[Cialug] OT: continuity test

[Thomas Kula]

On Thu, Jul 02, 2009 at 03:23:29PM -0500, David Champion wrote:
> For you electronics gurus out there...
> 
> For our SCCA autocross timers, we have a set of cables that go from the 
> timer head unit, to a photo sensor head out on the course (similar to a 
> garage door safety sensor, bounces a light off a reflector). The cables 
> have 3 conductors, we're using a fairly light gauge stranded wire, 
> probably about like what's in ethernet cable. The connectors are 1/4" 
> stereo headphone jack style (3 connectors). The cables may be up to 
> about 250 feet in length.
> 
> Occasionally someone will get off course and run over one of the cables 
> with their car, and they get some rough treatment at times, may get 
> kinked, and get dragged around parking lots. I'd like to have some kind 
> of way to do a quick cable test on them during setup to help shorten the 
> troubleshooting time, so we'd know whether or not it's a cable issue, or 
> another issue with the photo sensor.
> 
> Anyone have an idea of a simple device we could build to test the 
> cables, or would we be better off using a multi-meter, or another 
> pre-existing device?

Two things spring right to mind:

I'm guessing it might be nice to test the cables whilst they
are laid out already (helps you with "oh, hey, it stopped working,
did someone damage the cable or bump the photo sensor, let's test
the cable"). I'd build two boxes. The far end simply shorts both
ring and tip to sleve. The near end has a battery and two leds,
send power down the sleve, run an led between ring and between
tip to ground. If both lights are out, your cable is either totally
cut, or the sleve wire is damaged. If only one light is out, you
know which wire is damaged, if both lights are on, everything is
fine. I don't know if you need this granularity, but it seems like
it might be useful.

Even if you don't want the above lights, having the "loopback" box
and then replace the led box with one that simply has three easy
to use terminals, so that when you use a multimeter you don't have
to try to hold the probes against the headphone jack itself.


-- 
Thomas L. Kula | kula at tproa.net | http://kula.tproa.net/
Mathom House in Midtown, The People's Republic of Ames