Cable Tester Modification: Polarity Check

[FarrisGoldstein]

I'm working on a custom cable tester as a gift for someone. I came across this thread:

http://www.diystompboxes.com/smfforum/index.php?topic=19419.0

It references the Cable Detective: http://www.zzounds.com/media/ct1man-de2cac1d945986ab7dff98dca26451ca.pdf

I have put together a scaled down design that works and only has the features that my giftee wants (only 1 set of XLR, and no MIDI). But what's missing is a polarity check (sometimes erroneously called "phase"). I just want to add one more LED that will light up when pins 1 & 2 on the XLR or Sleeve & Tip on the 1/4" are cross-wired/reversed. Can anybody help me figure out how to add that to the above circuit? Can it be tacked on without needing an IC or transistors? Seems to me, just one more LED, strategically placed, is all that's needed.

[FarrisGoldstein]

Just to simplify it for myself, I drew my modification:

http://gentlenews.com/tmp/FG_cable_inspector.png

If anybody can help a noob figure out where to put that extra LED, or what else might be needed, I'd be eternally graetful.

[slacker]

For XLR 1 and 2 reversal I think you can put another LED in parallel with D1 but the other way round, D2, D3 and the new LED will then light up if 1 and 2 are reversed. I didn't check to see what affect this might have on the other faults, so it might be worth putting the extra LED on a switch, or have a switch that reverses D1 instead of adding an extra LED. Then if you plug in and get no LEDs flip the switch and if 1 and 2 are reversed all 3 LEDs will light up.

Not sure about tip and sleeve, did you want that for a mono or stereo jack?

[FarrisGoldstein]

Thanks. That makes sense electrically now that I think about it. Are there any heat/breakdown implications with just sticking a diode in backward like that so that when current goes through it doesn't light up?

I'm not all that interested in it for a mono jack, and now that I think of it, neither for the TRS. For this application, if L & R in a TRS cable are switched, we're not that interested.

[scratch]

I made a simple cable tester very similar to this, but I used momentary push button switches, one for each 'wire', GND, Pin1, pin 2, pin 3 and a battery test ... each with their own LED built into a plastic case (so I can check the GND connection.

you can tell if you have a phase flip when u press the switch for pin 2, and the LED for pin 3 lights up, ditto pressing the switch for pin 3 and the LED for pin 2 lights up ... also can tell you if someone connected the GND to Pin 1, or if pin 1 has been left open ...

[Fredenando]

I use the Behringer CT100 from about five or six years ago without problems and is very inexpensive:



http://www.amazon.com/dp/B000CCIVCA

[FarrisGoldstein]

I thought about using an LED on each side of the tester with momentaries, but this is going to be used on a bunch of cables at once. It's important that the user will be able to quickly identify the main problem with the cable.

As for buying the Behringer, yeah, I thought about that too. But it does way more than I need it to, and since it's a gift I wanted to put some time & thought into the build.