Making a Pulse Dial (also called Loop Disconnect) to DTMF Converter

[Napalm]

Hi Everyone,
So one of my current projects to to make a converter for an old BT #332 phone. I want to make it work with the newer telephone system.

There is a little room in the case for some small electronics without removing the old components. So I was thinking of small board running a ATtiny todo the detection of the pulses from the rotary encoder and convert those to DTMF. This is not a problem for me. I probably could find a DTMF generator but then I'd need to use say some 7400 series IC's to decode the rotary encoder. So I'm thinking the micro is best to keep the component count low. The part which I have limited experience with is the power supply side.. coming up with a design that will fit the requirements.

If the electronics are placed inside the the case and they get quite hot the old Bakelite case might be at risk. So I'm thinking some type of switch-mode power supply. I don't want the telephone expending too much power for no-reason since it will be idle most of the time. So when the phone rings or the handset is picked up the power is turned on. The BT spec's say the potential between the 2 telephone line terminals will not exceed 70V when off-line. The 'Call Arrival Indication' (Ringing) will be between 100V and 40V AC rms. The receiver for the handset has a switch to detect pick up.. so detecting that is not much of an issue. But still need to detect the Ringing. The BT line specs are linked to at the bottom for full details if anyone is interested.

So currently I need a way to step-down the line voltage which can vary.. to 5V DC for the micro, and detect when the phone is ringing so the micro can alert the user in some way.

I have not yet decided if I want to keep the rest of the internals. I could replace the current microphone and the speaker with newer parts (this would be preferable). The ringing bells would probably have to go.. since I have not yet worked out how I would interface this with the micro.

I have attached some images, as well as the original wiring diagram (which is printed on a label on the inside). Gone are the days manufacturers would do that!   I have purchased a new braided wire which is insulated properly and put that on you can see that in the image of the inside.

For those that are interested I have wired up a new BT phone connector to the phone and it still works! this phone is about 40 years old! They don't make them like they used to

Any help/ideas on where to go with this would be appreciated.

Thanks,
Napalm


Links:
British Telephones - BPO Telephone No. 332 and variants
BT Public Switched Telephone Network (PSTN): Technical Characteristics Of The Single Analogue Line Interface (400KB PDF)

[Kiriakos-GR]

Well I have another approach to suggest ..  

Build or buy an old small telephone center unit, that accepts analog and " digital " telephones on it,
this would possibly do the conversion , about the dialing part.

I had work for Panasonic, telephone centers installations and servicing photocopiers ,  for more than three years.

In some of the customers that I had install an modern Panasonic center , I had see such devices,
but I do not remember any brand of those.
And actually after the installation of the new center , there was no need for them , so I was totally removing them.

[NiHaoMike]

A regular analog phone line is 50V DC open circuit, pulled down to 10-20V or so by the phone when it is active.

A simple solution is to use a shunt regulator.

[cybergibbons]

If it works, why replace any of it?

[TopherTheME]

Sparkfun has something similar to this except they converted the rotary phone to wireless so its compatible with GSM.

http://www.sparkfun.com/commerce/product_info.php?products_id=286

[Napalm]

Hi,
Thanks for the replies.

@NiHaoMike: If I used a shut regulator like a 7805 wouldn't that got hot quickly?
@cybergibbons: Most of our phone services in the UK require DTMF.
@TopherTheME: One of the reasons for doing this project was not to spend lots of money.

At the moment I think I might be approching the problem wrong. Perhaps I could leave the phone as is and pass-thru the outside line to the phone. Then just detect the dialing pulses somehow? and convert those to DTMF.

Still requires further thought.


Napalm

[cybergibbons]

Napalm - really? All System X and AXE10 exchanges still support it, the 21CN network is likely to still support it. It certainly still works fine in at least 4 locations in the South East. No idea if you move away from BT, but then you aren't talking about most services.

There's quite a few pulse -> DTMF converters about - Dialgizmo is one.

[JohnS_AZ]

Dial phone still work here as well (Arizona). However if you need to get at your voicemail you'd be kinda screwed.

[NiHaoMike]

The interesting part is that if you're designing a converter box to turn your old analog phones into VoIP phones, pulse dialing is much easier to decode than DTMF. Might even be possible to have the box based on a PIC and some kind of Ethernet chip.

And it might be possible to modify the dial mechanism to speed it up and reduce one of its main disadvantages.

[Excavatoree]

...And it might be possible to modify the dial mechanism to speed it up and reduce one of its main disadvantages.

I know that the governor on a "500" set can be modified by using a tighter spring (requires more centrifugal force to move the brake shoes against the stationary drum and thus the dial moves faster) I think the dial in this older model is similar.

I used to play with these, and after you cleaned up the mechanism (many were slower than stock due to dirt in the works) I would get the fastest dial possible that would work with the local exchange.

This looks like an interesting project. 

[Zero999]

@NiHaoMike: If I used a shut regulator like a 7805 wouldn't that got hot quickly?

I would've thought the LM7805 would use too much power?

To be honest, I haven't read the phone spec's but from what I gather, the standby current when the phone is hung up needs to be tiny but I don't know what the limit is when the phone is off the hook, it could be higher. Fortunately your circuit only needs to be powered when the phone is off the hook but you need to disable it when the phone is not in use.