Familiar UK telephone dial - dimensions

[Karen O]

Hi All,

I'm interested in knowing the dimensions of the UK telephone dial that we're all familiar with. Specifically, over all diameter, finger hole diameter, angular spacing of the finger holes, etc.

I'm hoping to implement a dial using modern parts

Many Thanks.

[60 oldjohn]

Will this help? Finger dial is 79.3mm across Finger holes 11.9 mm D. 2.5 mm of plastic between the finger holes. 2.1mm plastic to outer edge of dial. Distance between 0 to 1 (straight line) cutting corners as to speak 45.8mm. Disk In middle 43.6mm D.


John.

[Dave Moll]

I assume you do mean the plastic dial, as shown by John, used on the 700 series rather than the metal dial of the earlier 200/300 series.

[Karen O]

Thanks John,

I should be able to glean the measurements from your photos.

Yes, it was the plastic dial I wanted, Dave.

I recall that metal dials were used in phone boxes, presumably for vandal-proof-ness.

Wasn't the mechanical dial a wonderfully elegant digital entry device in a pre-digital age?

[Herald1360]

Wasn't digital first? The telegraph predated all this analogue stuff!

And optical, semaphore.

[mark_in_manc]

Chapter one of Hunt, 'Electroacoustics' gives a great summary of Benjamin Franklin right up to 1950s electronics, with a focus on telecomms and radio as you might guess from the title. Although a quick google credits Reeves with PCM in 1938, in a discussion of the electrostatic telegraph Hunt says the following:

"The curious fixation on the notion that a separate wire must be provided for each transmitted character adhered to most of the telegraph proposals made before 1830, although single-channel systems began to make their appearance as early as 1795. In that year, Tiberius Cavallo experimented with such a system, an there is a prophesy of modern pulse-time modulation in his suggestion that 'by sending a number of sparks at different intervals of time according to a settled plan, any sort of intelligence might be conveyed instantaneously'".

Plus ca change!

[Karen O]

You are all right, of course

I just find the telephone dial inspiring as an electro mechanical digit entry device.

I had this idea of using a modern rotary encoder. There would be a finger stop such that the number of pulses it generates depends on which finger hole you start from.

It wouldn't return to its original position (it doesn't need to) and the pulse spacing won't be precisely timed, but it would work.

Ca plane pour moi

[TonyDuell]

I have, somewhere a modern-ish telephone with a rotary dial (I do not mean a ring of buttons that looks like a dial) that does tone dialing (DTMF). It's quite ingenious. The dial does return to the normal position and has a governor to get this to occur at about the right speed, but that is really just for appearance.

The trick is that the finger stop operates a microswitch. The dial has a disk with a hole on the back (inside the telephone), on one side of the disk is an LED which is flashed when the finger stop is moved. On the other side of the disk is a ring of LDRs, connected as the key matrix for a common DTMF dialer IC. The operation is obvious, you dial a number, the hole lines up with the approriate LDR, when you hit the finger stop the LED flashes and the DTMF chip acts as if a key was pressed.

As for making a dial with a rotary encoder, you may find the pulse timing is a lot more critical than you think. I've had problems with exchanges not recognising out-of-spec pulse trains. I think it's for this reason that dials throughout the world generate pulses on the return motion (where the speed can be controlled by a governor).

[Karen O]

Hi Tony,

Very ingenious tone dial! Probably more reliable than rotating contacts.

The application I had in mind for my ersatz telephone dial is a home project with a vintage bias. A real telephone dial would be ideal but they're too valuable to true vintage gear. And my PIC microcontrollers don't mind what rate the pulses come in at.

Yes, I'm sure it would be too inaccurate for use with a telephone exchange.

Which reminds me of an incident on the school pay phone back in the 70s: a boy claimed to be able to dial by tapping the hook rest. 'Let me show you - I'll call the talking clock (98081)'

Next thing we hear: 'Emergency, which service please?'

and the pulse spacing won't be precisely timed, but it would work.

Chuck a micro at it and you could dial at any speed you like and get near perfect pulses out. I have a 706 telephone and fitted a rotatone http://www.rotatone.co.uk/ to generate DTMF tones from the pulse dial, I wish a label was supplied to stick on the bottom of the phone as a cheat sheet.

I like the idea of it not returning, instead of a microswitch on the finger stop you could use the capacitive sensor input available on many PIC micros. I taught my girlfriend (now wife of just over 25 years) to tap out numbers on the locked 'phone at her shared house, the secret is to leave a large gap twixt pulsings.

[TonyDuell]

Unless you want something very exotic/old, mechanical telephone dials are neither rare nor particularly expensive. I am sure a lot of telephone enthusiasts have at least one spare in the junk box. If you want to use a real dial, then do so.

As for the capacitive sensor rather than a microswitch, the thing I liked about this rotary tone dialer was how simple it was. I was expecting a custom microcontroller in there (it appears nobody can design anything nowadays without using a million transistors), but it was just the LED, LDRs, a common (very common) tone dialer IC and some discrete components. Nice and simple, and easy to repair (not that you were supposed to, I guess).

[Dave Moll]

I have to admit that my eyebrows flicked up a bit at Karen's comment that a real telephone dial would be ideal but they're too valuable to true vintage gear - and I would echo what Tony says. I, for one, have spares of both plastic and chrome dials - and would not see the plastic ones as being of any great value. They can probably be purchased for less money than one would pay for the parts to make one from scratch.

If, however, the joy is to rise to the challenge of designing and building one, that's an entirely different matter!

it appears nobody can design anything nowadays without using a million transistors

The advantage is you can design a very robust mechanical interface (mind you that isn't very commonly done ) with loads of fuctionality. I enjoy programming micros to my preferences.

[Karen O]

When a million transistors costs 5p you'd be mad not to use them!

I was hoping I could miniaturise my rotary encoder based dial but that would necessitate use of a stylus. Either that or sharpened fingernails.

I don't want to risk angering the vintage gear spirits by using a genuine telephone dial in a modern application. Superstitious I know, but you've got to be superstitious. Awful things happen when you're not superstitious.

[julie_m]

I always say, believing in superstitions is unlucky .....

Seriously, though, real dials are still plentiful enough; and if you can find an example with, say, faulty off-normal contacts or a governor that won't adjust to the proper speed for 10 pulses per second (which is less of a big deal for us if we are not dependent on an accurate return speed), nobody's likely to miss it -- although nobody's likely to have hung onto a dial in that state either, more probably having stripped it down to get the most useful parts and throwing the rest away.

And I think you'd be missing a big part of the "user experience" by not having the dial spring-returned.

For a miniaturised dial that could be operated without a stylus, have you considered extending the finger holes into slots reaching all the way to the edge? The "finger surface" then would be the end of the "spoke", which would be a bit easier to handle than a hole too small for a finger.

I do quite like the optical dial idea for its audacious elegance, though it sounds a bit expensive having ten separate light sensors. Last I checked, LDRs were not exactly cheap. If I was using a microcontroller, I might consider using a clock frequency that was a multiple of the NTSC standard (all the DTMF frequencies can be got by dividing this down), and generating an interrupt by dividing this down; then all four "row" tones and all three (four?) "column" tones can be generated with simple software counters, and the use of an interrupt frees us from having to care about how long all the necessary increase, compare, test/skip and reset instructions will take in practice. Between interrupts, we can do software debouncing on the impulse contacts; and by timing the operation of the off-normal contacts, switch to a mode allowing the "digits" *, #, A, B, C and D to be dialled -- e.g. dial 1, hold 2 seconds, let go to send *.

[TonyDuell]

Actually, a lot of old telephone enthusiasts are worse pack-rats than many people here and keep just about anything. Finding an old dial is not going to be hard. And nobody is going to moan if you use it like this. It's like using a 'scrapper' radio for parts. Nobody would do that to a round Ekco, just as nobody would raid an orginal candlestick telephone for the dial. But a common 706 or 746 telephone? They are not rare. I can see a dozen from where I am sitting and I am not a collector.

As for the 'optical dial', believe it or not it came in a telephone badged 'GPO'. Not an old Post Office one, but the new company also known for cheap and very nasty record players. There are actually 13 LDRs in it, it has 12 holes in the finger wheel as you can 'dial' # and *. The thirteenth one would be illuminated by the LED with the dial at the normal (rest) position, it would give last number redial if you blipped the stop without moving the dial. I say 'would' as a wire on an interconnecting ribbon cable was cut to disable this when I got the phone (new). I reconnected it, of course.

I suspect you can get LDRs a lot more cheaply in the Far East than over here...

You can make a smaller dial by having the finger holes equally(ish) spaced, not with the big gap between 1 and 0. To get that to work, you have a moving finger stop. Look at the American 'Trimline' telephone and its dial. No, I do not suggest stripping one of those beautiful units for the dial.

[60 oldjohn]

Originally Posted by Karen O

Which reminds me of an incident on the school pay phone back in the 70s: a boy claimed to be able to dial by tapping the hook rest. 'Let me show you - I'll call the talking clock (98081)'
Next thing we hear: 'Emergency, which service please?'

Yes exactly same thing happened to me a couple of years since. intermittent phone in my workshop, thought I would brighten the switch contacts by pressing cradle switch a few times. I won't be doing that again, at least while it is in circuit!

John.