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.
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.
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... :)
Quote from: twocvbloke 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... :)
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.
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 ...
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.
Quote from: Owain on August 20, 2013, 05:45:30 PM
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.
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.
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.
Quote from: Owain on August 23, 2013, 06:27:04 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.
QuoteA 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.
These are 12VDC to 70VAC 20~ ring generators: 2 for $10
http://www.ebay.com/itm/121058890727
^ 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.
Quote from: GTC 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.
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
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.
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.
For a dial tone, all you really need is something that plays a tone that you can identify as being the dial tone for the system, rather than being something like what you'd hear on a current landline or on something like a Panasonic PBX, and make it something different so other people can identify it as not being a landline dial tone... :)
Though I don't think a "mains hum" dial tone as per the design of the system specifies would really sound good, it'd just sound like, well, mains hum, which makes phones sound like they're faulty... :D
Shame you couldn't incorporate something like this though:
https://www.youtube.com/watch?v=VzIXvO6RahQ
Quote from: twocvbloke on August 24, 2013, 04:27:08 AM
For a dial tone, all you really need is something that plays a tone that you can identify as being the dial tone for the system, rather than being something like what you'd hear on a current landline or on something like a Panasonic PBX, and make it something different so other people can identify it as not being a landline dial tone... :)
Though I don't think a "mains hum" dial tone as per the design of the system specifies would really sound good, it'd just sound like, well, mains hum, which makes phones sound like they're faulty... :D
I agree with what you say about mains hum, and I'd love to have a modulated tone a la exchanges, but I'm looking for a simple solution. I'll probably just use a simple oscillator.
However, if there's a cheap and available PBX module out there I'll use it.
QuoteShame you couldn't incorporate something like this though:
https://www.youtube.com/watch?v=VzIXvO6RahQ
I love those old motorized tone generators.
How about this one:
(https://i2.sndcdn.com/artworks-000024163567-14i03b-t500x500.jpg)
Quote from: GTC on August 23, 2013, 11:22:24 PM
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.
No need to do both. If you encounter a short, or a shorted coil on a ringer, the 330 ohm resistor will present a proper load on the device, and protect it from damage.
Dave
Quote from: DavePEI on August 24, 2013, 06:05:22 AM
Quote from: GTC on August 23, 2013, 11:22:24 PM
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.
No need to do both. If you encounter a short, or a shorted coil on a ringer, the 330 ohm resistor will present a proper load on the device, and protect it from damage.
Dave
True, but a fuse gives a hard indication of a fault condition.
I mentioned earlier that I'm constructing the 1993 Maplin Ring Simulator project and that I outsourced manufacture of the PCB. Well, the board cam today and the etching is very well done (as it should be for the price charged) but there's a problem ... the guy making the board enlarged my layout image to fit the board size that I specified without referring that decision to me beforehand. I had mentioned in an earlier email that I specified a larger board to allow for mounting.
Result is the component drill holes are about 10% further apart than they should be. Not such a problem for resistors and capacitors but fatal for DIL chips.
He's apologised and undertaken to do the job again at his cost, this time using the image transfer at 1:1 as I expected the first time around.
Oh dear.
If you get some wirewrap DIL sockets you can maybe spread the legs on them to fit the enlarged board; it'll look like a spider perched on the board but might enable you to use the 'free' board for a second module.
http://www.rapidonline.com/cables-connectors/wirewrap-turned-pin-dil-sockets-truconnect-63770/
Quote from: Owain on August 27, 2013, 04:31:16 PM
Oh dear.
If you get some wirewrap DIL sockets you can maybe spread the legs on them to fit the enlarged board; it'll look like a spider perched on the board but might enable you to use the 'free' board for a second module.
http://www.rapidonline.com/cables-connectors/wirewrap-turned-pin-dil-sockets-truconnect-63770/
Thanks, that's good idea. Hopefully the replacement will be correct, otherwise I may have to adopt the "spider" approach.
Quote from: GTC on August 24, 2013, 05:00:42 AM
I agree with what you say about mains hum, and I'd love to have a modulated tone a la exchanges, but I'm looking for a simple solution. I'll probably just use a simple oscillator.
Audio store/replay chips/modules are so cheap now, especially as you only need a second of tone and can loop it, that you could easily capture some dialling tone from somewhere.
For a rather comprehensive recording of British telephone tones, listen to
https://www.youtube.com/watch?v=rYa9DPgWSk8
At about 7' there is interrupted 50Hz AC as a ringing tone on a small railway PAX
I like the doorbell dial tone... :D
Quote from: Owain on August 28, 2013, 10:59:02 AM
Audio store/replay chips/modules are so cheap now, especially as you only need a second of tone and can loop it, that you could easily capture some dialling tone from somewhere.
For a rather comprehensive recording of British telephone tones, listen to
https://www.youtube.com/watch?v=rYa9DPgWSk8
At about 7' there is interrupted 50Hz AC as a ringing tone on a small railway PAX
Thanks again. That's quite a comprehensive sound clip.
Those are all good ideas, but I have just bought this:
http://tinyurl.com/qxegc8s
I think it would be nice to built the relay PBX actually as designed; it's kinda vintage in its own way.
Quote from: twocvbloke on August 28, 2013, 11:08:21 AM
I like the doorbell dial tone... :D
That's bizarre! It made me think of a potential alternative: a cuckoo clock chiming. That would've been a very applicable tone for a couple of companies that I've worked in. LOL!!!
Quote from: Owain on August 29, 2013, 03:21:32 AM
I think it would be nice to built the relay PBX actually as designed; it's kinda vintage in its own way.
True, but as I had to replace the power supply owing to the unavailability of a suitable transformer for the original design at anything like a reasonable cost, I needed a new dial tone option. There was no way I'd tap off the 240 volt mains to use 50Hz hum, and the idea of using a (say) 240 to 12.6 volt transformer simply to get (uninterrupted) low voltage hum wasn't very attractive to me either.
Looking at that tone generator chip prompted me to notice that this design apparently doesn't cater for busy tone. If not, then I'll try to figure out how to do that now that I have a suitable generator on hand.
When I get the time, I should combine the schematic into one diagram so that I can trace out the various signal and control paths more easily than I can over two pages. If I can digitize it then it will also enable production of a wiring diagram.
I realize that I'm wandering a bit from the vintage electromechanical design but, with 22 relays in it, it's still vintage and electromechanical enough for me.
I made a dial tone generator with a small brush type dc motor in series with a filtered power supply feeding a small transformer on the low impeadance side
My PAX dial tone and ringing here:
http://www.classicrotaryphones.com/forum/index.php?action=dlattach;topic=4185.0;attach=34107 (http://www.classicrotaryphones.com/forum/index.php?action=dlattach;topic=4185.0;attach=34107)
One vibrator making the dial tone (and busy), another the 25Hz ringing.
dsk
Quote from: rdelius on August 29, 2013, 11:45:13 AM
I made a dial tone generator with a small brush type dc motor in series with a filtered power supply feeding a small transformer on the low impeadance side
Nice idea.
I had thought of using a small genemotor and looked around for one running on 24v DC, but to no avail.
Quote from: dsk on August 29, 2013, 12:00:04 PM
One vibrator making the dial tone (and busy), another the 25Hz ringing.
That's quite a PAX you have there!
;D I'm really proud of that one, not only saved from the container, but I have got it working, and that's more than the Norwegian telemusum have with their bigger version.
http://www.youtube.com/watch?v=JdGaCSdnLAg (http://www.youtube.com/watch?v=JdGaCSdnLAg)
dsk
Quote from: GTC on August 27, 2013, 08:07:42 PM
Quote from: Owain on August 27, 2013, 04:31:16 PM
Oh dear.
If you get some wirewrap DIL sockets you can maybe spread the legs on them to fit the enlarged board; it'll look like a spider perched on the board but might enable you to use the 'free' board for a second module.
http://www.rapidonline.com/cables-connectors/wirewrap-turned-pin-dil-sockets-truconnect-63770/
Thanks, that's good idea. Hopefully the replacement will be correct, otherwise I may have to adopt the "spider" approach.
I received the replacement board today and happily the chip sockets fit perfectly. So, I'll proceed to construct this little project while I wait for the ordered parts for the mini exchange to arrive in my mailbox from various parts of the world.
The dial tone at my old pax is just generated by a buzzer (SUM) at the diagram: (http://www.classicrotaryphones.com/forum/index.php?action=dlattach;topic=10104.0;attach=57591;image)
The coil has an extra winding, acting as a transformer, and giving you the sound without contact to the power-supply.
A rough sound with many frequencies, but mainly near 400 Hz. It disturbs all tone to pulse boxes except for the Mitel SMart-1.
dsk
The'res a dial tone generator circuit here
http://www.theremin.us/Circuit_Library/dial_tone_generator.html
At page 90 here: http://www.scribd.com/doc/163713446/Manual-T-N-50-6-exchange-1964
You may see a circuit providing ringing current and dial tone.
dsk
Well, I think there may be a curse on this project.
Four weeks ago I placed a large order of 24 volt relays on an American company. They offered PayPal and as that is my preferred method I created the order only to find on the checkout page that PayPal was not available.
So, I contacted them and they said that, due to back office server connections, PayPal is only available during certain hours. Huh? Never struck that situation before. So I asked them to send me a PayPal invoice. They replied that they cannot do that and would have to cancel my order and I would have to recreate it. Something about computer systems not linked.
Okay, as they were the only company with stock of the required relays at a reasonable price I went ahead and created a new order, and paid with PayPal.
Next, I get an email from their credit person telling me that the order number associated with my payment was not mine but belonged to somebody in another country altogether. Huh?
I referred that to their customer support who emailed back to say the credit person was wrong and that it was indeed my order.
Next I was advised that the goods have been shipped.
Today, as I cleared my PO box of various overseas deliveries, I wondered where my relay shipment is. So, I checked the invoice for the shipment date and noticed that the box number in the delivery address is incorrect making the shipment undeliverable at my end. >:(
(My correct shipping address is in my PayPal payment details, as per usual.)
So, presumably my relays have been stamped "No such number. Return to sender" and are somewhere in the international mail system on a return journey to the supplier.
I have reported this snafu to the supplier and asked for a status report.
Okay, the gods of telephone exchange projects have smiled on me.
I traced the delivery via the Post Office who confirmed that the package had been collected by the owner of the incorrect PO box, so I wrote to the owner via registered letter and today got a call saying they have been holding my package pending contact from me.
My phone number is on the shipping documentation, so they could have called me weeks ago and saved me the angst, but as they say: all's well that ends well.
Congratulations. :D
We will follow your project.
The idea of making a course of this are really good.
Please show us print-card layout, parts-lists, and how things works out etc.
Good luck
dsk
:) Happy new year!
How does your project turn out?
May a music tuning device work as a dial tone?
dsk
Happy New Year to you, too!
The project is still in its shipping cartons. I am in the process of moving to a new and larger workshop. Once I'm settled in I will re-prioritize my long list of projects, both construction and restoration, and work my way through them.
I will get to this one in due course and will update this thread accordingly. I know a couple of kids who will want to have fun with this exchange -- and I'm one of them. :)
A music tuning device may indeed suffice, but I have purchased a number of chip based telephone equipment solutions and will use the one that sounds best.
Hello GTC,
I am wondering if you ever got around to finishing this project as there has been since January 2014. If you did how did it work?
I am considering building it myself.
Ross
Not active on the forum since August 27, 2016 so he quite likely won't see your question. You could try sending him a PM as he may be set up to receive an email notifying him of your PM.
Terry
Thanks Terry... I should have thought of that myself ;D
Have now PM'ed the OP.
Ross
Good luck! Otherwise you can maybe explore some of the many other options discussed on the forum.
Terry
I had a quick PM from GTC, who hasn't made the PBX yet but still hopes to get around to it.
I received a similar reply to my PM...
I am also in the early stage of building this little exchange since I have most of the relays which will do the job - I only have to source a few on Ebay. I will be using all 48V relays and a PBX 48V power supply with integral isolated 80V,25Hz ringing supply.
On studying the schematic of the 10 line exchange I was rather perturbed by the rather crude design of the called line battery feed relay which also serves as the answer ring detect relay. Insofar as the speech battery feed goes I have no problem but the ring detect function just can't work - at least not in any of my bench experiments and certainly not with 25Hz. There is no way that a standard off-the-shelf relay serving as relay D can be made to be AC immune to the ringing current simply by shunting one half cycle of ring current through a diode-resistor shunt arrangement - it still buzzes no matter what value of resistor I try.
My other concern is that the way the circuit is arranged... if a called telephone answers during a 2 second ring burst it will produce a loud 50/60/25Hz buzz in the receiver which could blast the eardrum depending on how quickly it is placed to the ear. The only time relay D can operate to an answer loop is during the quiet period between ring bursts when the -VE supply line is connected behind the D relay.
I am going to use the standard DC biased ring feed arrangement similar to the old Strowger final selector where the ringing supply has +VE behind it and returns via the called telephone to 200 Ohm -VE. In the case of the 10 line exchange all of the telephones use a common +VE on one side of the line which demands that the ringing supply has -VE behind it and this is shown in my sketch of the ring feed/trip arrangement below. In the final selector a double winding heavily slugged relay (usually F) has ringing fed to line via one winding while the other winding is held short circuited by an 'x' (early operate) normally closed contact which in conjunction with the slug, makes it immune from operating to the AC ringing current. However, when the called telephone answers the DC loop DC current flows through the ring feed winding which causes it to operate sufficiently to remove the short circuit from its other winding which then locks solidly to external circuitry via both of its windings. Relay F thus acts as the ring trip relay and it will operate reliably during bursts of ringing or during the breaks between thereby preventing any possibility of painful bursts of ringing in the eardrum.
The only problem with this is that short of pulling a rather large F relay out of an old selector I have not been able to find a small PCB mount relay with the right characteristics to replicate the function of the F relay used in the selector. Not to be deterred, I found I had a good number of Electrol TLS1A18A10 line sense relays which have two 18 Ohm windings and a single NO reed contact inside. Admittedly, not a slugged relay but I figured I could get it to do the trick (I actually experimented with a line sense relay made by Erg but the only difference is that it has two 25 Ohm windings). By connecting the windings in series opposing and shunting one winding with a diode I was able to make the relay immune to the AC ringing current through it but still allow it to operate to an answer loop which causes the windings to become unbalanced with respect to each other and thus to operate to the DC loop current. Sure, the line sense relay will still chatter at the ringing frequency but at the first make of its contact a standard PCB relay will operate and lock thus cutting the ringing current and causing the D relay of the exchange to operate and perform its normal battery feed function.
This is the circuit I came up with.
Sounds exciting.
I must confess that following the circuit diagram to understand it is a bit beyond me!
I don't think I've found this page before, but the author discusses the problems of ring trip at 20 Hz on home exchanges (scroll down to Ring Trip section)
http://www.thompdale.com/pbx/home_pbx.htm
Owain,
Thanks for the link to Dale Thompson's little exchange. I must admit I hadn't come across the web page before despite many Google searches - I guess my search terms didn't include Home PBX or the like. Perhaps he should have titled it to include the terms 'telephone" or 'exchange' which is what I would have searched for.
I must say that he has done a superb job on the design but is certainly looks a lot more complex than Andrew Holme's 8 line exchange. Unfortunately, the page format on my old monitor makes it necessary to scroll the page horizontally in order to read the text and then scroll back to refer to the circuits.
He does cover the Ring Trip problem very well but doing the task with semiconductor devices makes it very complicated when just 2 relays and a diode make it easy to do. The sketch I attached to my last post is only what I did on the bench to prove that it worked and doesn't show how it hooks into the 10 line exchange circuit, but this detail is easy to work out. I will draw out the relevant section of the circuit to show how it can be done and post back later.
I will takes copies and print out the text so I can study it more in depth when time permits.
Quote from: rherber1 on June 26, 2018, 10:26:08 AMdoing the task with semiconductor devices makes it very complicated when just 2 relays and a diode make it easy to do
Generations of telephone people would agree with that.
Further to my previous post on a modified ring trip arrangement for this 10 line exchange I have come up with a sketch showing how it would be incorporated into the original circuit. During my study of the circuit diagram in conjunction with the description (this is rather vague and difficult to follow btw) I noted a couple of inconsistencies which require circuit changes. Firstly, to prevent an undesirable momentary operation of relay FC during clear down following hang up, and secondly, the sending of ring tone to the calling line before dialling is completed.
Taking the 2nd point first - when a call is initiated relays A, B and C (among others) operate. At the first break of the dial impulsing contact relay A releases and FC operates via B2 and the diode. Contact FC3 connects ringing via RI1 normal - FC1 operated - D coil - via the 1u8 bridging capacitor and 100nF + 3K3 (ring tone bleed) directly to the top of the A relay and out to the calling line via contact CA2 operated. This means that ring tone is heard by the caller even before dialling is completed. To correct this anomaly it is necessary that the 1u8 bridge capacitor is connected at the other side of the C2 contact to that shown in the circuit. Thus, only when relay C releases at the end of dialling does ringing get connected to the called line, and at the same time ring tone is fed via the 1u8 capacitor and tone bleed to the calling line. Only after the called line answers and relay D operates does the 1u8 capacitor get connected directly between the calling and called lines when speech can take place.
The other point I noticed was that at call completion, because relay B is slow to release due to the capacitor across it, a re-operate path is established for relay FC via the now released contact A1 - via contact B2 (slow release) - Diode at B2 - relay FC and the -VE rail, thus allowing FC to operate for the time it takes for B to fully release. While this momentary operation of FC doesn't appear to do anything untoward it is unnecessary and not good practice. The solution is to introduce a break contact into the initial operate path for FC via the diode at contact B2 which is only closed again after relay B has finally released.
I have incorporated both of these solutions into my modification. I have included only the relevant section of the circuit to show how the modification is applied.
Note that relay LD is a line sense reed relay (Erg PM55/3 or Electrol TLS1A18A10) - I experimented with the former which has 25 ohm coils while the Electrol has 18 ohm coils.
The windings of LD are connected in series opposing so that it will not operate to the AC ringing current (25Hz) going to line. Only when the called telephone answers does DC current flow via the windings of relay LD from +VE via the answering telephone loop - C2 released - RT2 released - diode on top coil of LD - lower coil of LD - FC3 oper - RI1 (either normal or operated) and back to -VE. The DC path for LD is completed to the -VE rail behind the ringing supply when RI1 is released or direct when RI1 is operated thereby allowing ring trip to occur as soon as the called line answers (does not have to be during the silent period as in original cct). Due to the shunting effect of the diode on the top winding of LD the current in the coils is now unbalanced and LD will operate. At the first make of LD1 contact relay RT (new) will operate and lock via RT1 to +VE at B1. RT2 switches the called line over to relay D - which operates, while at the same time opening the ringing supply to the called line. Contact D1 establishes the speech bridge via the 1u8 capacitor (I show 2uF).
After a call is completed relay B will remain operated as long as the called line has not yet hung up. When the called telephone hangs up relay D releases and breaks the holding path for relay B at D2 (now normal) and relay B releases slowly due to the shunt capacitor. Only after B1 is open will relay RT release and via RT3 re-establish the initial operate path for relay FC at the next call establishment.
It also occurred to me that the ringing interrupter flip-flop - due to the same timing components being used on both sides, would seem to indicate that rather than the cadence being 2 seconds on and 4 seconds off (as stated in the description) it would be more like 2 seconds on and 2 seconds off.
That's really interasting the other day I was designing a circuit using 4 relays for a small phone exchange I was going to use in my new service called NPSTN for New Public Switched Telephone Network its similar to C*NET.
If anyone wants a number on the network just PM me :)
But thats for posting this I will see if maybe I can use this circuit! :)
I should have spotted it earlier but it occurred to me after posting my circuit that contact FC3 is now redundant since its function (disconnect ringing supply at answer) is now performed by contact RT2. Therefore FC3 should be taken out of the circuit.
The first part of the Maplin Digi-Tel electronic exchange from the 1980s is in this online version of the Project Book 04. It uses the 45100 crosspoint IC.
https://www.americanradiohistory.com/UK/Mapelin/Maplin-Project-Book-04.pdf
Quote from: Owain on July 01, 2018, 08:19:45 AM
The first part of the Maplin Digi-Tel electronic exchange from the 1980s is in this online version of the Project Book 04. It uses the 45100 crosspoint IC.
https://www.americanradiohistory.com/UK/Mapelin/Maplin-Project-Book-04.pdf
Thanks for the link.
It is quite a sophisticated project and it wouldn't be cheap to build - even assuming all the parts were available. For instance, I can't find one hit on "45100 crosspoint switch" when searching for a datasheet with Google.
I have spent some considerable time scouring through the magazines and journals on this website and it is quite extraordinary what you can find. It is a great resource.
Quote from: rherber1 on July 01, 2018, 11:44:52 PM
It is quite a sophisticated project and it wouldn't be cheap to build - even assuming all the parts were available.
It wasn't (and I didn't!). Even their "low cost" telephones were about £30-40. I wouldn't expect that many people actually did build it, but it's amazing the complexity of some of their projects using the technology of the time. Maplin Matinee electronic organ, MOSFET amplifiers. They launched a build-your-own modem kit and had a dial-up ordering/bbs system.
For non-UK readers who don't know, Maplin finally went into administration this year and closed the last of their shops a few weeks ago.
There's a small uniselector exchange described here
http://www.samhallas.co.uk/repository/documents/miniature_exchange.pdf
G F Goddard, Wireless World, February 1972
Update on progress:
I have successfully built the dial pulse counter section along with the called line decoder contact tree. I used Panasonic/Aromat DS4E-M-DC48V relays for the SA,SB,SC,SD relays and Meisei M4-H-48 (Now Maluska Elektronik) for the 2 C/O type relays since I had a hundred or more on hand.
I am now in the process of designing the calling line selector circuit. The original circuit uses 4 relays with a diode encoder to produce a 1,2,4,8 BCD encoder circuit around CA, CB, CC and CD relays. When I looked at this arrangement it occurred to me that if 2 phones happened to be lifted simultaneously (unlikely, but possible) ie, Tel 1 and Tel 2, the calling line selector would operate relays CA and CB together and this would decode as Tel 3 originating the call and operation would be halted until both phones hung up. While the selector circuit saves on relays it doesn't provide an exclusive 1 of 10 selection. I decided to scrap it and use 10 individual line relays with a chain of contacts wired so as to allow only one relay to latch if more than one line is looped simultaneously. This means the smaller inexpensive M4 relays can be used instead of the expensive 4 contact types (CA, CC, CD).
Also, the original circuit did not guard against the calling line dialling its own number. The call would progress as normal and a burst of ringing would go to line but would be tripped immediately due to the calling line loop. I think the caller would still receive a short loud burst of ringing in the ear. By arranging for one contact on each line relay to be inserted in the called line contact tree formed by the SA - SD relays then when a calling line is identified by its line relay operating, it disconnects its own line from the called line tree and at the same time connects the line through to the A relay.
Further reports to come...
This is getting quite exciting :-)
Has anyone made this? If so can they post a video to YouTube and link it here?
I am thinking of putting this together..
I can now report that the 10 line exchange is now working...tests so far have shown no operational issues.
The final circuit now uses 32 relays mainly due to dispensing with the BCD calling line identifier and substituting it with individual calling line relays with lockout function. The ring cadence generator is now a double multivibrator which produces the standard UK/AUS 0.4 on- 0.2 off - 0.4 on - 2.0 off pattern. The original dial tone and ring tone feed arrangement is totally useless and has been replaced with a transformer feed arrangement. My 48V DC power supply also incorporates a 25Hz @ 80V ringing supply so the DT and RT are at this frequency.
My final arrangement consists of 4 boards all using wire-wrap technique. Board 1 holds the 10 line relays and the 8 relays in the dial pulse counter. Board 2 holds the A and D relays plus all other control relays. Board 3 is the ring cadence generator. The 4th board simply holds the tone feed transformer because there was no room available on the control board - where it would normally have been located.
When I get all the circuits tidied up I will post again.
Wow.
Cool!
I would LOVE to see a video on that if possible!
Sounds Great! When it is all ready would you post some pictures? I for one, would like to see it. I am sure others would be interested too.
https://www.americanradiohistory.com/Archive-Wireless-World/80s/Wireless-World-1980-08.pdf
Go to page 41
has a LOT more information with more high quality images for this little diagram!
=================
EDIT: Entire document and Pages 41-45 attached as a PDF files
Quote from: dylanc4 on February 12, 2019, 01:24:00 PM
https://www.americanradiohistory.com/Archive-Wireless-World/80s/Wireless-World-1980-08.pdf
Go to page 41
has a LOT more information with more high quality images for this little diagram!
Note also the corrigenda in the following issue, which I included in my original scan.
Well, I have been prompted to do something about getting something posted on my arrangement of this project so I will start with a sketch of the line relay and binary counter stage. I will post other docs and pics as I can get around to it. It is a pdf with 2 pages so you will have to join them to see the full diagram.
You may find some relays for this type of project from Bob833
Here is a link to some parts he has:
http://www.classicrotaryphones.com/forum/index.php?topic=24271.msg241352#msg241352
Quote from: rherber1 on May 29, 2020, 07:55:06 AMWell, I have been prompted to do something about getting something posted on my arrangement of this project so I will start with a sketch of the line relay and binary counter stage. I will post other docs and pics as I can get around to it. It is a pdf with 2 pages so you will have to join them to see the full diagram.
Did you ever take photos and do any more documentation for your adaptation of the 22 relay exchange rherber1 as its been a couple of years now?
Quote from: Mark.R on March 13, 2023, 04:14:26 PMDid you ever take photos and do any more documentation for your adaptation . . .
Probably won't reply. Hasn't logged on since February 2021.
Quote from: TelePlay on March 13, 2023, 08:09:15 PMProbably won't reply. Hasn't logged on since February 2021.
You could be right there which is a shame.