Re: [CAD_CAM_EDM_DRO] digitizing probe

Hi Tom,

I'm not your "electronics genius", but I'd suggest more like 5+ and a 1K
resistor pull-up (the parallel port may/may not have one). I believe
you need a MINIMUM amount of current through the contacts to make them
operate properly. I don't think you'll get arcing, and I wouldn't use
the cap. RG-174U (small, .125" coax cable) makes good leads for this,
if the probe doesn't already have something. Shielded audio cable
should also work. Since the "opening" of the contacts is a "terminal"
event (meaning you only need to see the first one), I don't believe the
debounce is necessary. You could use a Schmitt trigger gate to buffer
the line (and give a little drive to the parallel port input), but it's
probably not necessary. I would recommend keeping this probe line AWAY
from motor circuits, to keep the noise out! Just my opinions. YMMV.

Alan KM6VV


Tom Eldredge wrote:

>
> Gentlemen,
>
> I am not trying to get things going on the digitizing thread again, but I have a question for you electronics geniuses ( I say that respectfully, and gratefully):
>
> I am using a digitizing probe made by Fowler. It is a copy of a Renishaw cmm probe that the patents have now run out on. It has no debounce electronics or anything like that, just a set of contacts configured as the Renishaw probes are. My question is:
>
> I am applying + 12 volts through the probe contacts then through a 100 kohm resistor then to ground to protect the contacts from too much arcing. Should I put a capacitor accross the contacts to further protect them from arcing? It seems as though a .1 ceramic might help.
>
> By the way, I am using 12 volts, because when I tried to use 5 volts, the contact resistance sometimes gave me erroneous indications. I compare the signal coming from the probe to a reference of 8 volts using an LM339 to avoid erroneous indications produced by contact resistance. I also use software to debounce the contacts.
>
> Thanks again for your help.
>
> Tom Eldredge