Fletchtronics

On the train on my way to work yesterday morning I was hit with an inspiration on how to build something like the hotpot and softpot potentiometers that sparkfun.com sells. Only, I wanted something that cost a bit less than $19. See? Commuting really is good for something!

Remembering back to my days as a troublemaking youngster, I recalled that putting 12 volts across the lead of a pencil caused it to glow brightly red (and then stink up the basement). So I knew that it would conduct and that it has resistance.

So I wondered if something as simple as pencil on paper could be used as a resistor. As it turns out, yes, yes it can.

Conceptual

Conceptual Drawing

Layers 1 and 5 are paper with a lot of pencil lead scraped on. In a 3" pot (so 2x3" sections = 6" long) I used half of a pencil. These are the two resistors that make a voltage divider.

Layers 2 and 4 are thick double sided tape with a recangle x-acto cut out of the center. Leave a nice wide margin or the paper won't spring back into shape after you touch it. These layers insulate everything until you press down in the center.

Layer 3 is tinfoil. This is our "wiper".

Paper Resistor

The paper resistors are very simple in concept. A wire is attached to one end of it, and contact with different points at various distances away causes different resistances. In general, the farther the point of contact is from the wire, the more resistance.

Paper Voltage Divider

When the device is pressed with your finger, contact is made between the 3 plates, top lead, tinfoil, bottom lead. A wire connected to the tinfoil is the output of the voltage divider, the wiper. Contact creates two resistors. The length of the first resistor is equal to the length of whole thing minus the length the second resistor.

The wires connected to the two resistors are connected at opposite ends, so that the total length of the conducting area of the two resistors is always the full length of the device. With this inverse relationship between resistors, we get linear output from the device.

Wikipedia tells us that the equasion for voltage dividers is: Vout = Vin * (r2 / (r1 + r2)). Ignoring Vin for the moment (assume it's 1 volt and so doesn't affect the second part of the equasion), let's look at the values that come out of this inverse relationship:

>>> def f (r1, r2):
... print(str(float(r2) / float(r1+r2)))
>>> f(1000, 0)
0.0
>>> f(900, 100)
0.1
>>> f(800, 200)
0.2
>>> f(700, 300)
0.3
>>> ...
...
>>> f(0, 1000)
1.0


So as long as the two numbers in this equasion add up to the same amount, the output of the pot should be linear.

Materials

• Paper. Strong paper is good but too much glossy coating could stop the pencil lead from sticking. Needs to be flexible enough to bend when you press it.
• Pencil. Use a #2 pencil. Anyone not using a #2 pencil should report to my office immediately.
• Double stick tape, the thick foam kind. Get a nice wide version so the middle can be easily cut. The kind that comes in a roll can be tricky because it's sticky while you're cutting (only one side has the waxed paper strip on it).
• Tinfoil.
• Three (3) wire leads. For connecting it to things.
• Electricians tape. Optional. I used it to seal off the edges and keep the wires from ripping out too easily.

Total materials cost: About 30 cents per unit.

Construction

Cover the two resistor plates entirely with pencil. Give it a nice thick coat, watch for thin areas. Check periodically with an ohmmeter to see if the resistance is consistent. I checked it with the leads about 1" apart in several places to make sure the resistance was consistent. If one area has more resistance than others, add more lead to that area. From end to end, both resistors had a range of about 60k to 500k ohms.

Cut the middle out of the double sided tape. Leave a nice wide margin and a small-ish gutter so that everything will spring back into place when you stop touching it. A gutter that is too wide will result in permanent contact between the layers, it will not have enough tensile strength to spring back to shape.

Strip your three lead wires. Braided copper will work best here, the strands will stick down better than a single solid wire and make contact with more surface area of the resistor.

Connect one wire each to the end of the two resistors. Pinch it in place with your finger and thumb, then seal it with the tape layer.

Connect the third lead to the tinfoil. Fold the edge a bit and tuck the exposed copper wire in there.

At this point, I arranged the wires how I wanted. Even though technically I could have had all the wires poking out near to where they connect, I wanted them to all come out of the same corner.

After you are happy with the positions of all the wires and they seem solid, make a tinfoil sandwich with the two plates, making sure the wires are connected at opposite ends. Make sure the plates are the same size and aligned well. Don't leave any slack in the tinfoil, it should be taut.

Tie the leads together and cover the gaps in the edges with electrical tape.

Trim and tin the exposed ends of the leads.

First Test Pot

Congratulations, you're all done!

This first one I made as a test works ok. I made the gutter too wide (hence the advice above) and it gets bent out of shape a bit easily. Measuring the voltage output at the wiper with 5v input, it ranges from 4v to 5v as I slide my finger up and down it.

Since the resistances are so high, there is not enough current flowing to drive a LED or anything with any appreciable current requirements. The output can be connected to the base of an NPN or mosfet transistor to amplify this signal. Alternatively, an ADC pin of a microcontroller should be able to read values from it.

At first I was somewhat worried about heat as well, lots of resistance on a paper substrate. However, at 5v the device doesn't even get warm when active. I suspect that the tinfoil wiper is actually helping to dissipate heat as well.

Happy home-brew resisting! Enjoy!