Cable tester? xlr, jack, phono etc

It looks pretty simple: http://www.studiospares.com/test-gear/studiospares-ultimate-cable-tester/invt/458260/

i was wondering whether anyone could help me with how i would go about building one of these from scratch within a hobby box? i have components (sockets and plugs, veriboard etc) but i've never designed a circuit before so any idea would be great! im thinking two rows of LEDs that match pins 1 to 1, 2 to 2, and 3 to 3 at each end of the cable being tested (maybe 4 and 5 too) refering to tip, ring sleeve on jacks, centre pin and ground sheath on phono, and pins 1, 2, 3 on xlrs. any schematics/idea please! take a look at the link for a better idea. LEDs to indicate shorts and phase switch would be cool too.

Picture of Cable tester? xlr, jack, phono etc
orksecurity6 years ago
A cable tester mostly checks for two things:

1) That the expected connections are being made from one end to the other. (No open circuits.)

2) That no other connections are being made. (No short circuits.)

The easiest way to do this is that rotary switch. In position 1, it applies power to pin 1 on one end and checks that there is power on pin 1 and not on anything else at the other end. Repeat for 2, 3, ... as many pins as needed. Light up or buzz (or not) as appropriate to indicate that result.

Next easiest (but what I'd probably do) is to wire up a bit of simple TTL circuitry (or, these days, slap in a microcontroller) which sequenced through those combinations and displayed good or bad (and, ideally, what kind of bad and on what pins). A microcontroller which has pins that can be made either inputs or outputs under software control would be ideal for that scan.

"phase switch" ... There are some cables which have legitimate "backward" connections. Not many of them, but they do exist. In that case, what you want to do is either have the microcontroller recognize that pattern and report it as backward, or have a switch feed into the logic and cause the variant wiring to be recognized and accepted.

All pretty easy to do if you have some experience with basic logic and/or with microcontrollers, or are willing to spend a bit of time learning how to work with those.

If you want a fancier tester, you could also report how good or bad each connection was, but that's a less common failure mode.
thank you
one bajillion plus.

Some options are 'plug both ends into the tester' -- which is only good if the cable isn't installed in something -- or having a box at the other end that has expected connections from data pins to ground with specific resistors. Using a bit of logic you can identify if each pin is 'returning' the correct resistance, or if it's way out to lunch, you can deduce which shorts or opens there are.
The one other thing I'd add is that there are some fairly standard adapter conventions from one type of cable to another, and it would be nice if you wired things so you could plug the two (different) ends of the cable into the two (different) connectors on the box(es) and get the right result. That's mostly a matter of finding out what those combinations are and making sure corresponding wires are used for each type of connector. Or, again, using the reverse mode to handle at least one of the other combinations.

Or just using the appropriate adapter back to the original kind of connection. But that requires having an adapter you've already tested and know you can trust. Of course, if you got that far there's always the question of whether you can trust the cable tester. Generally, if all your cables fail the same way, the adapter or tester are probably at fault, whereas if it's just one cable bet on the cable being the problem.

(I've generally just wired up the sound system and "walked the mikes", flushing out bad cables that way... but the tester is useful for figuring out what's bad about a cable, or for going thru a lot of cables relatively quickly. Of course you _can_ just test the cables with a continuity meter -- which may be as simple as a battery, bulb, and buzzer or light -- but again, having the dedicated tool is faster and easier, especially on some of the tiny little annoying connectors.)