Sparkfun Button Pad Matrix, Autotel


Sparkfun Button Pad Matrix

I’ve been using this product for development of my interface. Here some things I’ve learnt about it

Making a velocity sensitive controller

The contact of this product works using conductive silicone. The conductive silicone has the property of changing the resistance when it is under pressure. I measured the range to be between 2 and 0.008 K.ohm. This was tested using the same pcb etch that is used in the original sparkfun board, but in a custom built board that is using tin plated contacts.

The 2 K.ohm resistance happens when just one side is slightly touching the contacts. The 0.008 K.ohms happens when the button is being pressed firmly, but within a normal range. Anyways I think that the relation between resistance and pressure slope will get weaker and weaker. According to my understading, lower resistance values will result in a larger voltage variation.

I have used this effect. In that use case, I was measuring each button behind a multiplexor, and I applied a pull-down resistor of 470 ohms in the common pin. I believe that any pull-down resistor between 1.8 and 400 ohms will result in a pressure sensitive matrix, assuming that the matrix output is collected by an analog pin, of course. I don’t recommend using this setup if you expect to use a digital pin, because slight pressures could result in button detection, leading to bounces and a poor overall interaction.

Importing sparkfun’s eagle pads to Kicad

Using a text editor it is possible to import sparkfun’s routes. This process will be explained at changing the layer of routes on kicad.


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