Incorrect bell?
 

Hi all - I set myself a project on the weekend - to resuscitate an old telephone I bought at a market and connect to my BT landline at home to allow incoming calls - and got on quite well, but I can't get the bell to ring! I'd be grateful if you could help me.

It's an old unidentified telephone but has the following etched on the back-plate:

Circuit Operateur S63
Sous licence de la
Societe Francais des
telephones Ericsson

I believe it used to be positioned in public locations to allow direct communications with the fire brigade in France. There is no dial/pulse facility, and there's an external bell (which is marked 6-12v 50 ohm).

The wiring to it is quite peculiar: a 3 pair cable provides:

• 1 pair connected directly to the bell
• 1 pair connected to the terminal board (line I assume)
• 1 pair connected to the hook

I guess that this is something similar to a Socotel S63 telephone. In any technical documents I can find, a bell with a completely different resistance value is mentioned as suitable.

I have connected the terminals to A/B from my BT line (as per photos) and tested the phone fine with incoming calls. I've wired the bell onto effectively pins 3 and 5 of the BT plug (again, as per photos) while plugged directly into a BT master socket and the bell rings only very faintly...

I wonder if the installed bell is completely inappropriate to be operated by the bell voltage on a BT line?

Any help/pointers would be much appreciated!

Many thanks

Re: Incorrect bell?
 

Is the electrical part of the bell just the coil, or are there interrupter contacts as you would find on a DC bell?

50 Ohms is far too low a resistance for a normal telephone bell. An REN of 1 corresponds (roughly) to 4k Ohms. Old telephone bells were around 1k.

I am wondering if this bell (given the totally separate connections) was rung by a DC voltage from the exchange.

Re: Incorrect bell?
 

Regarding the bell, your descriction sounds like a DC trembler bell. Have you tried feeding it with a DC voltage between 6V and 12V independently of the rest of the circuitry?

Looking at your third image, I cannot see the wiring clearly. The blue wire of the bell appears to be connected (via the 5th terminal of the telephone) to the green(?) wire of the black cable that I take to be the line cord, and the yellow wire of the bell is connected to the yellow of the line cord (I assume the component at the bottom is just a connector block). If this is correct, how is the other end of the line cord wired? Also, to what, if anything, is the black wire of the line cord connected?

N.B. Tony posted while I was composing the above.

Thank you Tony - that makes sense: this must be a DC bell. I can see interrupter contacts on the arm of the bell (having read up on a Wikipedia page about electric bells!)

That's a good idea Dave. I am certain this is a DC bell now.

That's correct

Also correct

Green and Red go to the master socket via pin 2 and 5, and yellow to pin 3 (on a BT plug)


It is not connected at either end.

So my options are to work out how to make the DC bell work, or to exchange it for an AC bell.

To make the existing one work I'd be looking at using an additional power supply and some kind of relay. Or perhaps by using a voltage rectifier to convert the AC supply to DC - is this feasible?

Re: Incorrect bell?
 

Your best option would be to use a correct bell rather than trying to get an appropriate DC voltage from the 75VAC ringing current on pin 3, even though the latter is theoretically possible.

Save the DC bell for a more appropriate function.

Re: Incorrect bell?
 

I will guess the phone was without bell, or with a buzzer equal to those used in the cobra telephones.

Re: Incorrect bell?
 

As has been said the bell is completely the wrong type.

Re: Incorrect bell?
 

Excellent. Thank you all for your responses. So it must have been rung by a separate voltage applied across another pair of wires.

I will have a look around for a similar bell rated at ~4000 Ohm and suitable for use on an AC circuit.

Can someone indulge me as to what would be required to make this DC bell work with an AC line?

And also; is the pair of wires wired back to the exchange from the hook an unusual thing too? I assume that would have been used to indicate when the line was in use. If so I'm surprised by that. Presumably it's possible to discern this from the "A/B" line?

Re: Incorrect bell?
 

Perhaps something like this...
https://uk.rs-online.com/web/p/embed...-smps/1812110/

It'd be slightly out of range on the input side, but hopefully there's more than 75V available at my BT socket

Re: Incorrect bell?
 

Here's what I did: http://joefreeman.weebly.com/ringing-detector.html

Re: Incorrect bell?
 

Thank you Joe! That's really interesting. It's above my head but will be interesting to study.

What do you think of the little switched-mode power supply I referenced in my previous post? Obviously it's a bit of a shortcut in comparison to your creation! But can you see any problems with its use in this case?

Re: Incorrect bell?
 

One part of Joe's circuit which you ought to include with anything beyond a replacement bell is the opto-isolator (IC1). It may be simpler, as I said previously, to replace the bell, either with a small bell internally - there are ones produced that are used to add a bell to telephones, such as GPO 232, which don't have an internal bell, but I can't find anything for sale currently - or use a separate plug-in bell/ringer, which is available for about £5.

Re: Incorrect bell?
 

I would expect to get a reasonable result from adding an ordinary bell transformer via the capacitor in the master socket. Those simple bells are not usually that fussy about AC or DC.

Re: Incorrect bell?
 

There's a Velleman kit K8086 to operate a relay when it detects ringing voltage on a telephone line, for example here :


https://quasarelectronics.co.uk/Item...lay-output-kit

I built one of those kits, and it seems to work well. You'd then need a suitable power supply for the bell.

Re: Incorrect bell?
 

The idea of using the ringing current to switch another power supply to supply the bell with about the same amount of energy as what is coming down the line anyway seems a bit over-complicated!

If you have a mains transformer in your parts drawer (any voltage output will do just for a first test; if your transformer has a centre tap, try end to end and centre to end), try connecting the secondary of that to the bell terminals, and the primary between pins 3 and 5 of the RJ-431. Even an old loudspeaker transformer from a valve radio might work, at least just for a try.

The trembler contacts rely purely on magnetic attraction, not repulsion, so polarity should not matter -- but this is the real world, where not everything pays any mind to "should". Try using a bridge rectifier, if you can't get the bell to operate satisfactorily from AC.

A ringing phone line is an energy-limited source; so the worst that is likely to happen is, it won't ring on an incoming call and it will pull down the voltage so much that none of your other phones on your line ring, either.

Have fun experimenting!

Re: Incorrect bell?
 

I think I follow Julie - sounds like fun... I'll give it a go!

Re: Incorrect bell?
 

I have a toroidal transfomer here, ptimary 15V 25Hz scondary 88.6V 50mA. If someone is interested. Could probably work in reverse.

dsk

Re: Incorrect bell?
 

Thanks dsk. Will keep that in mind.

Re: Incorrect bell?
 

Julie - presumably I'd need to try your suggestion using a wire-wound type transformer, rather than a switched mode type. Is that correct? I have hundreds of the latter lying around, but don't think I have any wire-wound ones...

Re: Incorrect bell?
 

Yes, a "traditional" transformer with copper wire and a laminated steel core is what I meant.

Though if you have a low power switched-mode PSU that will work down to about 90V on the input and puts out enough to ring the bell clearly, you might try it anyway just for a laugh and carry-on.

I wouldn't expect it to work very well if it works at all, because the ringing current comes in short bursts and the SMPSU might not have time to start up properly before the end of the ringing pulse. The telephone ringing voltage being at the very bottom end of the power supply's working range isn't going to help matters, either. (But this would be a good thing for a traditional steel core, since the frequency is only 25Hz; this means that a half-cycle lasts twice as long as on the mains, so you'd end up trying to put twice the amount of flux through the core unless you reduced the voltage.)

But, who knows? If you can find a switched-mode phone charger that will start up from the telephone ringing voltage, deliver enough power to sound the bell and not consume so much energy that there isn't enough left to ring the other phones on the line, it would almost be a shame not to use such a crazy solution!