SparkFun Forums

[sparky]

So after receiving some 300 emails about the rotary phone, I'm still trying to get the ringer to yell.

These old phone techs keep emailing me saying the ringer (old bell style) takes something like 80-100VAC to ring. Jeebus that's a lot.

Ringer coil characteristics - clicks over well at 25V DC so at 50-80V, it's going to make some noise. Short circuit current through the coil is 25-30mA - not too bad.

So how to do it?

One idea was to use capacitor charge pumps. I used a MAX232 chip, used a PIC to send 1 on one TX pin, and 0 on the other TX pin - and then flip both bits. On the output side, I got -10V and +10V - effectively 20VAC off of four caps and a PIC. Not bad! The ringer vibrates, but it's really soft. I really need something hefty-er.

Next idea was to use an EL backlight driver from a previous project. This baby puts out 80VAC at 100-110Hz off of a 3-5V input. Sounds perfect. I hook it up and zapped myself once . It bites like a bee sting. Unfortunately I just get a high pitch hum out of the ringer coil. Why doesn't it work? I think it should...

I tried building my own charge pump as an option. Using basic .7V drop diodes and 4 10uF 25V caps, I was able to generate 25V!!! off of two PIC pins toggling at 500-1kHz. I could increase the caps (10-15) and generate quite a bit of voltage. But this gets big - and it's only +~80V. how do I switch it on/off to generate the VAC for the ringer coil? It's gotta be small - no big mosfets or anything.

Finally, I tried a step-up transformer:

If the ringer coil only needs 25-30mA at 80V then I need to input ~540mA at 3.7V (lithium battery driving voltage) into the transformer. This is very feasible for a LiPoly battery capable of 6C (or nearly 12A!! continuous). This is actually the first transformer I've messed with - I wound it myself with a 22:1 ratio. I used an hbridge driver to supply the current into the primary coil. The PIC switched the hbridge back and forth, switching the current into the coil. For a split second I get a hi-pitched squeal, nearly 50VAC, the hbridge heats up, and the system shuts down. Who knows about transformers? I don't.

I'd really like to use the backlight driver solution as it is off the shelf and small.

Sorry for the long rant. Who knows how to make these ringers ring like crazy?

-Nathan

[SOI_Sentinel]

First things first, I must have better luck with google.

http://www.hut.fi/Misc/Electronics/circ ... inger.html

(go figure, it's in Finland)

Another useful document: ADSL filter spec with max voltages:

http://www.accesscomms.com.au/specs/k2320spec.pdf

Your current problem is that you're going TOO fast. The mechanism's just wiggling in the air, it has no time to swing and generate the momentum required to ring the bell. Take a ruler and wave it about fast, Faster you go, the more it bends and makes it's own sound. You really do need 20-30Hz for classic performance.

That also requires a fairly hefty "iron core" transformer. I'm assuming you wound something similar. The Hbridge is overheating because you're driving the transformer far faster than it can go and the inductive kickback is probably huge. Slow down your switching speed, and you may want to consider a heavy duty low pass filter between the hbridge and the transformer to sine-out the signal and also reduce the kickback's jolt. IF you can get out 8VDC or so, you may want to consider going with a Radioshack 300mA PCB transformer for others trying to duplicate your feat.

[ferrick]

Umm, maybe this obvious solution to the "ringer problem" has already ocurred to you, but you could always use a sound playback chip (maybe a SpeakJet with an appropriately "classic" ring downloaded to it?) running into an LM386 amp or some such.

Sure, it's nowhere near as cool as shaking a big 'ol bell, but as long as it sounds close I think you'll get the effect. [aside: normally _I_ would be the one advocating a really funky electromechanical solution, but I want to get my hands on one of these phones asap...and I figure this would probably speed up the process!!]

Patrick

[sparky]

Hey Patrick,

Oh yea, we could easily hook up a piezo or a old school buzzer, but I've got to have that really annoying, *loud*, classic ring.

Fooey. The phone is ready to go, just short the ringer.

-Nathan

[Guest]

Hey Patrick,

Oh yea, we could easily hook up a piezo or a old school buzzer, but I've got to have that really annoying, *loud*, classic ring.

Fooey. The phone is ready to go, just short the ringer.

-Nathan

Can you replace the ringer with a low-power solenoid?? For example, this one:

http://www.eepn.com/Locator/Products/Ar ... 20196.html

Keep the original bell, just replace the actuator part... They have 3V, 6V, 12V & 24V options.

[ferrick]

Hey, that's a good idea! Surely a little solenoid like that one could make plenty of noise...and I'd have to think that the shape/composition of the bells themselves is responsible for the "classic" sound that you're after.

God only knows why Ma Bell (raise your hand if you DON'T recognize that name!) used 80 volts AC or whatever...

cheers,
Patrick

[dontremember]

Hey, that's a good idea! Surely a little solenoid like that one could make plenty of noise...and I'd have to think that the shape/composition of the bells themselves is responsible for the "classic" sound that you're after.

God only knows why Ma Bell (raise your hand if you DON'T recognize that name!) used 80 volts AC or whatever...

cheers,
Patrick

Thanks!

I imagine they used high volts to be able to push the signal down several miles of copper. Plus, I think up to 4 phones could be on the same circuit, and it was a long time before self-powered phones came long...

The solenoid just has to pop out smartly enough to give the bell a good whack, then retreat quickly so it doesn't damp the "ring" too much.

[cj8scrambler]

Any chance of posting the v1.1 firmware that includes the expanded dialing support and ringer circuit driver code? I would like to build off of this and not re-invent the wheel where needed.

One thing I would like to add is a dial tone generated by a PWM output through a low-pass filter. Another possibility is removing the GSM module and replacing it with a bluetooth chipset so that a regular cell phone could be used (without stuffing it inside the case).

[NateW]

Bluetooth is a great idea... then you can leave the rotory phone behind (at your desk, in your car, etc) and just carry a regular cell phone when you want something less bulky.

According to http://www.blueradios.com/BR-SC30A.pdf, those BlueRadio modules support a few different bluetooth profiles, including "audio gateway," which sounds promising. Is the "headset" profile the same as "hands-free?"

[cj8scrambler]

Is the "headset" profile the same as "hands-free?"

Most phones that I've seen support both headset and hands-free profiles. I believe that the difference is that headset only provides the audio links and a very basic control channel. However the hands-free profile provides a control channel which can do a variety of things like like dial numbers, answer the phone, etc.

[sparky]

Ug, I'll see if we can post the schematic and firmware. It's be a blast building the phones. It may be time to unleash the design.

-Nathan