2

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.

brhans
  • 14,723
  • 3
  • 35
  • 51
Mike Brew
  • 23
  • 2
  • 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. – Hearth Mar 27 '21 at 04:08
  • Something like a SLIC (subscriber line interface chip/card). There should be a few examples on the interwebs. – Kartman Mar 27 '21 at 04:13
  • 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 '21 at 04:19
  • 25Hz, I thought they were all 16-2/3 - today I learned something. – Jasen Слава Україні Mar 27 '21 at 04:20
  • is this a one off you can possibly hack up an old VOIP-FXS adaptor, or trawl theatrical props websites. – Jasen Слава Україні Mar 27 '21 at 04:25
  • @Hearth - 60v at least. tried 31v AC transformer I had laying around and it's not enough to power the coil and make the bell ring. I'd be interested in controllers etc.. But is there anything that can send out enough juice to power the coil. Unfortunately my knowledge in this area of electronics is very amateurish. – Mike Brew Mar 27 '21 at 06:57
  • @Frog - You are right, the coil moves the arm in one direction on a + peak resulting in the first bell chiming and then moves the arm to the other bell on the - peak. So if my understanding is correct, the diode will filter out one of the above peaks and result in possibly just one bell chiming (there's no spring to reset the arm as the other peak would move it enough). Worth experimenting to have a listen to the outcome though. The bells are slightly different tunes so I think it may again make it sound not quite right. Thanks for the help and idea though. :) – Mike Brew Mar 27 '21 at 07:08
  • @Jasen - yeah the transformer I have is an old theatre prop for making stage phones ring and was probably built back in 1970s.. We were trying it today and the outcome of the phone bell just was quite not right. Some research and pulling the thing apart I found that the ideal Fq was 25 (for Australian anyway which is where the play is set). I don't think my skill is good enough to hack a VOIP-FXS but thanks ;-) – Mike Brew Mar 27 '21 at 07:11
  • @MikeBrew Okay, but how much power does it need? And how nice of a sine wave does it need to be? – Hearth Mar 27 '21 at 13:23

2 Answers2

0

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.

Prathik Prashanth
  • 1,016
  • 4
  • 17
  • 1
    At the power level needed to run a single phone a simple H bridge could be used instead. – Jasen Слава Україні Mar 27 '21 at 06:29
  • That's actually not a bad idea, @Jasen – Prathik Prashanth Mar 27 '21 at 07:22
  • @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. – Mike Brew Mar 27 '21 at 07:30
  • @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. – Mike Brew Mar 27 '21 at 07:44
  • @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 :-) – Mike Brew Mar 27 '21 at 08:04
  • the problem with a cycloconverter is they are not too good when the frequencies are close. Naval ships use them as their main generators produce 400Hz so to generate 50Hz is really trivial and pretty good. –  Mar 27 '21 at 10:58
0

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.

Andy aka
  • 456,226
  • 28
  • 367
  • 807