"N_Cook" wrote in message ...

On 15/05/2014 15:28, John-Del wrote:
So I had this membrane keyboard assy from a Swiss machining center in for
repair as a couple of the switches had failed, and intended to replace the
two with mechanical NO switches as the membrane is NLA from TESA. I
couldn't trace one of the printed switch conductors back to the main
circuit, so I had to peel the membrane apart to see what happened. One of
the printed traces had opened, so I picked up some Bare conductive paint
from Radio Shack to repair the trace.

http://www.amazon.com/Bare-Conductiv...nductive+paint

This product sold on the premise that it could be used to replace or
fabricate circuit traces or use to attach components without solder. On
the back of the package, they show a painted circuit connecting an LED to
a battery.

Since this is just low voltage/low current key matrix, I figured that this
would work fine. Before painting it on, I drew a two inch trace (maybe an
eighth wide, about the width of the printed traces) on a piece of plastic
and measured the resistance the next morning. The two inch trace read
over 1K. That's right, 500 ohms per inch on a fairly wide circuit trace.
The small blob I would need would read between 50 and 75 ohms. Maybe it
would work, maybe not, but I'm not taking a chance.

I can't imagine any use for this stuff whatsoever.

Has anyone used this stuff and measured the resistance of it? :

http://www.amazon.com/CircuitWriterT...nductive+paint




This may or may not be relevant, but useful to know:

I recently bought some of this stuff to repair an unobtainable fader on a
vintage synthesiser:
http://www.ebay.co.uk/itm/Conductive...item4aa5393d90

It turned out to have rather a higher resistance than I anticipated, but I
got it to work in the end.
I tried a line of it on a piece of paper, maybe 5mm wide, and it had
hundreds of ohms per millimetre when dry, which sounds similar to the OP's
experience.

However, it does work well as a glue, and it is a damn sight cheaper than
any of the other silver loaded conductive epoxy's I came across.


My problem was the fader's earth tag was not connected to the start of the
track, so I had to glue a very fine wire onto the metallised section of the
track end that connects the carbon track to the pin, and feed the wire
through a tiny drilled hole to the outside world. The contact wipers sit on
this metallised section at zero, so it had to be connected right on the edge
so as not to foul the wiper.
Simply dropping a blob of this glue onto the wire and track produced 300 to
400 ohms of connection resistance, meaning the fader would not zero.

I had to bend and hold the wire in such a way that the end couple of
millimetres were actually sitting flush on the track. Not easy. Dropping the
glue onto this arrangement eventually produced a solid join of around 20
ohms resistance, which was enough for the synthesiser to read the zero as a
zero. (faders used as potentiometers feeding a DAC)



Cheers,


Gareth.