Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

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?

share|improve this question
    
Be careful. The voltages can hurt. –  Brian Carlton Feb 28 '11 at 23:02
add comment

3 Answers 3

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.

share|improve this answer
1  
Wow. Thats exactly what I need. A more detailed description of their build is at sparkfun.com/commerce/tutorial_info.php?tutorials_id=50 –  CMP Oct 24 '10 at 7:46
add comment

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.

share|improve this answer
    
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? –  CMP 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
add comment

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.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.