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Tech Forum

April 2017

POTS Ring Generator

I need a circuit to generate 20 Hz 90 VAC to ring a POTS telephone on a theater stage. That’s Plain Old Telephone System vs. Pretty Advanced New Stuff (PANS) for the younger readers. If I could locate an old crank ring generator, it would work but would not be as convenient.

Dale Carlsen
Woodbridge, VA


POTS ring generator modules are readily available prebuilt. The Tele-Q 70-9100 at is a self contained ring generator with simple pushbutton operation.  Viking Electronics model DLE-200B  is a two way phone line simulator that will ring one phone when the other is picked up and provides loop current to power the phones.  

For theatrical use this will ring the phone on stage but the ability to also talk on the phones may help the person on the phone have a more realistic sounding phone conversation.

Erik von Seggern
via email

I found this at: Seems like it would work fine for stage use.

I had an old Post Office local system ring generator that did the job very simply. It had a transformer with a split winding on one side, and a single winding on the other. The split windings were linked in series by a 2uF paper capacitor, and a single diode (IN4007 style) was in series with one of the outside terminals. The diode half wave rectified the mains going in, providing the transformer with half mains frequency pulses (25Hz in the UK). The cap between the windings provided rough tuning, which took the waveform back to something approximating a sine wave. Out of the other side of the transformer came a reasonable sine wave at about 80V p-p.

I guess that the same thing could be reproduced now using a split primary power transformer, with a secondary of say 40 - 0 - 40. The value of the cap could be played with a bit for best wave shaping to suit the tranny. I doubt that you would notice the frequency being 30Hz US or 25Hz UK. Arfa Daily, Oct 13, 2005

Andrew Gutierrez
Tehachapi, CA

It's called a ringdown circuit. Here's one assembled that would work, but if you search using that term, you should find something.

Richard Allen
Perry, MI

eBay has a Black Magic Ringing Generator for $26. A pushbutton can be used to intermittently connect four “D” cells to the input of the Generator and it will ring the phone. Be sure to hook the battery to the Generator with the correct polarity shown on the label. Connect the output of the Generator to the phone’s red and green wires, but you must also put a 330 ohm resistor in series with one of the output wires to the phone.

To be realistic, the phone should be made to ring for 2 seconds, off for 4 seconds, on for 2, off for 4, etc. Be sure to test the setup at home before moving it to the stage.

David Goodsell
Apple Valley, CA

Well, years ago when I was involved in theater, we just connected the ringer to regular 110V 60 Hz utility power, through an offstage switch. You might have to futz with the clapper spacing and spring tension to get a good ring. We had a regular telephone bell in a small box which we rang, we hid it on the stage near the set phone, which wasn't connected. If you want to ring the actual bell in the phone on stage, I'd suggest using a 1:1 isolation transformer for safety. One time during a rehearsal when there was a phone on set that an actor was supposed to dial, for a spoof we connected it to a phone line. When the actor picked it up and heard the dial tone, he completely lost it...

John Schmidt
Hempsted, NY