... where are you? (I'm in Australia).

If you want to do it yourself, bear in mind that an analogue phone line has an AC ring voltage of at least 30 volts superimposed on the DC of the phone line. This is usually tapped off by a capacitor of 1 uF or so which drives the ringer. So you could put a circuit across the line (The phone line, that is, not the phone speaker) with a 1 uF capacitor, a bridge rectifier and a (fairly sensitive) relay which would switch the 12V to your piezo horn. The circuit must be high impedance to AC or it will reduce the speech level, but a relay being an inductor would do that.

Of course if this is a digital (VOIP) phone line you can't use this method

Thank you for this thread..

Hello.
The phone current is very low.
You should make a transistor amplifier and use an MCU.
The RING signal should be lowered to 5V and straightened. The MCU then does the rest, driving the transistor and, in turn, driving the speaker.
Giovanni

I found this which looks like it will do exactly what you want:
https://www.velleman.eu/products/view/?id=461752

This one is VERY cheap and all you need is one extra RJ12 phone cable:
https://www.ebay.com.au/itm/284341261599
I've used one very similar to this before and it worked great.

A circuit diagram here if you want to build it yourself:
http://www.circuitdiagram.org/telephone-ring-detector.html
This one is good, uses an opto-isolator so no possibility of connection between your horn and the phone line.

Google "phone ringer relay circuit" for more, though the above is probably the best I found.
Again, these will only work on Analogue phone lines.