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MeasurementBlues
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Audible continuity checkers
MeasurementBlues   4/7/2014 12:14:53 PM
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Those DMMs with audible continuity testers will sound when the resistance measured is below some threshhold, but not always that close to zero ohms. In same cases, you get the beep but the resistance is still too high for things to work.

Bill_Jaffa
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Re: Audible continuity checkers
Bill_Jaffa   4/7/2014 12:26:34 PM
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Good point--as usual, it's a good idea to stop and think first, decide if you need a basic short vs open test, or something a little more sophisticated.

MeasurementBlues
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Stories versus wires
MeasurementBlues   4/7/2014 12:45:02 PM
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"The one-wire continuity problem was one I had faced in the past, when I was helping a friend fixing up a very old two-story house."

Two story, one wire, but I ahve three stories. Does that mean I have two wires? Actually, I have 12 Ethernet lines with eight wires each. To locate and document the connections, I used a DMM after making a test cable that shorted the two middle wires together. Then I just had to run around with the sorting cable to each Ethernet jack and listen for hte beep in the wiring closer where all Ehternet lines come out of the wall.

Bill_Jaffa
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Re: Stories versus wires
Bill_Jaffa   4/7/2014 12:50:08 PM
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Sounds like you should go Kickstarter and get some up-front funding to make it into a small prject, then get some distributor of building-wiring products to carry it!

MeasurementBlues
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Re: Stories versus wires
MeasurementBlues   4/7/2014 1:10:02 PM
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I wrote about the whole thing in some deatil a while ago.

See Multimeter traces home wiring.

Using a multimeter is so uncool. Today, You've need to do the same thing using a phone app as a TDR.

Bill_Jaffa
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Re: Stories versus wires
Bill_Jaffa   4/7/2014 1:19:21 PM
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Sometimes it is both more than OK  and actually a smart thing to use an "uncool" technique; otherwise, you risk the solution being more complicated than the problem!

stevehgl
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single-ended testing
stevehgl   4/7/2014 1:59:50 PM
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We have a network cable tester which (correctly) reports the length of an unterminated cable... slightly different, but hey...

Sheepdoll
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One wire try a thousand
Sheepdoll   4/7/2014 2:56:33 PM
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I work on pipe organs.

It is not uncommon for someone to take an axe to the main cable.  These are the same as a telephone switch trunk, about 2 inches or 5cm in diameter.  Most modern installations use telephone 50 pair wire code.  The old stuff is cotton covered. 

With over 1000 wires in the bundle, it is usually easier to replace with the color coded stuff.  Otherwise it is toneing the lines out against a common.

I keep thinking there should be some sort of device which puts a different frequency on each line, then tells what connects to what.

tb100
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Re: One wire try a thousand
tb100   4/7/2014 3:21:49 PM
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Well, you can test 8 cables at a time with a 'cable toner', such as ones made by Aska or Fluke. They are meant for coax, but I don't see why you couldn't use them on other cables, as long as you have a common ground.

MeasurementBlues
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Re: One wire try a thousand
MeasurementBlues   4/8/2014 11:32:04 AM
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@tb100

I used a cable toner to identify cables running through my house. There are 9 jacks located through out the house. I'm sure glad I put one on every possible place where a TV might go. We ended up moving the TV a few months after we moved in. I'll I had to do was connect a different cable in the wiring closet. All cables are marked as to their destinations.

mhrackin
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Re: One wire try a thousand
mhrackin   4/8/2014 3:39:45 PM
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I immediately thought of my cable toner, which EVERY telecom tech or engineer is (or should be) familiar with.  This is a pair of devices: a signal source, with 2-wire output, and a sensor probe with a skinny nose.  Connect the 'hot" (red) wire of the source to the wire in question and the other to any handy ground. Connectivity of the ground to the far end is UNIMPORTANT for this tool!  Go to the other end of the suspect wire with the probe, and see if it picks up the signal. If so, there is continuity; if not, there isn't!  It's also useful for identifying pairs in a multi-pair bundle (its original intended primary use).

zeeglen
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Re: One wire try a thousand
zeeglen   4/8/2014 10:44:20 AM
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It is not uncommon for someone to take an axe to the main cable.

@Sheepdoll - just curious, why does a pipe organ have such a cable?  Is it used to activate air valves remotely from a keyboard?  And why would anyone want to take an axe to such a cable?


Sheepdoll
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Re: One wire try a thousand
Sheepdoll   4/8/2014 1:12:33 PM
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@zeeglen - The air valves are electromagnets.  The cable controls the signal flow from the keyboard to the coils.  A mechanincal, electric or electronic muliplexer called a relay is used to activate ranks of piles called stops.

These can be large instruments, the size of rooms.  Cables are cut when the organs are moved or the building pulled down.  An electromagnetic instrument can have a sizable relay, This looks like a telephone switch exchange.  Same inventor Robert Hope-Jones.

 

bellhop2
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Re: One wire try a thousand
bellhop2   4/8/2014 2:59:30 PM
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Robert Hope-Jones (1859 -1914) had a telephone background. He transfered this technology to the pipe organ. Nowadays, they multiplex everything on to one wire or fiber optic cable. Pipe organs, however, are built today with technologies ranging from mechanical linkages to fiber optics.

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.....

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.

MeasurementBlues
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Continuity in Apple cables
MeasurementBlues   4/8/2014 2:41:01 PM
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Bill,

Today I discovered yet another way that fake Apple cables are made. Put an ohmmeter across the USB connector metal shell and the shell of the Apple connector (I use the 30 pin, not the lightning connector). On genuine Apple cables,  the connector shields on both connectors are shorted. Some fakes leave off the internal wire or connection between connectors.

I have a few Dynex charge/sync cables and the connector shields are shorted together. That's clearly the better design and they are only $8 at Best Buy.

I tested a Belkin Apple charge/sync cable. The connector shields are shorted together.

The lack of shielding means no protection from EMI of from emitting EMI. That would make another good test, check eimissions of cables with the connectors chields connected.

mysterylectricity
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Pocket calculator and transistor radio.
mysterylectricity   4/9/2014 10:09:16 AM
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When I wired my first house at the age of 16 (about 30 years ago) I faced a somewhat similar prolem at some point. 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. I then traced the wire with a pocket transistor radio. The idea came to me years after I'd spent a little time listening to my "Dataman" calculator/game thinking to itself on the AM band while drifitng off to sleep one night. Not sure how I discovered that phenomenon, except to say that I was so into calculators and transitor radios at the time that it's likely I chanced upon it. Or maybe Forrrest M. Mims III mentioned it somewhere in Radio Electronics or the like. Now your only challenge is to find an LED or VFD calculator!

zeeglen
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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.


jimfordbroadcom
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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.

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
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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.

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.


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!.

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.

 

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.



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