# Convert 60V AC 50Hz to 60V AC 25Hz

The setup is a 240V 50Hz AC transformer outputting 60V 50Hz AC. Connect this power supply (with a push button) to an old telephone and it rings the bell on command, but unrealistically fast.

I need to take this 60V output and reduce the AC frequency to 25Hz so the phone sounds more realistic. As per what I've read about the signals for making phones ring (here in Australia anyway).

Any advice on what I can put inline between power supply and phone to achieve this?

Thanks.

• How much power does the bell need? How nice of a sine wave does it need to be? Frequency conversion of any appreciable amount of power is nontrivial, but driving it with a low power square wave from a microcontroller should be doable. Mar 27, 2021 at 4:08
• Something like a SLIC (subscriber line interface chip/card). There should be a few examples on the interwebs. Mar 27, 2021 at 4:13
• Doubling the frequency is a lot easier than halving it. However, if the bell rings on both positive and negative peaks (i.e. 120Hz) then adding a diode might do the trick.
– Frog
Mar 27, 2021 at 4:19
• 25Hz, I thought they were all 16-2/3 - today I learned something. Mar 27, 2021 at 4:20
• is this a one off you can possibly hack up an old VOIP-FXS adaptor, or trawl theatrical props websites. Mar 27, 2021 at 4:25

Any advice on what I can put inline between power supply and phone to achieve this?

Convert to a DC voltage and use an audio amplifier and a sinewave oscillator circuit to produce the new frequency.

Similar story: I had to design a modern magneto phone that ran from a 6 volt battery. I needed to replace the equivalent of this part (the magneto generator): -

Picture from here

So I used a TO-220 power amp (can't remember which one because it was back in the 1990s) and fed it from a low pass filtered 25 Hz square wave oscillator. I then stepped up the output voltage using a smallish mains transformer to 70 volts RMS.

Summarizing your problem, you need 25Hz AC from a 50Hz AC source. This is the job of a Step-Down Cycloconverter.

Check this out: https://everycircuit.com/circuit/5551505345871872/step-down-cycloconverter (Click on the graph window to expand)

In the linked example, 50Hz is reduced to 16.7Hz, or 1/3rd of the input frequency. By re-configuring the control circuit to meet your needs, you can get 1/2 of the input frequency.