old phone as doorbell

[Electromatic]

As a home project, I'm trying to convert the ringer of an old rotary phone for use as a doorbell. I thought it would be relatively simple, but I'm having no luck.

The phone is ca1970 Western Electric.
I know that the ring state for old POTS is typically around 90VAC, 20Hz. I've tried briefly applying 24VAC (from a transformer I have) and even straight 120VAC to what I believe to be the appropriate coil wires but get nothing. Is this to be expected? I thought I'd get some kind of reaction despite the out-of-spec V and Hz.

The coil assembly is (from research) apparently two coils for L1 and L2. Each one reads what I think is correct at ~1kΩ and ~2.6kΩ. There is also a capacitor within the phone's assembly that seems to be out of spec, reading ~1.2uF vs 0.5uF. My understanding is that this is more for actual phone line usage than the electromechanical operation of the ringer.

Am I completely missing something other than getting closer to 90V, 20Hz? I don't care about actually getting a "ring-ring" pattern--just for the bells to ring when someone presses the door bell button. (I know they sell ring generators, but I'm trying to keep it simple and cheap.)

TIA

 

[tthh]

as I recall from my telephony days, you also need the 48V DC and then *add* the 90V at 20 Hz or whatever ring cadence you want

 

[zbang]

If all you want is it to ring, 120vac thorough a small capacitor and maybe a dropping resistor will do it. Add a 1 amp fuse for protection. (The DC is for the talking circuit, not used to ring.)

 

[hbiss]

If all you want is it to ring, 120vac thorough a small capacitor

I believe he actually tried that. The .5uF cap is in series with the windings (to block the 48VDC). I never tried applying 60Hz to a ringer, but I suspect that it is "tuned" to 20Hz and will do little at 60 probably for a reason. Further, the 20Hz makes the ringer sound the way it does as it follows the slow 20Hz waveform.

Look on eBay for ring generators. I used to find old 1A2 system power supplies all over still plugged in after 50 years! Can't be more than a few dollars. Or keep your eyes open in old building telecom rooms.

-Hal

 

[Electromatic]

Thanks, all, so far.

Yes, I've tried briefly applying 120V and 24V both directly to either coil or through the phone's capacitor and get absolutely nothing. I expected a fast ring or at least a single clunk from the clapper.
My searches for ring generators yield either fancy stage production battery modules for ~$100 or differently fancy Arduino-based solutions. I'll do some searching for 1A2.
Alas, nothing is as easy or cheap as one would think!

 

[tortuga]

To make a telephone bell ring, you don't even need a sine wave, (VOIP / ATA adapters generate square wave) you make a H-brige with MOSFETS (or a 555 IC) or these days just use cheap mini robotics brushed DC motor driver like this:

Adafruit DRV8871 DC Motor Driver Breakout Board - 3.6A Max

Crank up your robotics with powerful Adafruit DRV8871 motor driver breakout board. This motor driver has a lot of great specs that make it useful for a wide variety of mechatronics. In ...

www.adafruit.com

You do need to boost it to ~90VAC so get a small 120v transformer with, say, a 12v secondary. Drive the secondary with the waveform from the H-bridge/motor driver and adjust the signal to be ~20 hz and the bell should ring:

 

[Joethemechanic]

Back when I had VOIP and wired phones there was some kind of ring generator built into the modem

 

[Electromatic]

Thanks for the high-tech, @tortuga . Adafruit has some clever things.
Will this drive the AC coils of the ringer? My electronics knowledge is short of H-bridge rectifiers. I was hoping to keep the project low-brow, but I'll take cheap and simple.

 

[tortuga]

Yeah you drive the ringer like a stepper motor.
I'd do something like meanwell 24V DC supply ---> Adafruit DRV8871 ---> (12V:120V transformer) drive the 12V secondary side --> primary side to bells. Set up the DRV8871 to 20HZ.
For control use a

Adafruit 555 PWM Output STEMMA - 1.5K~25 KHz or 1.5~250 Hz

How many times have we heard "hey you could have just replaced that microcontroller with a 555"? Maybe 555 times! But we always find wiring up a timer chip to be a bit of a pain, ...

www.adafruit.com

 

[zbang]

I never tried applying 60Hz to a ringer, but I suspect that it is "tuned" to 20Hz and will do little at 60 probably for a reason.

Dunno, it always worked for us, probably depends on the ringer itself. Granted it's not the right way to do things, but for a poor community theater in the 70s and 80s, it did the job. Now, I'll just grab a small ring gen off the shelf in the garage.

BTW, there were 30Hz ringers, too, as well as a bunch of other freq's used for frequency-selective ringing on party lines; those are quite uncommon.