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-   -   Pulse (Loop Disconnect) to Tone (DTMF) Converter? (

DoctorWho 22nd Feb 2007 12:51 pm

Pulse (Loop Disconnect) to Tone (DTMF) Converter?
Does anyone know where you can buy a converter that allow you to connect a pulse dialling telephone to a telephone line or system which only recognises tone dialling?

I do remember some years ago having an old BT internal phone system which supported dial phones and converted the pulses to tone dialling when accessing an outside line.

What I basically need is something that I can connbect between a dial phone and a telephone line so that each time I dial a number it is converted in to the correct tone for that number.

Any help greatly appreciated.


Biggles 22nd Feb 2007 1:33 pm

Re: Pulse (Loop Disconnect) to Tone (DTMF) Converter?
The device would have to count the pulses then activate a DTMF tone. I am not sure what time interval window between DTMF tones is acceptable to the PSTN. In other words, if you dialled a nine, this would create quite a delay while the circuit counted the pulses then generated the DTMF tone. I'm just guessing about this, and have never tried it though. I have seen DTMF tone chips and pulse dial chips but never a pulse to DTMF conversion chip. Maybe someone with direct experience of BT networks would be able to help.


DoctorWho 22nd Feb 2007 8:01 pm

Re: Pulse (Loop Disconnect) to Tone (DTMF) Converter?
Well after a lot of searching it seems that one company in the US does indeed make such a unit, they call it the Pulse to Tone Converter, it can be seen here about 3/4 of the way down the page:

Oddly enough the reason that I want this is mobile phone related, I found a unit for sale on eBay which allows you to connect a regular telephone to the mobile network, but most of these units seem to only accept DTMF, I want to be able to use a 200 or 300 series telephone as a car phone, and so using the 12 volt supply from the car this is easily possible, all that is needed is the pulse to tone converter.

I have ordered one of these American units, so it will be interesting to see how well it works.

This is the type of unit I purchased from eBay (this listing is in no way related to me, I do not know the seller):

It seems that the use of vintage phones on the mobile network is possible, which is great news. Just imagine a candlestick phone as your car phone!

AndiiT 22nd Feb 2007 10:04 pm

Re: Pulse (Loop Disconnect) to Tone (DTMF) Converter?
As suggested elsewhere, the THG are a useful group for this sort of thing and I seem to recall Bob Freshwater (of bobs telephone file ), who is a THG member, mentioning a while ago that he had some Pulse to DTMF converters for sale.

Alternatively the BT Revelation and Inspiration PBXs will convert Pulse trains to DTMF and also the BT (Siemens) Diverse 2 - line does the same, although the Diverse 2 line is difficult to obtain and usually goes for quite a high price on e-bay, although the Revelation (particularly) and the inspiration seem to sell for more sensible prices.

If you decide to go down the Revelation/inspiration route make sure that you obtain at least one sytem phone in order to do the programming.


beery 23rd Feb 2007 12:41 pm

Re: Pulse (Loop Disconnect) to Tone (DTMF) Converter?
1 Attachment(s)
Hi Peter,
I actually made my own one to enable me to use a 300 series on my desk at work.
It did mean I had to put the circuit in the phone, but I did not have to mess around with the original workings.
I cheated by not including a bridge rectifier which has made the phone polarity sensitive, but that is ok as line reversal is not used these days.
I've included a circuit. If anyone is interested I'll have a look in my phone to see how it is connected and I'll upload the PIC code.


AndiiT 26th Feb 2007 7:50 pm

Re: Pulse (Loop Disconnect) to Tone (DTMF) Converter?

Originally Posted by beery (Post 103935)
I cheated by not including a bridge rectifier which has made the phone polarity sensitive, but that is ok as line reversal is not used these days.

Whilst line reversal isn't used for metering purposes (like it used to be) I have seen line reversal on some switches (exchanges) on an incoming call, also what about protection of the semiconductors (unless I have missed something in the circuit) during the AC ringing cycle?
In some cases ringing current is generated by chopping (and reversing polarity at 25Hz) the DC line voltage.

Additionally although line polarity is supposed to be maintained some faults/linesmen aren't too fussy about this so you cannot guarantee that the 'B' Leg will always be the -ve one.

Personally I would always fit a bridge rectifier in any device which is to be powered from a POTS or (analogue) PBX line


jim_beacon 26th Feb 2007 7:51 pm

Re: Pulse (Loop Disconnect) to Tone (DTMF) Converter?
The first generation Mercury adaptors will do LD to DTMF, and there is a code to reprogram the unit, so that it doesn't dial the Mercury access code first.


DoctorWho 6th Mar 2007 12:12 pm

Re: Pulse (Loop Disconnect) to Tone (DTMF) Converter?
Well I can now report very successful findings in this area.

Firstly the pulse to tone converter from the USA works extremely well, you can use it on a landline or with a PremiCell/GMS Adaptor perfectly well.

Secondly, there are a number of PremiCells/GMS Adaptors available, there are some Nokia models from the late 90's which support pulse dialling, I have managed to track down two of these, one for 02 or Vodafone and the other for Orange or T-Mobile.

They are extremely simple, you insert your SIM card in to the unit and connect it to a power supply (either mains via an an adaptor at home, or else though a cigarette ligheter adaptor in your car, there is also a re-chargable battery unit which allow you to use it anywhere, even when walking around.

You can then connect up any vintage telephone you want, I've tried 200 series, 300 series and 700 series telephones, and they work perfectly well. You can dial out and make a call, just as you would on a landline, you hear the usual dialing tone and you just dial. When a call is received the telephone rings and you can just answer it, you can also use a caller display unit between the telephone and the Premicell/GSm Adaptor if you wish, this then allow you to see who is calling.

At last it is now possible to have a Bakelite mobile telephone!

I didn't want to risk damage to a 300 though, and so our car telephone is now a black 706 from 1961.

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