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I just obtained an old rotary phone at a garage sale. I am working on hooking it up to an arduino project, and I was able to get the dial and hook switches figured out pretty easily. I cannot really figure out how the ringer works, or how to wire it up.

It is an old western electric C4A ringer. It has two bells and a striker arm between them. There is a coil, and some magnetic plates that move the ringer, with four wires going into the coil. I have found schematics online for how the thing connects to the other phone components for normal operation, but I really just wanna figure out how to make the thing ring on its own.

I read that most phone lines run in the vicinity of 90 volts ac. Is there any possibility I can make this thing ring with a 12 volt wall wart, or am I gonna need a full 120 line and a relay or something?

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Be careful. The voltages can hurt. – Brian Carlton Feb 28 '11 at 23:02
up vote 7 down vote accepted

Look at page 2 and 3 of the schematic .pdf from this page. Sparkfun have done it using an H-bridge and voltage booster circuit. Quite nifty really.

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Wow. Thats exactly what I need. A more detailed description of their build is at sparkfun.com/commerce/tutorial_info.php?tutorials_id=50 – captncraig Oct 24 '10 at 7:46

Phone lines normally run on -48v DC (referenced to ground) when the line is idle.

During the ringing cycle (in the US, 2 seconds on, 4 seconds off), a ringing voltage of 75-90v AC (typically 20 Hz in the US) is superimposed on top of the -48v DC.

When you take your phone off-hook, the line card at the central office (CO) senses the current and disconnects the ringing voltage. Meanwhile the voltage at the phone drops down to -12v or so, mostly due to the voltage drop across the line from the CO to your house.

So you cannot ring an older style phone with a voltage lower than 75v AC or so. Also, do not use 60 Hz AC from your house outlet -- that won't work either.

You need some sort of circuit that will create a 20 Hz sine wave (square wave would probably also work), that is amplified to 90v. There is a circuit on this page, under "Telephone Ringer". (Note: I haven't built it, but it looks like it could work.)

You would need a relay connected to the Arduino to turn it on and off.

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I found that link as well. The problem is, that looks like it is meant to work externally to the phone through the phone line. I am already in the phone. It looks like that is just generating an oscillating high voltage. How would I connect this directly to my ringer, rather than through the phone line? – captncraig Oct 24 '10 at 5:22
"...do not use 60 Hz AC from your house outlet -- that won't work either." -- That's an understatement! – John Lopez Oct 24 '10 at 16:04
@captncraig -- as far as the interior wiring of the phone, here are some reference schematics. The first one ( porticus.org/bell/images/503b_md_schematic.gif ) is for a data phone but shows the actually wiring of the ringer terminals. The other two ( porticus.org/bell/images/500dm.gif, porticus.org/bell/images/500c_d_schematic.gif ) show how the four leads are connected: S-R and S from the ringer go to A and K of the network, which is a capacitor. R from the ringer connects to R of the telephone line, and BK connects to either Tip (G) or ground (Y). – tcrosley Oct 24 '10 at 20:53
(continued) If the ringer leads aren't labeled, here is one color scheme I have seen: BK is gray(dark almost black), S-R is gray(whitish)/red, S is gray(whitish)and R is red. I don't know what value capacitor is used in the network (across A and K), but would guess 0.1 or 0.33 uf at 250v. – tcrosley Oct 24 '10 at 20:54

You could take a small one of those 120V to 6.3V step-down transformers (like RS sells), and 'use it backwards' to step up a low voltage signal from some drive transistors to nearly the right voltage for the phone ringer mechanism.

If you run 5V AC into the 6.3V secondary, you should get about 5 * (120/6.3) = 95.2V out on the primary side, which is a little high, but not terribly so. You could probably get away with using a single switching transistor to drive the secondary from your 5VDC supply, but be sure to put a fly-back diode across the coil to keep the transistor from getting killed. Then it's just a matter of turning the drive transistor on/off at about 20Hz for the desired length of time.

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