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Discrete Device to Isolate a Telephone

11/24/2015 10:21 AM

I remember,years ago,there was a discrete device,a diode of some type,that you could place across the phone line to prevent anyone else from using the line,like when sending a fax.

It also made it possible to prevent anyone except the first party to pick up the phone to use it;no listening in on an extension.

Anyone remember what the part number of the diode was?

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#1

Re: Discrete device to isolate a telephone

11/24/2015 11:16 AM

Not finding much that fits your description but I did find a 'diode' on this website for techno-paranoid ninnies.

For ONLY $36.95 US, you too can save yourself from all those harmful EMF's radiated from your cell phone and other electrical and electronic devices.

http://www.energpolarit.com/details.aspx?pid=666

Here is a quote from this website:

"The harmful energy frequency will be changed to a more harmonious energy that will not harm or affect the polarities (energies) of the human body. "

There is no way to create an emoticon to describe this.

P.T. Barnum was so right for something he never said.

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#5
In reply to #1

Re: Discrete device to isolate a telephone

11/24/2015 12:36 PM

I don't mind hearing voices,I just wish they would reach a consensus.

As for the EMF,I have a tin foil hat that works very well,however it is not very safe around large Tesla Coils, or outside during thunderstorms.

And,shiny side out does make a difference.

(Don't ask me how I know).

Since cars are safe from lightning,I put used tires all over my roof for protection.

Makes more sense that putting up rods that attract it.

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#2

Re: Discrete device to isolate a telephone

11/24/2015 11:46 AM

Hello: this sounds like a bit more advanced version of what you are looking for, check out the Surfer's Preserver near the bottom of the article: http://www.techlib.com/electronics/telephone.html

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#3

Re: Discrete device to isolate a telephone

11/24/2015 12:27 PM

I remember it shorted out all phone lines except the first one to pick up.

It did not matter which one picked up first.

I think each extension phone had to have this device across the line.

I believe some fax machines of that era had them built in to prevent interruption of fax when sending or receiving signal.

It was a single device,like a diode or VDR.

I saw it in an electronics magazine about 20+ years ago.

Still looking.........

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#19
In reply to #3

Re: Discrete device to isolate a telephone

11/25/2015 10:25 AM

"I remember it shorted out all phone lines except the first one to pick up.

It did not matter which one picked up first.

I think each extension phone had to have this device across the line."

That sounds like a 'selector circuit' that would be monitoring all the extensions, and the first one that 'breaks' the ringer circuit gets the line, the others are disconnected from the line until the one that has 'priority' releases the line by hanging up. The basics are simple, just a 'race circuit' like you find on the buzzers for game shows.

"I believe some fax machines of that era had them built in to prevent interruption of fax when sending or receiving signal."

What the fax machines did was to have the phone plugged into the 'extension jack' on the back of the fax, and the phone line plugged into the 'line' jack. The fax machine would constantly 'eavesdrop' on every call, and when it detected the 'fax start' tones, or when the operator pressed the 'start fax' button on the machine, it would disconnect the extension from the line and proceed with transmitting/receiving the fax, releasing the line back to the extension when it was through.

Basic 'home extensions' use the 'party line' system: each phone is connected in parallel to the phone line, to get the 'private line' setup, you need to have a 'hub' controller that chooses which extension will have the line. The Base Unit of modern multi-handset cordless phones acts as the 'hub' for the phones that transmit to it.

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#4

Re: Discrete device to isolate a telephone

11/24/2015 12:36 PM

I do not see how what you describe is possible. Maybe with a commercial quality 4 wire system used with a central switch that each phone set is connected to, but with more than one POTS units connected in parallel, I doubt it.

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#6

Re: Discrete device to isolate a telephone

11/24/2015 1:00 PM

..."While primitive lockout devices to prevent two subscribers from picking up the same line at the same time were proposed relatively early,[20] multiple simultaneous calls did not become viable until the initial tests of transistorised pair gain devices in 1955.[21][22] Any handset off-hook therefore tied up the line for everyone."...

https://en.wikipedia.org/wiki/Party_line_(telephony)

https://news.google.com/newspapers?id=v80wAAAAIBAJ&sjid=-dwFAAAAIBAJ&pg=7359,2053290&hl=en

https://en.wikipedia.org/wiki/Darlington_transistor

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#7

Re: Discrete device to isolate a telephone

11/24/2015 2:54 PM
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#8

Re: Discrete device to isolate a telephone

11/24/2015 3:32 PM
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#9
In reply to #8

Re: Discrete device to isolate a telephone

11/24/2015 3:59 PM

All it takes to tap into that is a shoelace.

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#18
In reply to #8

Re: Discrete device to isolate a telephone

11/25/2015 6:47 AM

It worked,but the problem was reverb and feedback effects due to stretching of the string.

I got the best results using cans with both ends tack-soldered on,and tapping into the center with a stethoscope.

It was also single mode operation.

Also had trouble with cross talk when passing under clothes lines,and it was

necessary to transpose the string direction 90 degrees periodically to prevent

interference from the earth's magnetic field.

All of this was eliminated when I got my tin foil hat.

Reception is good,video is great too!

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#10

Re: Discrete device to isolate a telephone

11/24/2015 4:17 PM

The device is a HS20 bilateral silicon switch which is installed in series with the line to each telephone. It's basically a combined triac and a diac as a trigger, discrete components can be used to achieve the same effect.

Google HS20 and you will probably get some more info.

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#13
In reply to #10

Re: Discrete device to isolate a telephone

11/25/2015 4:41 AM

Thanks! And a GA for that.

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#20
In reply to #10

Re: Discrete device to isolate a telephone

11/25/2015 1:43 PM

Good find. You're much better than Google.

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#11

Re: Discrete device to isolate a telephone

11/25/2015 2:42 AM

I believe that this is what you are looking for:

www.epanorama.net/circuits/tele_privacy.html

There are a few typos on the page, but it seems to fill the bill.

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#12
In reply to #11

Re: Discrete device to isolate a telephone

11/25/2015 2:49 AM

That appears to be exactly what the OP needs....

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#14
In reply to #11

Re: Discrete device to isolate a telephone

11/25/2015 4:48 AM

Thanks! Exactly what I was seeking.

A single Diac should do it.

The circuit I remember was as single component.

You deserve a GA!

Spades also gave a reference to the same device,but your link had much other info.

A GA to Spades as well.!

This is old tech,so I will have to see if it plays well with my DSL as well as my phones.

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#15
In reply to #14

Re: Discrete device to isolate a telephone

11/25/2015 5:31 AM

A Diac by itself won't do the job. The essence of the operation is that the telephone in use causes a voltage drop on the line which then prevents the Diacs in the other circuits from attaining their breakover voltages, so not firing their relative Triacs, and so keeping those phones open circuit. The first Diac achieves this by firing the Triac which will then remain in conduction until the phone is disconnected thus reducing the holding current below the threshold.

If you were to just use a Diac, you will have two problems.

1. It would only conduct when its breakover voltage was reached, and would then cease to conduct when the amplitude of the holding current fell to a low level - something that happens repeatedly during a telephone call, the signal will be distorted and, if another phone were to be off the hook, it would be able to activate during these periods and actually hijack the call.

2. A Diac exhibits "negative resistance" characteristics, this means that once it goes into conduction it will constantly vary its voltage drop in inverse relationship, but not in direct proportion, to the current through it. This will interfere with the amplitude modulation of the telephone transmitter and will cause garbled signals.

The Triac by comparison, when turned on by the Diac, maintains a steady on state until it is switched off by the switchhook.

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#16
In reply to #15

Re: Discrete device to isolate a telephone

11/25/2015 5:52 AM

Oops! DUH! My bad! you are absolutely correct.

I do need the Triac for reasons you stated.

Thanks for the correction.

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#17
In reply to #15

Re: Discrete device to isolate a telephone

11/25/2015 6:31 AM

That also gives me an idea for bypassing a battery in series with other batteries, when it is fully charged,thus preventing over charging of ,say, golf cart batteries.

I realize the Triacs will have to handle much higher currents,but that is a matter of choosing the proper specs of the chosen components.

Thanks!

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