Author Topic: Miniature 10-line telephone exchange using 22 relays  (Read 19465 times)

Offline Owain

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Miniature 10-line telephone exchange using 22 relays
« on: January 29, 2012, 10:21:34 AM »
The attached is an article from Wireless World, August 1980, by L D Gunn for a 10-line telephone exchange using 22 relays.

Please note the corrigenda from the Sept 1980 edition on the last page.

I would be interested to hear from anyone who constructs this circuit. I think the scan is legible but please ask if it's not.

Offline GTC

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #1 on: August 20, 2013, 06:53:29 AM »
I found this site and this thread via Google.

Thanks for making the PDF of the article available along with the corrigenda.

I quite like the idea of building this little 10-line exchange, although miniature relays -- especially 48 volt ones -- are not as abundant these days as they were back in 1980 when the article was published, and their current pricing reflects this, too.

Also, it's very hard to find multi-tapped transformers these days, so I'd be looking to redesign the power supply around components more easily obtainable today, and at reasonable prices.

Given that we are dealing with rotary phone technology, a relay-based mini-exchange appeals to me far more than a microprocessor-controlled one.

I'd like to discuss this project with other members who may share this interest.

Offline twocvbloke

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #2 on: August 20, 2013, 10:57:28 AM »
I recall seeing this exchange a few years back, looks quite intriguing, but beyond my abilities to construct, it would be interesting seeing one built though... :)

Offline GTC

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #3 on: August 20, 2013, 11:19:17 AM »
I recall seeing this exchange a few years back, looks quite intriguing, but beyond my abilities to construct, it would be interesting seeing one built though... :)

I think it's a great way to put 10 rotary dial phones to practical use, so I'm giving serious thought to constructing one.

Am in the process of working out the availability and cost of the various parts, especially the relays.


Offline Owain

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #4 on: August 20, 2013, 05:45:30 PM »
I'm glad it's been found useful.

I also put up this article for a 2-line ringdown circuit / line simulator
http://www.classicrotaryphones.com/forum/index.php?topic=6141.0

IIRC when the original article was published, 'old' phones (probably Bakelite 300 series!) could be had for about £3 and 'new' phones (probably 706) about £5.

Mind you, a dot matrix printer for your 8K computer was probably about £1000 ...

Offline GTC

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #5 on: August 21, 2013, 02:40:23 AM »
Thanks for that extra link.

I'm in the process of building a telephone ringer based on the instructions for a Maplin kit (LT19) "AutoRing – Telephone Ring Simulator" listed in their 96/97 catalogue (Volume 12 No. 61)

I got the details from somebody quite a while ago, but only recently dug it out following a request for something to be used in amateur theatre. This IC-based device can simulate both UK and US ring patterns.

The ICs are still available via eBay sellers, but supplies of the PCB are long gone, so I'm having that made for me at great expense to the management (though cheaper and easier than my buying all the bits and pieces to go into bespoke PCB production myself).

Once I get that built and working, I may consider using it as "plug-in" module to the 22 line exchange instead of the crude 50Hz ringer approach specified in that article, or I may decide to stay with the the crude but simpler approach. Nonetheless, it would be nice to use something closer to 20Hz to ring the bells.

 

Offline GTC

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #6 on: August 21, 2013, 02:52:53 AM »
IIRC when the original article was published, 'old' phones (probably Bakelite 300 series!) could be had for about £3 and 'new' phones (probably 706) about £5.

Yes, I guess that prices for the older phones are being pushed by upwards scarcity and demand.

As for the relays, I'm tossing up whether to stick with the original 48 volt units, or move down to 24 or even 12 volt relays for the purposes of availability and affordability. Also, it's much easier and cheaper nowadays to get components for 12 or 24 volt power supplies than for higher voltages.

48 volts is of course faithful to the telephony standard, and permits long lines, but as I don't have a need for long lines I'm prepared to drop the rail voltage to accommodate more readily available and more affordable relays.

I could even opt to have a multiple voltage supply, with one for the relay control circuitry and another for the phone lines, plus some AC for the ringer.

I'm going to have to model this in a spreadsheet to see how the economics of each option pan out.
« Last Edit: August 21, 2013, 03:29:53 AM by GTC »

Offline GTC

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #7 on: August 23, 2013, 03:27:46 AM »
Well, after doing the numbers I have decided to go ahead and build this little exchange but as a 24 volt system rather than 48 volts. I can buy a 24 volt 10 amp switch mode power supply on eBay for US $20  (about £13) -- there's no way I could build a 48 volt unregulated power supply for anywhere near that price using bought components. The transformer alone is a very expensive item.

Also, the 24 volt versions of the required relays are cheaper and easier to find.

I still need to decide what to do about the ringer. My options appear to be: (a) find a 240 volt transformer with a secondary in the range 60 to 90 volts and use 50Hz to ring the bells, (b) use the Maplin ring simulator  that I'm in the process of constructing, (c) use a DIY inverter off the 24 volts DC, (d) some other solution yet to be determined.

Offline Owain

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #8 on: August 23, 2013, 06:27:04 AM »
You probably don't need anything like 90V to ring the bells over short lines.

A 70V/100V line output Public Address amplifier fed by a sine wave audio signal generator will give you a correct frequency. At 70V line a 2500 ohm load corresponds to 2 watts, so you don't need a big amp. 30-40 watt PA amps with nice chunky MOSFETS are pretty cheap second-hand.
« Last Edit: August 23, 2013, 06:31:03 AM by Owain »

Offline GTC

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #9 on: August 23, 2013, 06:34:50 AM »
You probably don't need anything like 90V to ring the bells over short lines.

Yes, the Maplin simulator supposedly puts out about 60 volts using a 6-0-6/240 volt transformer in reverse. When I see how well that works I'll use that as a guide to what I need.

Quote
A 70V/100V line output Public Address amplifier fed by a sine wave audio signal generator will give you a correct frequency. At 70V line a 2500 ohm load corresponds to 2 watts, so you don't need a big amp. 30-40 watt PA amps with nice chunky MOSFETS are pretty cheap second-hand.

Something else to consider.
« Last Edit: August 23, 2013, 06:38:03 AM by GTC »

Offline poplar1

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #10 on: August 23, 2013, 10:40:15 AM »
These are 12VDC to 70VAC 20~ ring generators: 2 for $10

http://www.ebay.com/itm/121058890727
« Last Edit: August 23, 2013, 10:41:56 AM by poplar1 »
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Offline GTC

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #11 on: August 23, 2013, 11:42:17 AM »
^ Well spotted, thanks. Sounds like a neat solution for me -- if they work!

I've just ordered some. I note from Googling around that they were manufactured in Israel in 1996.  Can't locate a datasheet online, so have asked the seller if he can provide.

Offline DavePEI

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #12 on: August 23, 2013, 01:33:23 PM »
^ Well spotted, thanks. Sounds like a neat solution for me -- if they work!

I've just ordered some. I note from Googling around that they were manufactured in Israel in 1996.  Can't locate a datasheet online, so have asked the seller if he can provide.
Also manufactured and sold under the PowerDsine name - same part number, same device.

P/N PCR-SIN03V12F20-C
Vin: 12 VDC (regulated or battery)

Vout 70V RMS
P Out 3 W RMS
Frequency 20 hz/cycles

The module has 5 wires:

V+ (12V) red
V- (GND) black
RNG1     blue
RNG2     white
INHIBIT  yellow

INHIBIT can be left floating. If pulled to +5V from GND, the output ring signal is inhibited.

A 330 ohm resistor should be provided in series with the load on Ring 1 or Ring 2 to protect the module against a bad ringer or shorts.

I have 10 of the PowerDSine version of these should the seller above run out. They will cost slightly more than he is charging (my cost, $5.50 ea), as I had to pay shipping for them to me and I have to recover my costs, but postage to the US would only be a couple of dollars for two.

Dave
« Last Edit: August 24, 2013, 01:49:27 PM by DavePEI »
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Offline GTC

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #13 on: August 23, 2013, 11:22:24 PM »
Wow, many thanks Dave!  :)  What a great forum this is.

I spent some time Googling around for that information. Although I got some hits on that part number, the associated sites had no datasheets.

Thanks for the tip regarding the 330 ohm resistor. I had it in mind to install a fuse in the ringer circuit. I may do both.

Offline GTC

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Re: Miniature 10-line telephone exchange using 22 relays
« Reply #14 on: August 24, 2013, 12:07:51 AM »
Now that I've dispensed with the need for a low voltage transformer, I need to decide how to provide dial tone. The original design uses mains frequency via a capacitor and resistor.

An encapsulated 12 volt oscillator module for PBXs similar to the ringing generator would be ideal.