8
\$\begingroup\$

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.

\$\endgroup\$
10
  • 2
    \$\begingroup\$ 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 \$\endgroup\$
    – user253751
    Mar 10 at 20:10
  • 3
    \$\begingroup\$ If you are in the UK it would just have been a ring wire disconnection switch. \$\endgroup\$
    – Andy aka
    Mar 10 at 20:16
  • 3
    \$\begingroup\$ there is no "ring" line ... only two wires are used \$\endgroup\$
    – jsotola
    Mar 10 at 20:39
  • 4
    \$\begingroup\$ 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. \$\endgroup\$ Mar 10 at 20:43
  • 2
    \$\begingroup\$ A parallel RLC 20 Hz filter in series with the line would work. I don’t know how Radio Shack did it. \$\endgroup\$
    – RussellH
    Mar 10 at 21:08

3 Answers 3

11
\$\begingroup\$

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.

\$\endgroup\$
3
  • 1
    \$\begingroup\$ A 35V MOV across a nominally 48V loop supply is not a nice idea. \$\endgroup\$
    – KalleMP
    Mar 11 at 15:20
  • 2
    \$\begingroup\$ @KalleMP The capacitor in series with the MOV should take care of that, shouldn't it? \$\endgroup\$ Mar 11 at 16:26
  • \$\begingroup\$ @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. \$\endgroup\$
    – KalleMP
    Mar 23 at 20:30
2
\$\begingroup\$

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.

\$\endgroup\$
2
\$\begingroup\$

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.

\$\endgroup\$
3
  • 1
    \$\begingroup\$ "aerial cable" on a landline? \$\endgroup\$
    – Transistor
    Mar 11 at 10:33
  • 1
    \$\begingroup\$ @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. \$\endgroup\$
    – KalleMP
    Mar 11 at 15:22
  • \$\begingroup\$ @prisoner_number_6 I think (but don't know) that you may be right. Looking at the device in the catalog (worldradiohistory.com/hd2/IDX-CATALOGS/IDX/Radio-Shack/…) and looking at a project from books.google.com/books/about/… I think a simple switch is what it was. \$\endgroup\$
    – DrG
    Mar 11 at 15:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.