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?


  • \$\begingroup\$ 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. \$\endgroup\$
    – Hearth
    Mar 27, 2021 at 4:08
  • \$\begingroup\$ Something like a SLIC (subscriber line interface chip/card). There should be a few examples on the interwebs. \$\endgroup\$
    – Kartman
    Mar 27, 2021 at 4:13
  • \$\begingroup\$ 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. \$\endgroup\$
    – Frog
    Mar 27, 2021 at 4:19
  • \$\begingroup\$ 25Hz, I thought they were all 16-2/3 - today I learned something. \$\endgroup\$ Mar 27, 2021 at 4:20
  • \$\begingroup\$ is this a one off you can possibly hack up an old VOIP-FXS adaptor, or trawl theatrical props websites. \$\endgroup\$ Mar 27, 2021 at 4:25

2 Answers 2


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): -

enter image description here

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.

You can find more information about Cycloconverters on wikipedia.

  • 1
    \$\begingroup\$ At the power level needed to run a single phone a simple H bridge could be used instead. \$\endgroup\$ Mar 27, 2021 at 6:29
  • \$\begingroup\$ That's actually not a bad idea, @Jasen \$\endgroup\$ Mar 27, 2021 at 7:22
  • \$\begingroup\$ @Jasen - interesting.. I'm also looking at using a timing controller to replicate the timing sequence for the bell ringing. i.e. here in Australia it is ON 0.4s - OFF 0.2s - ON 0.4s - OFF 2.0s then repeat. I was going to do this using a relay in line from the AC power source and the bell. But I'm intrigued with the H Bridge idea. I could make the above timer circuit that I'm planning on building using Arduino instead fire a H-Bridge type circuit. The coil in the old phone still needs 60V at least so wondering if physical relays will be quick enough to replicate 25Hz. \$\endgroup\$
    – Mike Brew
    Mar 27, 2021 at 7:30
  • \$\begingroup\$ @Jasen - The semiconductor sounds like it can operate in a H-Bridge probably fast enough to replicate 25hz, but now this is getting out of my limited level of electronics knowledge on how to hook it all together without blowing it all apart lol. I know the Arduino operates at 5v and could power and operate a H-Bridge semiconductor circuit, but I've no idea how to make this send 60v to the phone coil. (I know I'll need to rectify my 60v AC to 60v DC). Unfortunately my limited level of knowledge only understands how a 5v control can trigger a relay allowing 60v through mechanically. \$\endgroup\$
    – Mike Brew
    Mar 27, 2021 at 7:44
  • \$\begingroup\$ @PrathikPrashanth - thanks for the idea of the cycloconverter. I'm trying but failing to wrap my head around it 100%. I think I'm following it 50% of the way. The theory and the output makes total sense but to build from scratch I don't know where to start haha.. Interested to follow this H-Bridge concept :-) \$\endgroup\$
    – Mike Brew
    Mar 27, 2021 at 8:04

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