# Wiring up old phone ringer to arduino

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?

• Be careful. The voltages can hurt. Commented Feb 28, 2011 at 23:02
• youtube.com/watch?v=G_eNNzNB-hU check out this..... I am sure it is made pretty simple here... Commented May 26, 2017 at 5:29

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.

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.

• 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? Commented Oct 24, 2010 at 5:22
• "...do not use 60 Hz AC from your house outlet -- that won't work either." -- That's an understatement! Commented Oct 24, 2010 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). Commented Oct 24, 2010 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. Commented Oct 24, 2010 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.

You can ring a phone with 120 volts at 60 hertz. It sounds somewhat more like a buzz than the musical sound that 20 hertz produces, but you have probably heard 60-hertz ringing in many movies and didn't notice the difference. The bells have to be adjusted closer to the clapper than normal. Usually the attachment holes in the bells are drilled off-center, to allow adjustment by rotating them.

• An isolation transformer (that will also limit current flow) would be a good addition. It'd have to be a 1:1 transformer though, which might be more difficult to find. Commented Aug 27, 2016 at 1:11