ProSoundWeb Community
Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => The Basement => Topic started by: Frank Koenig on July 07, 2019, 08:11:25 PM
-
Everyone should know how to make a 3-wire cable tester with 2 LEDs. (DC is applied to wire 1 at end A. An LED bridges wire 1 to 2 at end B. Another LED bridges wire 2 to 3 at end A. Wire 3 at end B connects to the DC return.) Both LEDs illuminate only if there are no opens, shorts, or any of the 5 possible miswirings (3! - 1) that exclude connecting a wire directly back to the same end of the cable from which it came.
I was trying to extend this idea to 4-wire cables so that I could make a speaker cable tester (all my speaker cables are 4-wire). There are 23 (4! - 1) possible miswirings that fit the criterion above. The obvious thing to try is to use 3 LEDs and bridge pairs of wires at each end so as to connect all wires in series. It turns out that while this approach detects opens and shorts it will not detect all miswirings, usually missing one of the 23 depending on the specific topology. I believe there is one class of topologies, however, that detects all 23 miswirings, an example of which is illustrated below. Please try to poke holes in it. I may be deluding myself.
Anyone actually wanting to build one of these testers should be mindful that a miswired cable applies reverse voltage to the LEDs and their reverse breakdown voltage must be respected by keeping the open-circuit voltage low or using protection diodes. BTW, there are 40,319 ways to screw up an 8-wire cable :o
And yes, for those of you who don't find this sort of thing amusing (and sort of elegant), buy one of the dozens of commercially available brute-force cable testers.
--Frank
(http://4WireCableTester.jpg)
-
The most important feature of the cheapie Behringer I have is the intermittency tester. I wouldn't buy/build a tester that doesn't have that function. Any idea how to incorporate that feature into your design?
-
The most important feature of the cheapie Behringer I have is the intermittency tester. I wouldn't buy/build a tester that doesn't have that function. Any idea how to incorporate that feature into your design?
A SR flip-flop would work.
-
The most important feature of the cheapie Behringer I have is the intermittency tester. I wouldn't buy/build a tester that doesn't have that function. Any idea how to incorporate that feature into your design?
I'll second the value of intermittency tester function. Caught more crab cables with that function.
-
Here seems like a good place to post my version of an XLR/Phantom Power tester if anyone is interested...
(https://stevesmithphoto.webs.com/XLR-TEST-1.jpg)
Steve.
-
As much as I like a good DIY project and I have built many RAT already makes an NL4 version of their XLR "RAT sniffer" cable tester.
-
Here seems like a good place to post my version of an XLR/Phantom Power tester if anyone is interested...
(https://stevesmithphoto.webs.com/XLR-TEST-1.jpg)
Steve.
Wow, pretty clever design!
I just carry around a vastly simpler tester, two LEDs on a bare male XLR insert - not as comprehensive, but allows finding many problems. Plus I can give one away without missing it.
-
Wow, pretty clever design!
I just carry around a vastly simpler tester, two LEDs on a bare male XLR insert - not as comprehensive, but allows finding many problems. Plus I can give one away without missing it.
I do have another design which includes an opto transistor and just one LED. The LED only lights if everything is ok. All other possible faults cause the LED not to light. That's all you really need in a live situation.
I designed the three LED version before I was aware of the Dave Rat version. I have had a couple of conversations with Dave via e-mail who was very supportive, rather than most manufacturers who would be protective of their product. I pointed out that I wasn't competition and I was only making them for friends.
Steve.
-
Here seems like a good place to post my version of an XLR/Phantom Power tester if anyone is interested...
This is very nice as it is diagnostic and not just a go/no-go tester, and I could see it useful in the field. It does not, however, detect perhaps the most insidious cable of all, which is the one with 2 and 3 swapped. Then again, if you start off with correctly wired cables this is unlikely to happen on its own.
I wrote a little R program that can simulate a tester with all permutations of miswiring for any number* of wires and a fixed external arrangement of LEDs. The 4-wire tester above appears to pass.
*Well, not any number, as it's of N! complexity :o
--Frank
-
This is very nice as it is diagnostic and not just a go/no-go tester, and I could see it useful in the field. It does not, however, detect perhaps the most insidious cable of all, which is the one with 2 and 3 swapped. Then again, if you start off with correctly wired cables this is unlikely to happen on its own.
That is absolutely correct. The Dave Rat/Sound Tools XLR sniffer can't detect that either when used with ordinary phantom power. However, it can if you use the battery powered sender unit with it as it puts two different voltages on pins 2 and 3. I could also do that with mine but decided not to as in reality, as you point out, cables don't re-wire themselves, they just short out or go open circuit. The swapped connections it does indicate are just extras which might be useful sometimes.
Steve.
-
That is absolutely correct. The Dave Rat/Sound Tools XLR sniffer can't detect that either when used with ordinary phantom power. However, it can if you use the battery powered sender unit with it as it puts two different voltages on pins 2 and 3. I could also do that with mine but decided not to as in reality, as you point out, cables don't re-wire themselves, they just short out or go open circuit. The swapped connections it does indicate are just extras which might be useful sometimes.
Steve.
I have a couple of the RAT sender/receivers.
At the show, I don't really care what's wrong, just that the cable is "bad"
If I don't see 3 green, a knot get's tied at the end of the cable, and it is put aside for some shop time.
I always wondered what was in the RAT tool.
Thanks for sharing.
Simple, but brilliant all at the same time.
Chris.
-
I always wondered what was in the RAT tool.
Just to be clear, this isn't a copy of the Sound Tools device (I wouldn't do that). Mine is different. You can find the Sound Tools circuit by doing a patent search.
I actually think mine is more logical. The error codes can be remembered quite easily.
Steve.
-
Just to be clear, this isn't a copy of the Sound Tools device (I wouldn't do that). Mine is different. You can find the Sound Tools circuit by doing a patent search.
I actually think mine is more logical. The error codes can be remembered quite easily.
Steve.
Hi Steve.
Sorry, I didn't mean to imply anything.
YOUR design is simple, and brilliant.
I can't imagine the business end of the RAT tester is a lot different.
Chris.
-
Sorry, I didn't mean to imply anything.
No problem. I wasn’t presuming that you might have thought that. I just thought I should generally point out that it wasn’t a copy.
Steve.
-
Hi Steve,
I'm a live sound engineer based out of india. During this lockdown time, I have been trying to build a prototype of the XLR sniffer. After days of searches on the internet I was finally thrilled to see the circuit daigram posted by you. Just so you know I'm a complete dummy when it comes to electronics and components, though I have been doing a lot of reading in the down time to understand the working of different components. I was wondering if you could guide me in building this prototype.
Here are few questions I had after studying your daigram(Please excuse me if you find some of them very stupid ::))
1. Are you using 5mm leds which work on 5V dc? And is this the reason you are using a 5v6 zener diode?
2. What if I were to use a 3mm led with forward voltage of 2-2.1V, would the zener diode configuration change to 2v4?
3. Irrespective of whichever led I end up using does the value of my resistor change? If yes how do I calculate that?
4. Would the circuit change if the applied voltage changes to 12V?
5. Lastly, How would my connections change if I were to use a 3 PIN bi colour 5mm led? as in where do i connect the common terminal?
I kindly request you to please guide me through this, as I have been stuck with question for days. Any sort of help would be very kind of you.
Thank you. :)
Warm Regards,
Abhinav
+91 8805507035
-
1. Are you using 5mm leds which work on 5V dc? And is this the reason you are using a 5v6 zener diode?
2. What if I were to use a 3mm led with forward voltage of 2-2.1V, would the zener diode configuration change to 2v4?
3. Irrespective of whichever led I end up using does the value of my resistor change? If yes how do I calculate that?
4. Would the circuit change if the applied voltage changes to 12V?
5. Lastly, How would my connections change if I were to use a 3 PIN bi colour 5mm led? as in where do i connect the common terminal?
1. They are 3mm LEDs. Red and green. Forward voltage is about 1.7v. The 5.6v zener is there so that if pins 2 and 3 are shorted, there is not enough voltage to light the LED with the zener in series. The other LED limits the voltage to 1.7v so no current can flow through the zener.
2. A higher forward voltage LED is fine, no need to change the zener. However, any zener from 1v to 10v will work. I used 5.6v as it is a standard value which I have a lot of.
3. The resistor value is not critical. 470R to 4K7 will be fine. The current is limited to 7mA by the 6K8 series resistors in any mixer's phantm power circuit. The tester doesnt need that resistor to work. It was added at Dave Rat's suggestion to prevent damage if accidentally plugged into DMX or Cue light system, or anything non standard.
4. It will work on 12v, but you need some resistance in series to pins 2 and 3. I suggest 2K2.
5. You cant use a 3 terminal LED. It needs to be the two terminal type with one LED connected one way and the other, reversed. Like this... [size=78%]https://www.digikey.com/products/en?mpart=LTL-14CHJ&vendor=160 (https://www.digikey.com/products/en?mpart=LTL-14CHJ&vendor=160)[/size]
For personal use only.
*** CONTACT ME IF YOU WANT TO USE THE DESIGN COMMERCIALLY ***
Steve.
-
If anyone is interested, this is my simple, single LED version. In a live situation, you just want to know good or bad. The LED will only light if all three conductors are good and there are no shorts.
(https://stevesmithphoto.webs.com/Single-LED-XLR.jpg)
Steve.
-
If anyone is interested, this is my simple, single LED version. In a live situation, you just want to know good or bad. The LED will only light if all three conductors are good and there are no shorts.
Brilliant! Uses phantom, correct?
It looks like the only catch is that if pins 2/3 are shorted or swapped it would still show "good"?
-
Hi Steve,
I'm a live sound engineer based out of india.
Warm Regards,
Abhinav
+91 8805507035
Please go to your profile and change the "Name" field to your real first and last name as required by the posting rules displayed in the header at the top of the section, and in the Site Rules and Suggestions (http://forums.prosoundweb.com/index.php/board,36.0.html) in the Forum Announcements section, and on the registration page when you registered.
Mac
admin
-
If anyone is interested, this is my simple, single LED version. In a live situation, you just want to know good or bad. The LED will only light if all three conductors are good and there are no shorts.
(https://stevesmithphoto.webs.com/Single-LED-XLR.jpg)
Steve.
When I was getting started in the early 90's I made a super simple phantom tester that was just a Switchcraft male XLR with a green LED between 1 and 3 and a red between 2 and 3. I'd take off the strain relief boot and fill the inside of the connector with silicone to keep out dirt and junk. The LEDs would still glow just fine in the translucent silicone.
I didn't know enough to add resistors or any of that "fancy" stuff. :) Still worked just fine. And it was so simple and basic I made a bunch of them so we could all carry one in our pocket and if we lost it we just grabbed another out of the bin. As Andy said, it didn't show pin 2 and 3 shorts or swaps. I toyed with making a "sender" that would apply voltage to 2, 3, or both to test when phantom wasn't available and to show shorts and swaps, but honestly I just didn't find enough of a need for it.
In today's world I find myself needing to do a LOT less analog cable troubleshooting, so it's not as a big of a need. But it certainly was a handy thing to have back when we were dealing with hundreds of feet of wire and usually at least half a dozen connections on every input channel.
-
Brilliant! Uses phantom, correct?
It looks like the only catch is that if pins 2/3 are shorted or swapped it would still show "good"?
Yes. Uses phantom.
If pins 2 and 3 are shorted, the voltage across the LED in the opto isolator is 1.7 volts. This is also the voltage across the series connected LED and the transistor in the opto. The transistor will drop 0.7v and the LED will need 1.7v, so 2.4 volts is needed for current to flow and the LED to light. as there is only 1.7 volts across the pair, it won't light.
It's the same principle as used in my three LED tester, but uses the transistor for the extra voltage drop instead of the zener diode. You could put a low voltage zener or even an ordinary diode in setries with the LED as well just to make sure.
The only way to check 2 and 3 are not swapped is to use a battery powered sender unit instead of the mixer phantom power, which puts out different voltages on pins 2 and 3. This is what Dave Rat's sender unit does.
Steve.
-
If pins 2 and 3 are shorted, the voltage across the LED in the opto isolator is 1.7 volts. This is also the voltage across the series connected LED and the transistor in the opto. The transistor will drop 0.7v and the LED will need 1.7v, so 2.4 volts is needed for current to flow and the LED to light. as there is only 1.7 volts across the pair, it won't light.
Ah. Didn't catch that. Very slick.
The only way to check 2 and 3 are not swapped is to use a battery powered sender unit instead of the mixer phantom power, which puts out different voltages on pins 2 and 3. This is what Dave Rat's sender unit does.
Unfortuately, that's a common miswiring, 2nd only to Pin 1/2 swap. I can't imagine a passive 1-ended device that could detect the 2/3 swap.
-
Unfortuately, that's a common miswiring, 2nd only to Pin 1/2 swap. I can't imagine a passive 1-ended device that could detect the 2/3 swap.
True, but this is a simple test to check a cable is still working. It isn't going to rewire itself. Assuming it has been made correctly, it's only going to suffer from shorts or open circuits.
Steve.
-
When I was getting started in the early 90's I made a super simple phantom tester that was just a Switchcraft male XLR with a green LED between 1 and 3 and a red between 2 and 3. I'd take off the strain relief boot and fill the inside of the connector with silicone to keep out dirt and junk. The LEDs would still glow just fine in the translucent silicone.
I didn't know enough to add resistors or any of that "fancy" stuff. :)
The current-limiting resistors are built into the mic preamp - 6.8k, per the phantom power spec. Convenient, eh?
I also keep a few of the same design handy. One of the fastest ways to troubleshoot a suspect microphone/cable/input.
-
Hi Steve.
Sorry for the late reply. Thanks to you I was able to successfully make the Sniffer end of the device. Now I'm trying to build the SENDER side of the device and facing the following problems:
1. So in the sender side of the device, I do not understand as to where do I connect the Ground terminal (PIN1) to?
2. Also, when I'm connecting my 12V battery ( +ve terminal to pin 2 & -ve terminal to Pin3) and PIN1 open, the LEDs representing PIN3 tends to go in reverse polarity colour(red), which otherwise when connected to the phantom power is green. Could you please point out as to where I might be going wrong?
3. Also, as per your advice I tried to put 2.2k ohm resistance in series to pins 2 and 3, it seems that the LEDs were glowing quite dim as compared to when they were connected directly to the 12V battery or the phantom power supply.
I'm using a 12V 23A DC battery.
Kindly please guide me steve. Also, just so that you know this is a personal project and and not being used commercially. :)
Regards,
Abhinav
-
Hi Steve.
Sorry for the late reply.
Regards,
Abhinav
Please go to your profile and change your displayed name to your real full name to comply with the forum posting rules.
Mac
Admin