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Old 14th May 2019, 11:37 am   #1
dominicbeesley
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Default Homebrew Strowger exchange

Hello, I know next to nothing about telephones and would like some advice - please go easy on me!

My 4 year old daughter found my box of battered old 746 telephones I acquired over the years and was fascinated by them. She's been (fairly) patiently helping me strip them down and give them a deep clean - filthy nicotine soaked, paint spattered and grimy as they were, truly "mossy".

I've now rigged up a pair in her bedroom with a 9V battery and she loves them. I'd now like to take it to the next step and make them ring and, if possible, rig up a few more extensions and make a mini-exchange. I've not much more than a vague idea of how it all works but would it be as simple as one/two uniselector(s) and a few relays or is it a lot more complicated than that.

I'm thinking something simple that would allow any one out of ten 'phone be able to dial any other which would then ring.

Would making a homebrew strowger exchange be a possibility, a weekend project or a long term labour of love?

Does anyone have ideas where I should start to look for inspiration? I've done some googling and reading about linefinders and allocators but I'm not sure how/if I can do away with them in a simple exchange like this - would it just need a simple relay for each subscriber?

Any ideas or inspiration would be appreciated

D

D
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Old 14th May 2019, 11:53 am   #2
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Default Re: Homebrew strowger exchange

Quote:
Originally Posted by dominicbeesley View Post
Any ideas or inspiration would be appreciated
An Arduino and a few relays should be able to handle a job like that.
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Old 14th May 2019, 12:29 pm   #3
dominicbeesley
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Default Re: Homebrew strowger exchange

Yes, thanks, it's a good idea and I had given that some thought (I've got so many Raspberry PI, Arduino and PIC boards that are begging for a job). But it wouldn't be as much fun, either for me or my daughter but I think a lump of chattering rattling metal would!
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Old 14th May 2019, 1:08 pm   #4
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Default Re: Homebrew strowger exchange

Quote:
Originally Posted by dominicbeesley View Post

I'm thinking something that would allow any one out of ten 'phone be able to dial any other which would then ring.

Interesting you say ten extensions.

See if you can lay your hands on an old BT Minimaster 3 PABX, they were once very popular in offices etc. they support dialling between up to ten extensions, (and two exchange lines).
They are based on an 8 bit processer chip (Z80 I think) but they are full of relays that clatter like good-uns as you dial. A positive advantage in your application!
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Old 14th May 2019, 1:58 pm   #5
M0FYA Andy
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Default Re: Homebrew strowger exchange

Post Office 3000 relays and uniselectors only allowed, none of these new-fangled digits, just the Mk1 used to operate the dials!

Andy
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Old 14th May 2019, 9:40 pm   #6
Ed_Dinning
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Default Re: Homebrew strowger exchange

Hi Dom, there was an article in WW in the 70's that covered the design and construction of such a system using relays and uniselectors for home use.
It should be in one of the online ww indexes.

Ed
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Old 15th May 2019, 12:13 am   #7
Craig Sawyers
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Default Re: Homebrew strowger exchange

I suppose you guys already know the Strowager story? He was an undertaker in Kansas city. But his competitor's wife was a switchboard operator. So when someone tearfully told her that they needed an undertaker, his competitor got the job.

So Strowager reasoned that an impartial system had to do away with the switchboard, and set about automating it, eventually setting up a company to commercialise his patent, which covered pulse dialling and connection apparatus to route the call.

Craig
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Old 15th May 2019, 11:06 am   #8
dominicbeesley
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Default Re: Homebrew Strowger exchange

Thanks all,

Ed, good pointer - I've located a couple of WW articles for inspiration

Craig, I knew he was an undertaker - I'd not realised the motivation was to avoid anti-competitive practice!

Now to find some reasonably inexpensive uniselectors and relays...and sneak them past the missus!
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Old 15th May 2019, 5:17 pm   #9
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Default Re: Homebrew Strowger exchange

There is this one:

http://www.samhallas.co.uk/repositor...e_exchange.pdf

as well as the 10-line relay telephone exchange I have attached, which is also available here

https://www.americanradiohistory.com...ld-1980-08.pdf

(note the corrigenda in the following month's issue)

If I may link to another forum, the project has been discussed extensively here

http://www.classicrotaryphones.com/f...?topic=6218.60
Attached Files
File Type: pdf 10 line telephone exchange.pdf (1.40 MB, 25 views)
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Old 16th May 2019, 11:21 am   #10
dominicbeesley
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Default Re: Homebrew Strowger exchange

Thanks OFT,

I've had kind offers of uniselectors and relays and some more diagrams so the 1972 article looks like a good starting point. I suspect this will be a slow burn (with a lot of experimenting) while I work all this stuff out...and find space for it!

D
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Old 16th May 2019, 11:59 am   #11
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Default Re: Homebrew Strowger exchange

I did this a few years back, and used the ringer settings referred to in the comments:

https://www.instructables.com/id/Hac...hone-Intercom/

Essentially using unlocked Linsys voip adapters, one phone rings when the other is picked up

I used 2 Linksys boxes and changed the settings slightly so instead of 2 phones into 1 unit, I have 1 phone in each, and the calls go over IP to the other module / phone. But that was because I was linking 2 distant rooms that happened to have Ethernet..
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Old 17th May 2019, 1:11 pm   #12
dominicbeesley
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Default Re: Homebrew Strowger exchange

That might be an idea as a stop gap!

D
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