need help electronic/relay problem phone horn

Thread Starter


Joined Jul 7, 2020
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


Joined Jun 22, 2012
In the UK the phone ringing signal is a 70V pulsed AC , that is fed through a 1uF capacitor in the phone socket pins 3&5.

So you could put that into a bridge rectifier and smooth it with a capacitor and use that signal to feed the relay.

Audioguru again

Joined Oct 21, 2019
You do not have a clue about what you need.
The signal in the speaker is low level AC but the piezo horn needs 12VDC.
A relay needs a fairly high DC activation so it will not work.
Instead you need an audio amplifier feeding a rectifier circuit that turns on a transistor.
It is simpler to use an audio amplifier to drive a loudspeaker. The signal at the 8 ohm little speaker can feed the audio amplifier.

Phone ringers are sold. Some reviews say it is Very loud and others say it is not loud.


Joined Dec 31, 2017
Here's a simple circuit that should work. The optocoupler allows complete isolation between the two circuits. What 12 volt relay do you have?
Some component values may need changing depending on the ring signal characteristics and relay.
See Post #6
Last edited:

Audioguru again

Joined Oct 21, 2019
Yes, the 90VAC ringing on the phone line will activate the opto-coupler. But the relay is turned on and draws current all the time there is no ringing since the ringing turns it off.
Then maybe the transistor in the opto-coupler should be uses as an emitter-follower to turn on the output transistor and relay.

Thread Starter


Joined Jul 7, 2020
I'm going to scrap this project. Way over my head. Thank you for responding. I will dive into some tutorials on basics and go from there.


Joined Sep 24, 2015
I'm the guy at work that will take on anything. Seems I've met my match lol
Good for you. That's how I learn new things - by getting into trouble and then finding my way out of it. When I was 7 years old I took my dad's new gas lawn mower apart because I wanted to know what inside made it work. He explained it clearly and I then put it back together and cut the grass. Curiosity may kill the cat, but satisfaction can bring him back again.

Thread Starter


Joined Jul 7, 2020
Exactly correct. That's my bad, should have paid more attention to the design which I was copying.
The corrected version:
View attachment 255540
Am I near the ballpark or still across town? Thank you.

630V jb JFX PREMIUM Metallized Polypropylene Axial Film Caps
Size: 0.82 uF

22K ohm 1/2W Premium Carbon Comp Resistor (pkg of 5) red-red-orange-G

4.7K ohm 1/2W Carbon Comp Resistor yel-violet-red-G

1.0 uF 50V Axial leaded Ecap - Electrolytic

1N4148 DO-35 Fast Switching Signal Diodes
Voltage - DC Reverse (Vr) (Max):100V
Current - Average Rectified (Io):200mA
Voltage - Forward (Vf) (Max) @ If:1V @ 10mA
Speed:Small Signal =<200mA (Io), Any Speed
Reverse Recovery Time (trr):4ns
Current - Reverse Leakage @ Vr:5µA @ 100V
Capacitance @ Vr, F:4pF @ 0V, 1MHz
Mounting Type:Through Hole
Package / CaseO-204AH, DO-35, Axial
Supplier Device PackageO-35
Operating Temperature - Junction:-65°C ~ 175°C

Brand Name NTE Electronics
Current Rating 1 amps
Ean 0768249007296
Global Trade Identification Number 00768249007296
Model Number 1N4007
Number of Items 20
Part Number 1N4007
Specification Met Rohs
Temperature Range -65-125 degrees_celsius
UNSPSC Code 32111500
UPC 768249007296
Voltage 1000 volts

MCIGICM 2n3904 npn Transistor, 2n3904 Bipolar (BJT) Transistors NPN 40V 200mA 300MHz 625mW TO-92-3

Gikfun OPTOCOUPLER DIP-4 PC817C PC817 for Arduino DIY (Pack of 20pcs) AE1143
Number of Pin: 4
Pin Pitch: 2.54mm
Optocoupler Output Type: optical transistor

4-PIN NO 40/30 AMP 12VDC
Must Operate Voltage: 6~8 VDC
Must Release Voltage: 0.6~3.6 VDC
non latching
(which I think means when the ringing stops, the coil will no longer be energized and the contacts will change from closed to open and the Piezo will no longer be energized)