Finding a minor Votage drop on a 50V source... It is possible?

Thanks for answering, I'm trying to detect a voltage drop on a Telephone line when a 10MOhm load is placed across (parallel) the line. I've measured the Voltage of line before and after the "load", with a Fluke 197 and found it was about 10uV. It interests me very much if using a precision high impedance comparator may detect this drop. Any other ideas will be welcome of course :) TIA. Best regards, Nina

Not at all :-D I'm trying to understand how a "commercial" device is working. Take a look at the following links:

You WILL NOT be able to detect that your spouse has placed a "bug" on your telephone line ;-)

...Jim Thompson

Best way to detect a tapped phone line: Have a friend call from a pay phone (they still have those, don't they) and say, "Meet me at the usual place at midnight. I have the goods. Bring the money."

Then see who follows you.

Best regards, Spehro Pefhany

Sno-o-o-o-o-ort ;-)

Good one!

...Jim Thompson

Sorry about the mess,,, It was 2:30AM when I wrote the message and I was really very tired... The Fluke is 187. The line voltage is exactly 48V, and it's not taken directly from a PS, but from Very large batteries on the public exchange, Charged all the time and they are VERY stabile, and as they are actimg as large capacitors too - there is not "voltage unstability". I'm working as a manager of our national Telecomm Lab, using the most sophisticated TDRs and OTDRs + Spectrum Analysers on the market. It's really "a piece of cake" to detect most (not all !!!) of the "bugs" on line for my staff and for me, and NO, I'm not paranoid ;-D. Disconnecting a line from the exchange part and doing all the tests - reveals almost any bug on line. (yes, I know about inductive and capacitative coupled ones...)

you would need a little voltage divider to get the 50 volts down to at least the safe range of a very high gain op-amp to perform a comparator function. what i would do is calculate the divider some where in the 5 volt range, have that network drive an op-amp for gain and the next op-amp as the comparator.

phone lines are very unstable, you can get changes in line voltage from the service much more than

Thanx a lot Jamie, that's *exactly* what I had in mind, I've even made a sketch for the circuit you've suggested, using a LM1458. I have the same one - on sketch - using an Opamp with JFET inputs as well, changing a few components. I'm using 2x9V batteries, even I can "split" 1 x 9V battery, a vothage divider.... I can up it somewhere if you wish - I've just drawn it by hand and I can use Orcad to do a nicer schematic. But, as I see, (after visiting your webpage... LOL) - we transmit at about the same "wave length" and you really don't need my schematic :D The tricky part is the voltage divider, - and here I'm stuck... :( I'm not able to think or to calculate the divider for this device and nor how to connect the input of the OpAmp to it, so I need help please. I'll be very thankful to get it :) == Sorry about my english == TIA, Nina

I've checked the values again, and here are the results: Clear: 48.003 V and "Loaded" (Parallel): - 48.003V, when the "3" of it become from time to time a "2".... I've checked the connections several times, trying to keep them as clean and tight as possible, and even I know there is a loss on the connections, I can still feel this minor drop.

There is any circuit in mind that may detect even this drop? I'll be very thankful for help :) TIA, Nina

"Phil"

Sorry about that :( I'm relative new to NGs and didn't know about that.

As English is not my native language, I did and do some mistakes, sorry about that... Thick..... does it mean ..."dumb" ?

and

As I understand this is your "description". I have drawn a sketch with an OpAmp and a comparator, but I don't know how to connect it to the line, eg how to make a suitable voltage divider and connect it to the OpAmp. Sorry again for the misunderstanding... Regards, Nina

@Ken Thanx for responding,

particular line

You are right, I have a good pair, and the distance from my exchange to my house is about 4Km. I've measured the voltage at the exchange and in my home - they are about the same.

Yes, right again. I know how the line's parameteres are changing due corrosion, temperature, moisture etc. I think that the devices I've mentioned use a changeable "relative point", that compares a given value of voltage, with a minor drop. Eg, a "bug" that has been placed BEFORE the "detector" cannot be detected. The circuit I'm looking for is for detecting a device placed AFTER the monitor's installation, parallel or series. I've seen such a device on my last trip to England, on a "spystore" that detected an automatic recorder activator placed in parallel, that it's input was 4.4MOhms. As well, it detected a line transmitter (Series) that used a 47 Ohm resistor over the diode bridge. I've seen it but I haven't buy it, even it interested me how's done....

With all the respect, here you are wrong... I've worked in many modern exchanges. The audio surveillance is definitely done ONLY by software, but this is the LEGAL TAPPING !!! All the bugs that private people or so called "detectives" ARE physical connected (except inductive), so this is the point that interests me. The minor CHANGE on the line voltage.

Of course, DEFEATING those *simple* bugs is a mistery for me too, (not by white/pink noise or other noises...) How can be done ? If you have any practical ideas and wish to help me, I'll be very thankful :) Regards, Nina

There was I thinking wow what is this 50v supply with better than 1 part in

Google poster - shoulda known better.

@ John Woodgate, Thanx for responding,

A Bright idea !!! Never thought about "sampling"... ;-S (ashamed)

Yes, you are right of course, and several NG members actually mentioned it... I'm guilty and ashamed.... It seems I must smoke better stuff.... LOL :-D

Even so, I still need a practical schematic from a "PRO", as I cannot find a solution to connect a 9V (or 18V - 2x9V) comparator to the line via a voltage divider... Tia for further helping Regards, Nina

@John Woodgate, Thank you very much for the ideas and explanations, I'll try to build a device, and try to find out if it works. The NGs are very new to me, it's the second time ever posted, (The first time was a Photoshop question...). Usually, up today, if I had an electronis problem, I've posted to Electronics Forums, exchanging *schematics* and ideas. As I could see, here is a higher "league", and even there are (or is) a member that blamed me (justifiably) for my mistakes, but using a real bad language, and even I don't really know how to use the > etc., I"d like to thank you all. :) Regards, Nina

@Phil Allison,

I'm sorry about the misundersanding, on my first post.

usenet !!

I'm stucked with my ISP for 2 years more, and I know nothing about usenet. I'll ask my kid when he'll return from the army next time, he is the internet "pro" in my family. I'll also ask my ISP about usenet.... I've heard this word before, but was never curious what it means. Anyway, even the cold showers I've got, I'd like to thank you very much for the "lesson" Regards, Nina

@ John Woodgate

EXACTLY !!! :)

Now, have you any idea how those devices, the simpler ones, 1-2MOhm Parallel devices, or simple Recorder activators (not VOXes) or simple Serial "bugs" can be defeated? No one from the members could answered this issue...

Regards, Nina

@martin,

Sorry martin, I cannot reach the link you've sent - don't know why... I have 2 TDRs and fixing TDRs and OTDRs are the "speciality" of my LAB, among many other devices. A TDR can detect a bug, but cannot defeat it. How those simple instruments can *defeat* a tap on line? It's very misterious for me, and I hope that any member may explain the theory behind them. :) Regards, Nina