For this week's assignment I wanted to explore to possibility of creating a cardboard-based potentiometer from scratch.
The premise was simple: have a moving part over a resistive material and by changing the length of the contact area, change the resistance of it.
Top: preliminary design of a linear variable resistor.
Bottom: *cooler circular design. Shaded area is resistive material.
Bottom left: most resistance possible, more material to travel through.
Bottom right: least resistance possible, least material to travel through.
As you can see, the center part would rotate changing the contact area's position within the graphite surface, increasing or decreasing the total material to travel through.
The resistive material need to be flexible and easy to apply to a cardboard surface. Graphite was my first option.
The main issue I found with graphite is that within very short distance, a high resistance is found. I tried different graphite widths, different sketched "wires" and the least resistance I could find in around 20KOhms, and the analog circuit we are building would not work under this current's settings, because:
V = R*I; If Vin = 5V and the LEDv = 2, a V=3 drop will happen at the resistor. At 20KOhm:
V/R = I; 3/20,000 = 0.15mA. The LED barely flashed, and the modulation according to rotated angle was way too small.
I decided to turn this into a boolean ON/OFF switch. With the help of Bare Conductive's electric paint, a laser cutter and some conductive tape, this switch was created:
Key findings:
Cardboard, while great for prototyping, is not always good for electronics contact areas because its very lightweight.
Different graphite types (2, 2H, 2HB) have different resistance.
Electric paint changes resistance as it dries up.
Instead of graphite, some resistive fabrics can be used to achieve what I wanted.