Ringing bells in old telephones

[Relay Automatic]

Reviewing both old and recent posts there seems to be a recurring need to ring the bells in old phones for theatrical productions and other reasons. The most recent request by mikeswynes prompted me to look through my ‘archives’ for various circuits that I have collected over the years. I could not find all that I thought I had but I have found three that may be of use.

Looking at the requirement there would seem to be two parts; the first to produce a high enough voltage at the correct frequency to drive the bell motor and the second to recreate the correct cadence to get the ‘ring, ring, pause’ effect.

In the old BPO/PMG auto exchanges there were usually motor driven ‘ringer’ generators that ran continuously to produce 70-90V at 20-30Hz. These had cams on the shaft that operated contacts to give the required sequence. Smaller exchanges and some PABX/PAX had mains driven sub-cycle ringer transformers that produced 70-90V at a half or a third of the mains frequency. Other PABX/PAX had pole-changers while PMBX had transformers that just produced 75V at mains frequency. Ring cadence was achieved by relay switching or by an operator flipping a key-switch.

The bell motors in 200 series, 300 series and 700 series plus Australian 800 series phones can be adjusted to operate at 50Hz but sound dreadful. The later phones with electronic ringers such as the Trimphone and the Australian 807AT and 809AT seem to work well on 35-90V at 50Hz. This means that a small mains transformer with a 30V RMS winding (approximately 42V peak) can be used to ring these phones but a 330 ohm, 5 watt resistor should be in series with the line for protection against accidental shorts. See 50Hz-Ringer diagram.

The most common approach to generating a suitable voltage at an appropriate frequency is to drive a mains transformer ‘backwards’ by passing a varying low DC voltage through the secondary windings and getting a higher AC voltage from the primary winding. This is of course a simple inverter circuit. Typically (or from my experiments at least) pulsing 12V DC through a 15V secondary winding will induce 90-120V on the 230V primary winding. So using a multivibrator running at 20-30Hz will generate a corresponding high voltage square wave output. However the efficiency of the transformer seems to drop off drastically when driven below its design frequency so a 50Hz mains transformer does not work well at 25Hz. A test unit that I made generated 120V at 25Hz with no load but the output fell to just 20V when I connected a single Tele 332 phone. Using a larger 60VA transformer gave more output which could ring the bell but only just. I also tried a PA line transformer which worked but was not what I considered usable.

With the forgoing reservations in mind, the following circuit by Julian James was published in Silicon Chip magazine, January 2009, page 41. (Reprints of the whole page are available from siliconchip.com.au) The circuit uses a 4069 hex inverter as a 25Hz oscillator and a 230V/18V transformer. (The Model 2154 transformer is an 18VA type and is available from Jaycar in Australia.) I have not built this circuit myself but James states that the output is 36V AC which will ring a PMG 800 type phone or a BPO Tele 746. A wireless remote control is used to ring the bell and the required cadence is produced manually.

A simple circuit for generating the ‘ring, ring, pause’ effect by W. Graham was published in Electronics Australia magazine, July 1978, page 93. (Reprints of the whole page are also available from siliconchip.com.au) The circuit was a modification to another circuit for a 10-line uniselector PAX which used a 32V supply. This circuit is intended to operate from 9V but 12V would be fine. The F relay was a 3000 type but a miniature 12V relay should be used instead. The NO contacts of this relay switch the ringing and could be connected in series with the 30V RMS winding of a transformer to ring a Trimphone or in place of the S1 push switch on James’ circuit.

An alternative method of making the bell motor used in the 200 series, 300 series, 400 series or some of the Tele 706 phone work on just 12V DC was first published in the 1970s. I don’t now have a copy of the original article but did try it at the time. It used the novel approach of treating the 1000 ohm bell motor as two separate 500 ohm coils and alternatively driving each coil with a simple two transistor multivibrator. I cannot remember the component values but it worked very well. The following circuit by G. Durant was published in Practical Electronics magazine, October 1985, page 40. (I do not know if reprints of the whole page are available anywhere.) It was designed for use in amateur theatricals and is an improved version of the same idea two coils idea. It incorporates two oscillators, one to generate 25Hz to drive the motor coils and another to drive a counter to get the correct cadence. Note that there is a minor drafting error and the BC558 is a PNP transistor, not an NPN as shown.

Andrew

[Nickthedentist]

Very interesting and useful.

Another addition to your collection would be the one published as a telephone tester in Television magazine circa 1990-1995 at a guess, which used a relay driven by a 555 ISTR. Perhaps someone with a set of mags to hand could scan and post for us?

Nick.

[Radio Wrangler]

In the Museum of Communications, there is a desk-top strowger demonstrator in a glass case with two periof phones. Visitors can pick up a handset and dial the number of the other phone while watching the switches seek and select. Inside the cabinet there is also a modified relay with a 25Hz resonator bob weight buzzing away all the time, and there is a ring of relays oscillating to give the ring-ring-pause cadence. This all runs off a -48v supply.

David

[Relay Automatic]

Nick,

I have a long standing interest in restoring/building small PAX/PABX units since I made a 10-line uniselector PAX as a teenager in the '60s. (I started collecting old phones in 1963 despite parental opposition.) Over the years I grabbed any magazine with something alone those lines whenever I saw it regardless if I intended to make it or not. Unfortunately the 'library' has had to be purged several times and I tried to make photocopies of the 'good bits' before disposing of the magazine. The resulting 'archive' has also been trimmed too and this may be why articles I can remember once having are no longer there. I look at the UK magazines in the local newsagents but in recent years these magazines have been few and fewer. I think I know the Television magazine but I did not see the article. I would like to see the circuit if it can be found.

I did find a link to EPE June 2006 that had a telephone tester that would do the job nicely. (http://david.alfa-romeo.eu/elektro/E...6_EPE_2006.pdf)

Something similar is at http://www.sm0vpo.com/_visitors/inst/ring-gen.htm


The thing that puzzles me is that other people get a good result with small transformers but as I noted in my previous post, they don't seem to work for me. I noted that the unit in the second link used a speaker output transformer so I will try that too. I also liked his way of using the mains as a frequency reference.

Andrew

[Relay Automatic]

David,

Yes those small PAX used a modified relay as a pole-changer or some form of low frequency vibrator driving a transformer. I am (very slowly) restoring an ATM 10-line PAX that has modified relays to generate a ringing supply and the various tones required. The problem is that those parts are hard to find hence the emphasis on something that can be made with modern components. In another post I (with much help) looked at modelling circuits using 3000 type relays. One outcome of this exercise was a three relay design that should theoretically work as 17Hz pole-changer. When I get the time to build it I will confirm if it actually works.

Andrew

[lesmw0sec]

Surely the easiest method would be to use a PIC to get the appropriate timing for ringing and cadence and use this to switch a bridge or totem-pole pair of transistors or fet's using a 50V rail? This would eliminate the need for a transformer.

[Relay Automatic]

Yes lesmw0sec I agreed it is easy to program a PIC chip to generate the correct cadence and I have seen circuits where this has been done. Certainly I do not discount the use of microprocessors and have used them in tasks like this. However it does require a knowledge of micro programming whereas the 555 plus 4017 approach just requires careful soldering. (Something that I do not always achieve in my projects!) The other minor disadvantage is that PIC chips require an additional 5V supply where as the others will work on 12V.

As for my 3-relay pole-changer design, it was just an exercise in keeping the PAX purely electromechanical rather than a hybrid. Note that I make no claim for efficiency. Curiously I have recently seen a pole-changer design made with two relays driven by a Arduino.

I also agree that a transistor/MOSFET H-bridge would drive a bell motor. These two links show two implementations of this approach.

http://veryuseful.info/linesim/

http://www.bowdenshobbycircuits.info....htm#ring2.gif

One disadvantage however is testing for the phone going ‘off-hook’ so that the AC is cut off when the handset is lifted. In the other transformer method the AC ring has a DC offset imposed on it so that the transformer ‘secondary’ is in series with the DC supply. When the handset is lifted a relay or equivalent circuitry detects the loop and stops the AC. In the notes with the Durant circuit it was suggested that the wiring in the phone be modified so that the normally closed contacts on hook-switch were in series with the bell motor. This way lifting the handset would immediately open the circuit to stop the bells ringing and also the poor actor getting a loud ‘raspberry’ from the receiver. If however the phone is part of a collection rather than a theatrical prop then modifying the internal wiring may not be desirable. The Linesim design uses an opamp to detect the loop condition.

The following link to Cambridge Labs gives a free comprehensive document on the subject of ringing telephones.

http://www.camblab.com/lit/trix_v_2.pdf

I can also vouch for their 12V ring generator module which worked very well until I managed to kill it. (See observation on soldering in first paragraph of this post.)

http://www.camblab.com/datashts/lnuspec.pdf

Andrew

[radiotechnician]

The ringer in a phone has a capacitor in series to increase the current
at the ringing frequency, and on some types of circuits, (party) to ensure
only the intended phone rings, when a single line has multiple stations
sharing the line, with different numbers. The other thing about the ringer
is the magnet which causes the current to exert attraction on one coil,
and repulsion on the other, which doubles the force to tip the armature,
such that the positive part of the cycle rings one gong, the negative, the
other. If the assembly is adjusted right, the clapper will bounce
off the gong allowing a proper ring.

The inductance of the coils, the capacitance, mass of the clapper
and the flexing of the shaft is adjusted for efficient ringing.

The ringing circuit should be at specified voltage, but with no
capacitor, it won't ring properly. A square wave isn't found after
a mile of cable, at the coils.

The forgoing is of no new news to telephone people, however the
correct ringing of the gong type phone on stage must be considered
as a musical source, and if forced by a contrived circuit, may sound odd.

This would come to the attention of the sound crew, and especially so if
the performance was being filmed.

I was raised in a family where father worked for Bell and have messing
with them for 60 years.

[LRS0001]

Hi All

Does anyone happen to know if you can buy one of these telephone test ringers ready-made anywhere?

The eyes and soldering skills are not so hot anymore, so a ready-made, off the shelf, ring testing unit would be most helpful.

Si

[Pellseinydd]

We have a quick easy way to ring our old telephones at the cost of around £16.00 UKP. We just use a Linksys Analogue Terminal Adapter (PAP2, SPA1001, SPA2102, SPA3000 etc) connected to the old GPO replica telephone network (all free to connect and use http://www.ckts.info/192/uk/ ). Then we can dial the old Faultsmans Ring Back number '174' (or whatever it was on your exchange) and we get called back after putting the phone down. The Linksys range of ATAs can have the ring cadence, voltage and frequency set up to suite your country - so no problems there! Very flexible and easy to set up. All you need is an ATA connected to your router and a DTMF phone to dial into the ATA (or a Loop Dis phone with a pulse to tone converter if you want to be able to dial out using that phone).

[dagskarlsen]

I guess this article may help:
http://www.instructables.com/id/Hack...hone-Intercom/
The risk is of course to be interested in VOIP and want to read this:
http://voipstuff.net.au/

dsk

[Brigham]

Originally Posted by Pellseinydd

We have a quick easy way to ring our old telephones at the cost of around £16.00 UKP. We just use a Linksys Analogue Terminal Adapter (PAP2, SPA1001, SPA2102, SPA3000 etc) connected to the old GPO replica telephone network (all free to connect and use

Thanks for the link. Sounds great. I just wish I knew what they were talking about!

[dagskarlsen]

Brigham, I understand you.

When you have a hobby, and forgets that others may be not have come (went) so far it may be easy to forget the long way of experience and learning, and this may kill others interest.

I really hope what I have posted are understandable, but even here it is a need of knowing some terminology, and have a little experience.

To all out there who want us to step down to the ground, and start from a lower level, let us know, and together we figure out where to start.

I will love to guide you from the start to my rather moderate level.

It is much more fun to be able to use your old telephones, even when people in the other end dont understand why you are using those old .....

dsk

[rherber1]

Originally Posted by LRS0001

Does anyone happen to know if you can buy one of these telephone test ringers ready-made anywhere?

The easiest way is to find an old BPO ringing converter as used for PMBX's.
http://www.britishtelephones.com/menuconr.htm
The little No.7 would be ideal for your purpose but any of them would be suitable.

If you needed a 50V DC power supply as well as the ringing supply then a Power Unit such as the 100A would give both supplies.
http://www.britishtelephones.com/pugpo1.htm

[bobsterkent]

When my friends am dram group need a 'practical phone' they borrow my line simulator (from my testing days) so they can do their own ringing if its a manual board or dial the second line on it for a cadence ring. I know they turn up now and then on eBay.

How about a modern mobile 'phone with a ringing bell ring tone near the period one?

[Nickthedentist]

Yes, but get it right! Most are American i.e. different gongs and different cadence.

[matthewhouse]

I still have a basic ringing generator here if it's any use...