# What is this resistor strip on my helicopter circuit?

RM: Smaller photo substituted - larger slower loading one here if desired: http://i.stack.imgur.com/RPM8l.jpg

I am building a helicopter and would like to use a linear servo like the one on the right in this picture. The exact same servo has been removed from the left side, revealing two strips, one bare copper and the other one black with variable resistive properties. The black one is connected to positive voltage on top, ground on the bottom.

The back side of the slider on the servo is touching these two strips, causing a variable voltage input on the microcontroller on board. I need to get me one of those black strips, and having searched for membrane resistor/potentiometer, I can not find anything that will suit my circuit. It is supposed to be about 20mm long, and the only thing I can find that actually looks like something I need is this one from SparkFun but that is obviosly to large.

All help on the mather is greately appreciated

• That strip looks like Carbon Conductive Ink or what theses guys are calling thermoset ink henkel.com/com/content_data/LT-5980_HI_REV_p1-2.pdf Basically it's a resistive ink laid down after the pcb board is fabbed. You could probably find an assembly house that can do it. Why not just buy an off the shelf slider pot? – Some Hardware Guy Oct 4 '12 at 12:38
• Slider pot will add extra weight, which is critical. Thank you for explaining what the strip is – chwi Oct 4 '12 at 13:16
• You are probably going to be better off buying the miniature linear servos than trying to make them - you might make one experimentally (been there, done that) but will have a hard time duplicating the process factors that go into getting something reliable to actually fly, especially that is mass competitive with existing solutions. – Chris Stratton Oct 4 '12 at 16:02
• Who said anything about making them myself? Of course.I buy them ;) – chwi Oct 4 '12 at 18:04

## 2 Answers

That will be a linear resistive track for servo feedback.
Measure track resistance end to end. Then ...

You may be able to drive a standard linear mechanical pot instead using the mechanism.

Looks doable. Gear on end of existing linear resistor needs to be fitted to multiturn end of a linear pot.

You can get linear posts with screw thread drive. Or you could add a simple thread and nut drive to something like this linear pot from Mouser

• Making a helicopter I need to think about the weight of the hardware, making an extra linear potmeter extra weight. Thank you for the different solutions. – chwi Oct 4 '12 at 13:14
• Having done this, it takes a lot of mechanical rework of a multiturn pot to get the friction low enough. I ended up replacing the screw and the plastic "nut" with miniature metal hardware, though modern commercial RC linear servos use a lower friction plastic nut design. – Chris Stratton Oct 4 '12 at 16:01

Here's some of those paints I was talking about. I've never used them myself so you'd have to experiment but take a look. Looks like they range from $9.95 to$29.95 (and up I guess)

http://lessemf.com/paint.html

http://www.tedpella.com/semmisc_html/SEMpaint.htm

http://www.tedpella.com/adhesive_html/Adhesive-Comparison.htm

Here's a interesting MIT site that talks about it as well: http://web.media.mit.edu/~plusea/?p=183

Good Luck.