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DCH0
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Tic Tracer
DCH0   4/11/2014 2:33:36 PM
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Commonly called a Tic Tracer, Triplett makes a version Sniff-It 2.  With a variable setting, it beeps and flashes a light near an energized circuit.  Beeping becomes more frequent the nearer you get.  Rated for 5V - 600VAC, it is a non contact device great for finding live wires behind dry wall.

I think your time domain reflectometer will bounce signals off of every wire nut connection.

MeasurementBlues
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Followup story on continuity
MeasurementBlues   4/10/2014 10:24:08 PM
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See DMM Shows Differences in Real, Fake, and Good Substitute Apple Cables checking for continuity turned up some differences.

 

salbayeng
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Re: RF continuity tester
salbayeng   4/10/2014 3:38:31 AM
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Hi 

I just use RF out on the spec analyser, but it's kind of big to lug around. 

Years ago I remember there were hand made testers that generated swept signals around 455kHz or 10.7MHz or 45MHz. You could test all the way from the antenna through to the speaker / TV screen, (AM /FM didn't matter) They relied on leakage of the IF through the front end so you didn't need to tune the radio. (If you have a spec analyserswith a tracking generator you can make a similar test signal)

For locating the other end of a single wire , I just hook up a 20.00MHz crystal oscilator to 4.5v worth of battery and attach the crystal oscillator output to the cable. You can do all this with 3 jumper cables. Then just use a FM radio tuned to 100MHz to follow the wire. You can use any crystal oscillator that has an odd harmonic in the FM band. 

If I need a TDR , I just connect a Tee to the scope input, and put the "1kHz probe comp" signal into one end of the tee through say a 100ohm resistor and connect the other end to the cable. Do a calibration check on a known piece of cable and you are away!.

Sheepdoll
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Re: One wire try a thousand
Sheepdoll   4/9/2014 11:58:44 PM
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 @David Ashton - I'm not sure how many pipe-organ axe-murderers there are out there.....

It is more a theoretical exercise. In practice, given the age of the wire it is easier to just make a new cable.

David Ashton
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Re: One wire try a thousand
David Ashton   4/9/2014 4:12:21 PM
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@Sheepdoll... you can get RS-232 cable testers which do 25 wires at a time.  Or there are companies which make bigger ones....try here....

http://www.cablescan.com/products/products.htm

The small 8-wire network cable testers usually use a CD4017 counter chip to put a + on one wire at a time and use any or all of the other wires for return...so you need at least 2 wires working.  You can use the same technique and a bunch of 74HC154 dedoders to expand that to 16 x n lines.    So you could build yourself a custom cable checker quite cheaply if you don't mind picking up a soldering iron.

Of course, I don't know if would be worth your while doing this - I'm not sure how many pipe-organ axe-murderers there are out there.....

zeeglen
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Re: RF continuity tester
zeeglen   4/9/2014 2:23:57 PM
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@Bill the continuity threshold was about 5 ohms. Of course, your value may differ!

Our lab DMMs buzz at 33 ohms - useless!  So I built a bunch of "milliohm squawkers" for our techs to use.  This changes delta-R of a few milliohms into a variable pitch tone so a reverse-engineering tech can audibly distinguish correct PCB connections in spite of closed relay contacts or low resistance transformer windings.  Watch for it in upcoming EDN Design Ideas.


jimfordbroadcom
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CEO
Re: RF continuity tester
jimfordbroadcom   4/9/2014 1:12:28 PM
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Yeah, that's the problem!  You may think things are OK with one meter, but another tells a different story.  Usually it's best to see the value and make your own conclusion.

Bill_Jaffa
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Re: RF continuity tester
Bill_Jaffa   4/9/2014 1:09:55 PM
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I actually measuredthe buzz-out ohms value on my el-cheapo Radio Shack multimeter (easy enoug to do)--the continuity threshold was about 5 ohms. Of course, your value may differ!

jimfordbroadcom
User Rank
CEO
RF continuity tester
jimfordbroadcom   4/9/2014 1:01:15 PM
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Speaking of continutity testing, the other day I was frustrated because I needed to check RF continuity through a path with series AC coupling caps.  DMM is going to show near open circuit, and I wondered if a device to inject an RF tone at a certain frequency (user settable of course) and detect it at the other end of the path would be a marketable product.  What do you think?  I'd buy one!

What about step attenuators with pre- or post-amplifier or both to compensate for non-0 dB insertion loss, bypassable for those situations where you don't want the noise or distortion from the amp(s)?  And/or built-in power detector for ALC (automatic level control)?  Any takers?

I'm just throwing things out there, lest you think I'm doing my own marketing in a public forum.  If anyone from Mini-Circuits or Telemakus or wherever is listening, maybe this is an opportunity to make some useful instruments.

@MeasurementBlues, you make an excellent point about the audible (DC) continuity testers - what is the resistance threshold for the beep?  I often say, "I don't trust the beep." and set the DMM to read the ohms, thank you very much.  Now, if I could set the ohm threshold, I might be inclined to buzz my circuits out.

zeeglen
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Blogger
Re: Pocket calculator and transistor radio.
zeeglen   4/9/2014 11:07:56 AM
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@mysterylectricity My solution was to loosely wrap one end of the wire around an LED pocket calculator. This was guaranteed to inject a good deal of distinctive RF hash in the line.

What a great idea

A few years ago some squirrels got into the walls and chewed through an alarm cable leading to a group of windows.  I knew the cable was open from ohmeter checks, but finding it's route through the wall was another matter.  I connected an RF signal generator to one end in the attic, then used a scope probe as a small antenna (did not have a transistor radio handy) to locate the cable in the wall.  Then used a 4" hole saw to get at the cable.  The nice thing about cutting into drywall with a hole saw is it is so easy to repair afterwards.


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