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Conductive inks on PCB for variable resistance 1

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BrianE22

BrianE22

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Mar 21, 2010
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I'm looking to make a switch with a variable resistance - much like what you see on power tools. Our space is very confined so I can't order a standard power tool trigger assembly. I've taken apart a few switches and it looks like maybe a conductive ink is used on the circuit board with a movable contact on the trigger. Am I assuming correctly that conductive ink is used? I've also noticed that potentiometers look to be using what I'd guess is graphite?

Since my application is so close to a power tool trigger I'm guessing conductive ink is the way to go. Our quantities will be about 500 pcs/year. Are there prototype board makers out there that will print conductive ink? Or for prototypes can I buy the ink and somehow apply it myself?
 
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Why not use a linear slide potentiometer? That would seem to be substantially more controllable and consistent than something you codge up in your back lab.


While you can roll your own, there's no guarantee that you'd have the right formulation to have sufficient wear resistance to meet your product life requirements.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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I've looked at linear slide pots. It's hard to find them short enough but I think I could fit one in with a little effort.
 
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Piezo-resistive sensors are low cost and can be made very small. No moving parts, all you need is variable force. Force-controlled devices are easy to Control with good precision. Sliding things are more difficult.

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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The resistors will be printed using screen-printed conductive inks. We used to use these in the thick film hybrids industry where the resistor values were fired and then trimmed using a laser for high accuracy applications. Can't remember the ink manufacturer's name - print & fire wasn't my area of the plant - but here's some examples from Henkel:
Any PCB house with silk screen capability should be able to do this for you.
 
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You could do it digitally. You make a no-moving-parts variable touch sensor with a micro that reads the setting and provides the remembered setting permanently. It can be a pretty small micro. You can also use the micro to control everything else and add useful features.

Keith Cress
kcress -
 
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You could also consider capacitive or inductive sensors on the board. They can found in planar form. A quick peek at Digikey finds many choices.
The advantage of these types is that you get away from the direct contact of one element sliding against another. Friction and wear and all that... life's too short, so to speak.



STF
 
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IRstuff - I ordered a couple of those to play with. I like that fact that the linear pot idea already has the sliding contact portion designed and tested.

Gunnar - Neat idea - do you mean something like this, ? That's too much money for me. I'm hoping to keep the sensor part under $8.00. While snooping Digikey, I did find these:


They looked kind of cool so I ordered a few of them.

Scotty - I don't know why I didn't ask my board supplier. Turns out he has done printed resistors before. He's going to get back to me on who manufacturers the ink he's used. I'm looking for design info on the sliding contact part of it (contact shape, pressure and ?).

Keith - Another neat idea. I wasn't sure what to search for but I found this:


Is that what you had in mind? Looks promising - I ordered and evaluation kit. I haven't found much info yet on the sensing part of it. I think you just put some pads on a PCB and slide a metal plate over them. Their examples show 4 pads but it looks like you can put 2 in series for 8 bit resolution. That's more than good enough for me.

So now it's a week or so of play time with the gadgets. Thanks for the ideas -
 
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There are other force-sensing solutions. Like this:
I have also used "soot foil" (the kind of plastic that you find in certain anti-static bags) to improvise force sensors. Then the cost is nil. The flexi-force sensors can be customized so you get exactly what you need. The actuator can be part of the housing, like a beam with three "weak" sides and one "strong". Movement doesn't need to be more than a mil or two.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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HI Brian.
Brian said:
Is that what you had in mind? Looks promising - I ordered and evaluation kit.
Click to expand...

Sort of.. I was think more along the lines of a Microchip uP with touch capabilities. You can use a uP to do any of the other logic your product needs too and in a flexible manner. For instance Milwaukee's newer cordless tools are using Brushless motors that the uP runs. They do things like have a work light that comes on when you key up the drill and then the LED light stays on after you stop operating the drill for about 10 seconds then it coolly fades over about 3 seconds. The uP also keeps the Lithium batteries from being over discharged, etc. etc.

What you chose is a specific chip that does the touch detection and outputs signals stating "channel x active". That works fine too but is less flexible unless you already have a uP involved.

Sadly Microchip has disgustingly slurped up it's only direct competitor Atmel and in an effort to combine their monopoly into one website they've created one of the buggiest POS sites I've seen in years. I used to be able to find everything in a minute or two, now I can find nothing in hours but; "that page is not available", "there is a server issue", "404", and I am utterly unable to find, for you, their touch development board that was very cool.

Keith Cress
kcress -
 
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"it looks like you can put 2 in series for 8 bit resolution" - While I didn't read the datasheet, I expect using two will will shift you from 4:1 to 8:1 so you'll move from 2-bit resolution to 3-bit resolution. Double the fun means 1 more bit!

Z
 
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