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'[EE] Membrane potentiometers'
2011\05\10@185810 by Sean Breheny

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Hi all,

A colleague of mine is looking at using a membrane strip potentiometer
as a touch sensor. This is a three-terminal device, just like a
typical pot, except that the wiper is only connected when pressure is
applied. In effect, the wiper is actually whatever point or points of
contact are made between the top foil layer and the bottom resistive
layer.

We were both puzzled to discover that the datasheet for this part
lists both a max and a MIN voltage. Max voltage is obvious for thermal
reasons. However, we are not sure why a minimum voltage is listed.
They list their standard product as rated from 5V to 30V and then they
state that custom products can be made with ratings ranging from 1V to
60V. This seems to indicate that the 5V lower spec is not simply a
typical value listed for convenience but that there would really be
some undesirable condition to using a lower voltage.

My colleague would like to run this at 3.3V since that is the system
voltage in his device. Does anyone here have any idea why there would
be a minimum voltage spec and what the impact of operating this device
at 3.3V (when 5V is the minimum rating) would be? The only idea I was
able to come up with is that perhaps there is a certain amount of
voltage noise present due to operation of the part and that 5V is the
lowest voltage at which the accuracy specs would not be affected by
the ratio of the supply voltage to the voltage noise.

Sea

2011\05\10@192110 by Brent Brown

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I suspect it could relate to operation of the contact between conductive top wiper contact and lower resistive element... similar to minimum "wetting current" required for switches and relays to keep contacts "clean", without meeting the minimum spec it might work for a while but with reduced lifetime (contact resistance increases to the point of being un-usable). As 3.3V is so common it would be worth asking the manufacturer specifically, there may be a trick such as adding capacitance to the wiper to increase wetting current. On the other hand I could be completely going down a wrong track here in which my comments are worthless~!

On 10 May 2011 at 18:58, Sean Breheny wrote:

{Quote hidden}

> -

2011\05\11@010214 by Sean Breheny

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Hi Brent,

Thanks for the reply. Wetting current was my first thought, but I
ruled it out because the minimum voltage spec is NOT between the wiper
and the resistive element, but across the resistive element, so the
minimum voltage spec would not guarantee any current flow through the
contacts which make and break.


Sean


On Tue, May 10, 2011 at 7:21 PM, Brent Brown <spam_OUTbrent.brownTakeThisOuTspamclear.net.nz> wrote:
{Quote hidden}

>> -

2011\05\11@044251 by alan.b.pearce

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{Quote hidden}

My reaction is why not use a capacitive touch sensor? What requirements specifically call for this sort of sensor?
Microchip have a capacitive touch sensor demonstration kit that has a slider type element as one of the possible.
-- Scanned by iCritical.

2011\05\11@082808 by Sean Breheny
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Well, I was not directly involved in choosing this part but at this
point it is one of several being tried, of different technologies.
This is not actually going to be used to sense a human finger but
rather contact by cardboard boxes and other items in an automation
system, and he doesn't really care too much about the ability to tell
where (along the length) it was bumped, only whether contact was made
or not. This part is nice because it is sealed, cheap, rugged, and
very long (about 50cm) - the contact area he needs to sense is long
and not a point.

Sean


On Wed, May 11, 2011 at 4:42 AM,  <.....alan.b.pearceKILLspamspam@spam@stfc.ac.uk> wrote:
> My reaction is why not use a capacitive touch sensor? What requirements specifically call for this sort of sensor?
> Microchip have a capacitive touch sensor demonstration kit that has a slider type element as one of the possible.
> --
> Scanned by iCritical.
>
>

2011\05\11@085334 by alan.b.pearce

face picon face
> Well, I was not directly involved in choosing this part but at this
> point it is one of several being tried, of different technologies.
> This is not actually going to be used to sense a human finger but
> rather contact by cardboard boxes and other items in an automation
> system, and he doesn't really care too much about the ability to tell
> where (along the length) it was bumped, only whether contact was made
> or not. This part is nice because it is sealed, cheap, rugged, and
> very long (about 50cm) - the contact area he needs to sense is long
> and not a point.
>
> Sean

OK, so it sounds rather like he really needs a large button shaped to suit the touch area. I would still think in terms of trying a capacitive touch sensor, using a metalized Mylar film as the 'button' element, and measuring the capacitance change from the rest position. With the film at a suitable distance above the PC electrode any bump would change the capacitance enough to sense. Maybe use the capacitance as the capacitor in an oscillator and sense the frequency change.
-- Scanned by iCritical.

2011\05\11@152036 by Dwayne Reid

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face
At 06:28 AM 5/11/2011, Sean Breheny wrote:
>Well, I was not directly involved in choosing this part but at this
>point it is one of several being tried, of different technologies.
>This is not actually going to be used to sense a human finger but
>rather contact by cardboard boxes and other items in an automation
>system, and he doesn't really care too much about the ability to tell
>where (along the length) it was bumped, only whether contact was made
>or not. This part is nice because it is sealed, cheap, rugged, and
>very long (about 50cm) - the contact area he needs to sense is long
>and not a point.

What happens if there is NO voltage applied across the strip?

In other words, just use it as a contact sensor.  Connect both ends of the strip together and to one supply rail (I would use Gnd), use a high-value pull-up resistor on the center conductor.

It might be worth checking with the manufacturer to see if this is reliable..

dwayne

-- Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

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