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and very rarely ever needs repairs, once you fix them." - Dan/Panther

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Miniature 10-line telephone exchange using 22 relays

Started by Owain, January 29, 2012, 10:21:34 AM

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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.



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.

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:

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.





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



 :) Happy new year!
How does your project turn out?

May a music tuning device work as a dial tone?



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.



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.



Thanks Terry... I should have thought of that myself  ;D

Have now PM'ed the OP.