need help electronic/relay problem phone horn

I need to ring a phone that activates a loud 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

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Hi George

That is very low. I would probably expect the meter to read something like that with nothing connected, just stray mains pick up. Think you should have another look at your measuring techniques. I doubt that level would produce any sound from the speaker.

I doubt if anything like that exists, 12VDC yes but 0.03VAC ??? I would say no.
Cheers Bob

I really don’t know what sort of level the ring generator produces. With older (ancient) systems the ring tone on the phone line could be in the order of 180V at about 17Hz. You could actually get a bite if playing with a line and somebody rang the number.


The signal to the speaker would be audio, it varies by the same amount above and below zero volts (usually), hard to measure with a multimeter, you would need an oscilloscope. Because it is audio you need a circuit to detect this audio level, which then triggers a relay.

Possibly you could do this with a simple circuit, or you could use a micro; analog input pin to detect the audio level, process this and a digital pin to trigger a relay when the right audio signal is present.

I am making a lot of assumptions as to what the phone device is and how it works.

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Hi All
Some years ago one had to be very careful what one connected to a phone line and making any modifications to any device that connected to a phone line. Engineering (PMG, TELECOM, TELSTRA, AUSTEL) approvals and all sorts of paperwork.
Some retailers of devices like ring tone detectors etc actually published disclaimers and other similar documents to prevent any comeback on themselves.

Now all this has probably changed since I had anything to do with anything like this but I just thought I would bring it up in case something still applied. Worth checking to save some inconvenience later.

I think there are commercial devices (which should have all the required approvals) available to do just this. Try hearing aid people, I am sure there is something for the hearing impaired or there is certainly something for the factory environment.
Cheers Bob