Turning a rotary phone dial

Using Arduino Project Guidance
1

Hey all,

Dreaming up a project that involves turning a rotary phone dial to call a number. I imagined using a servo motor connected to a 3d printed part that would fit into all the holes, then rotating the servo some calibrated amount per number. But it seems like servos don't have much strength, at least your standard servo. What's my solution here? I have lots of Arduino experience but almost none using heavy motors or alternate power sources.

2

You may need to do a bit more research. The old rotary phone dialed the number when you release the dial after indexing it to the stop, not while it is being moved to the stop. How would you design something to actually release the dial? It needs to return to the resting position at a specific rate so the switch opening steps are all the same length of time.

Paul

3

But it seems like servos don't have much strength, at least your standard servo.

That would depend on the "size" of the servo.

Servos are geared-down so they typically have more torque than a direct drive motor of similar size and similar voltage/current rating.

Another option might be a stepper motor.

You might want to estimate the amount of torque you need, or measure the force needed with a fish scale and calculate the torque.

Old automatic telephone switchboards used rotary relays and I'd guess they used a special stepper motor with big steps.

The old rotary phone dialed the number when you release the dial after indexing it to the stop, not while it is being moved to the stop. How would you design something to actually release the dial?

I was thinking about that too, but if you drive it in reverse at the right speed that shouldn't be a problem.

4

DVDdoug:
Old automatic telephone switchboards used rotary relays and I'd guess they used a special stepper motor with big steps.

Actually they used a solenoid to advance the ratcheting mechanism one step each time the telephone dial switch closed the circuit. A separate switch assembly was used for each of the 8 possible digits in the phone number. When the phone was "hung up", a second solenoid in each switch assembly reset the whole thing back to rest.

Paul

5

Yes, they used to be called uniselectors in the UK. Try looking it up on Wikipedia.

Russell.

6

Stronger servo. They come in all kinds of sizes.
https://www.servocity.com/html/servos___accessories.html

7

If you are going to use the motor to return the dial you could remove the return spring from the dial and it would take very little effort to turn.
You could also do it not even using the dial.
All it involves is a segmented wheel actuating a set of points.
Worked on this stuff for 13 years back in the 60s.

8

I suppose I could have the 3d printed part "pull out" after it turned to the correct number/position, then go back to default position and push in. Thanks for the help all, really appreciate it! Maybe alternatively I could wrap a gear around the dial, and have a stepper motor drive it, then disengage once it hits the target. Can a stepper "disengage" and have the rotary dial pull it without injury?

9

Here's another larger servo option.

10

forget the servos and lookup what the dial does (did). a quick succession of breaks in the circuit, one for each count if the digit dialed.
dial '9', you get 9 clicks when the circuit was broken nine times.....

11

I don't understand why you're using the dial. I can only come up with a few possibilities:

  • You're interfacing with an old piece of telephone equipment that only recognizes pulse dialing - in this case, it is probably easier to emulate the dial.
  • You're trying to dial a number on the actual phone system - I'm not sure how many telephone systems even support pulse dialing. It would be easier to just use the touch-tone system (DTMF).
  • You want the "look" of the old dial - in this case, the servo sort of ruins the look. And brings up the question of whether a user would need to be able to also use the dial.

Also keep in mind that there are quite a few other details if you are planning on attaching this to the public phone system.

A more common use of phone dials with Arduinos is to build retro Bluetooth phones. But in those, a user is turning the dial, not a servo.

--Doug

12

Ah yes, to clarify, this is an art project in which I feel the turning of the dial is important.

13

prismspecs:
Ah yes, to clarify, this is an art project in which I feel the turning of the dial is important.

Then disconnect the dial from its inner workings so it looks like you're turning it but really the number is being dialed behind the scenes. Then your servo idea will work much easier.

14

For a very old-school method of how to do this:

"Phone Tattles on Burglers" - Popular Mechanics, December 1966 - Page 174

15

Delta_G, I like the way you think. So to dial the number "behind the scenes" would involve breaking this dialing circuit? The number 1 is one broken circuit, number 2 is two, and so on? And I imagine this has to happen at a specific interval. Would you recommend using a relay to facilitate breaking the circuit?

16

prismspecs:
Delta_G, I like the way you think. So to dial the number "behind the scenes" would involve breaking this dialing circuit? The number 1 is one broken circuit, number 2 is two, and so on? And I imagine this has to happen at a specific interval. Would you recommend using a relay to facilitate breaking the circuit?

To illustrate this for yourself and maybe get an idea of how to do it, you can dial a phone without the dial from just the hook. Even works on pushbutton phones. Pick up the receiver and rapidly tap the hook the number of times you want for the number. So to dial 4-1-1, tap-tap-tap-tap - pause - tap - pause -tap. If you put the receiver to your ear and dial the number, you can hear it clicking to give you an idea of the speed of the taps. It's a great thing to know if you've got a pushbutton phone that won't work in a power outage because it allows you still to dial a number.

I don't know if a relay would be the best thing or not. Worth a try though.

17

The impulse speed is 10 Hz. Virtually all phone phone systems will still respond to it. Can't recall offhand the mark-space spec but 50% will do.

On the other hand, you could fake the dialling and use TouchTone to make the actual call. :grinning:

One interesting way to grab and release the dial would be an electromagnet. This would require driving the mechanism on one direction only.

russellz:
Yes, they used to be called uniselectors in the UK. Try looking it up on Wikipedia.

Well, that was one, but in fact, most of the switching was berformed by, and what Paul_KD7HB was trying to describe was a Strowger or "Step-by-Step" two-axis selector.

18

Ah yes, to clarify, this is an art project in which I feel the turning of the dial is important.

You could attach a servo to the dial face fairly easily just using a hot glue connection (or maybe velcro if the return spring is removed). You would need a servo that would be capable of rotating the amount required rotation, such as a sail winch servo. Removing the dial return spring would take the load off of the servo.