Low-Tech Touchpad

touchpad

In this project, Peter and I had to combine high and low tech materials.  The inspiration for this project came from the High-Low Tech lab at MIT as well as Squishy Circuits among many others which you can find at the course webpage.

We constructed a low-tech touchpad that you could use to control a mouse, or anything else that could use one or two dimensions of input. Here’s a video showing it in action.

How does it work?

The touchpad has three layers. The top and bottom layer are resistive fabrics, which means they conduct electricity but not quite as easily as copper wire does – so more fabric between two leads increases the resistance between those leads. The middle layer is a conductive fabric, which means it conducts electricity just like a wire – with very little resistance between any two points on the sheet.  Here’s Peter’s concept sketch.

Concept Sketch of the Low-tech touchpadSo how does it produce an (X, Y) point reading?

Circuit diagram of one layer of the touchpad

Circuit diagram of one layer of the touchpad

Take a look at the image above.  It shows the top and middle layer of the touch pad.  The middle layer is connected to ground and remember since its very conductive connecting it to ground makes the whole sheet basically the same voltage.

The top layer is connected as if it were a resistor in a voltage divider. The sweet trick is that second resistor in this faux-divider is dependent on how much material is between the voltage source (top of the sheet) and ground (where you touch the top sheet to the middle sheet.

One technical detail is that the edge of the top sheet which has the power applied to it has conductive thread running through it so that the voltage is the same in the horizontal direction and only changes in the vertical direction.

Hopefully you see where this is going, the bottom layer is exactly the same as the top layer except rotated 90 degrees and bam, the reading from one layer gives you a vertical position and the other layer gives you a horizontal position.

Here’s a few more photos; some of early prototypes.  Thanks for reading!

Earliest Prototype - Graphite on paper with a copper tape layer.

Earliest Prototype – Graphite on paper with a copper tape layer.

Prototype - Conductive paint on normal fabric

Prototype – Conductive paint on normal fabric

Resistive fabric taped to cardboard with conductive fabric in between

Resistive fabric taped to cardboard with conductive fabric in between

Final prototype next to a graphite on paper switch

Final prototype next to a graphite on paper switch

Peter demonstrates the low-tech touchpad

Peter demonstrates the low-tech touchpad

 

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