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Years ago I bought a device at Radio Shack that disabled ringing on household phones. It was a very small box with a modular receptacle on one side, a pigtail on the other with a modular plug, and a switch on the box. The switch was "ring on" and "ring off". It was easy to put into the phone wiring (serially).

What was in that box? Did the switch just cut the "ring" line? Or was there some sort of a frequency trap to inhibit the ring signal? Oh, and with the device set to "ring off", it was still possible to pick up a phone and make an outgoing call.

ocrdu
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Roger
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    probably a frequency or voltage "trap" as you put it. Ringing adds quite a big voltage onto phone lines for historical reasons, in fact you heard of people holding telephone lines for maintenance and then suddenly getting zapped when they rang – user253751 Mar 10 '23 at 20:10
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    If you are in the UK it would just have been a ring wire disconnection switch. – Andy aka Mar 10 '23 at 20:16
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    there is no "ring" line ... only two wires are used – jsotola Mar 10 '23 at 20:39
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    In a two wire subscriber loop, the wires are "tip" and "ring". What these terms refer to is how the wires are connected to the plug/jack in the central office. The "ring" is the conductve sleeve. It is unrelated to "ringing" the phone. – Math Keeps Me Busy Mar 10 '23 at 20:43
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    A parallel RLC 20 Hz filter in series with the line would work. I don’t know how Radio Shack did it. – RussellH Mar 10 '23 at 21:08
  • @RussellH ah right, Radio Shack is a North American local chain, right? So they would not have to build it multi-frequency. Probably want to have quite (dimensionally) large L, considering the resonant voltages that ringing might produce – Marcus Müller Mar 10 '23 at 21:56
  • The Radio Shack part number was 43-127, you can see a picture of it here: https://worldradiohistory.com/Archive-Catalogs/Radio-Shack/Radio-Shack-1989.pdf There's a company called Sandman that is selling a product that appears to do the same thing. – GodJihyo Mar 10 '23 at 22:23
  • @jsotola It's not always been so. In the UK it used to be that when more than one phone was installed the bells were connected in series on a third wire, fed via a capacitor from one of the two incoming 'legs'. – peterG Mar 11 '23 at 14:14
  • A North American "POTS" subscriber line consists of a two-wire loop. Idle is about 48V; when the telephone set goes off-hook, it introduces a lower impedance and the voltage drops. To ring the phone, the central office applies an alternating signal (nominally 90V, but could be more) at about 20Hz, which drives the ringer. Presumably, a device could absorb the ring signal by selectively filtering to shunt the 20Hz and/or passing current to clamp the line voltage to the highest expected for an idle state. – Anthony X Mar 11 '23 at 15:40
  • That's it, GodJihyo!!!! The photo is exactly the Radio Shack device I had. I wonder if it is possible to get another one, or make one myself (but I'm a complete electronics amateur)? Maybe there is a circuit diagram for it (or similar) somewhere? – Roger Mar 12 '23 at 02:06

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I'm taking a wild-assed-guess here, but I suspect that the circuit consists of a simple Metal-Oxide Varistor (MOV) with the appropriate voltage rating in series with a largish capacitor, all connected across the telephone line. It would clamp the ringing voltage to a low value but not affect either the oh-hook DC voltage nor affect the speech AC signal.

You would have to experiment with different voltage MOVs but I'm thinking a 35V MOV is a good starting point.

Marcus Müller
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Dwayne Reid
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    A 35V MOV across a nominally 48V loop supply is not a nice idea. – KalleMP Mar 11 '23 at 15:20
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    @KalleMP The capacitor in series with the MOV should take care of that, shouldn't it? – Tanner Swett Mar 11 '23 at 16:26
  • @CassieSwett On reflection you are most likely correct. The DC line voltage would hold the MOV in conduction for longer than it takes the capacitor to charge. The large (about 90V) AC ring signal would get clamped quite effectively, a series resistor before the MOV+CAP might still be prudent rather than relying on the source impedance. – KalleMP Mar 23 '23 at 20:30
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There's many ways this could have been done; I wager it was a simple device that triggered on the relatively high voltage the ring signal had in old-timey landlines.

In the simplest case, you would have used the same mechanism as in mechanically ringing phones – simply a capacitor together with a relatively large solenoid – to actuate a relay instead of hitting bells with a hammer. That relay would simply disconnect the phone from the line.

A simpler, solid-state solution would have been to use an appropriately sized clipping circuit: ringing works by overlaying the DC base bias on the two telephone lines with a high-amplitude AC. Add a thyristor that fires the first time the added AC and DC exceed some defined voltage and keeps conducting through the whole ringing and only gets slowly "snubbed" after would probably work.

Marcus Müller
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My recollection was that -way back- while the aerial cable may have been just a pair the phone cordset (500 set or similar) had the ringer separated on a third wire which was then connected to one side at the bell block on the wall. Opening that third wire would stop the mechanical ringer from operating.

prisoner_number_6
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    "aerial cable" on a landline? – Transistor Mar 11 '23 at 10:33
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    @Transistor The landline is used to distinguish from cordless phones these days, Even back in the day land line was used for any POTS service no matter if it was elevated over the land or buried in the land. – KalleMP Mar 11 '23 at 15:22
  • @prisoner_number_6 I think (but don't know) that you may be right. Looking at the device in the catalog (https://worldradiohistory.com/hd2/IDX-CATALOGS/IDX/Radio-Shack/Radio-Shack-1990-OCR-Page-0088.pdf) and looking at a project from https://books.google.com/books/about/Telephone_Accessories_You_Can_Build.html?id=_qzgAAAAMAAJ&source=kp_book_description I think a simple switch is what it was. – DrG Mar 11 '23 at 15:56