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.
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!.
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.....
@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.
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.
@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.
January 2016 Cartoon Caption ContestBob's punishment for missing his deadline was to be tied to his chair tantalizingly close to a disconnected cable, with one hand superglued to his desk and another to his chin, while the pages from his wall calendar were slowly torn away.122 comments