Sunday, August 28, 2011

DIY Touch pannel

Here you can see an experimental device I made some time ago. It's for testing a home made 1-dimensional touch input device. The panel is made from an antistatic bag, some ribbon for a nice touch surface, wood and a piece of wire.

The "slider-contact" is connected to an ADC able io-port of the microcontroller with an external 2 MOhm pullup (must be much higher than the stripe resistance to keep linearity). At first both sides of the resistive stripe are pulled to low. The voltage level at the "slider-contact" indicates if the touch pad is touched (lo) or not (hi). When touched-state is indicated one end of the stripe is pulled to hi-level. Now the ADC can measure a position proportional voltage like on an ordinary potentiometer.

The metalized bags seems to have an isolating layer on both sides of the conductive layer, so they are not usable for that application (wikipedia: "forming a Faraday cage around the item"). I used the type of bag you can see here. There are only the black stripes (obviously carbon powder with binder) conductive, so you can only make use of it for 1D and not 2D panels.

It is to note that there is an constancy problem with the carbon/binder-layer. It is water resistant but gets solved in no-polar solvents, even fat from finger that seems to diffuse through the polymer layer and soak the carbon/binder. Maybe an additionally protection layer that is les permeable can fix that issue.