Multitouch

My journey to the final project

In the input devices week, I experimented with the step response.

Multitouch Board

To design a multitouch panel, I learned from Matt Blackshaw. Schematic design for 18 resolutions multitouch step-response board:

schematic

Components:

I milled and stuffed the board.

touchBoard18Res

Touchpad with Copper

I drew the touchpad in Illustrator and used vinyl cutter to cut out the copper sticker.

ai

But the outcome was quite rough at the beginning because the blade was almost done.

waste

Here is what I got for the first version.

roughcut

Ideally, three pads connected to the same pin should give different range of values when I touch so that finger can be located precisely. However, they provided similar values. I assume it was caused by noises.

Later, Kris replaced the blade and suggested to replace the cardboard with epoxy layer to cancel some noises.

sandwich finecut

Setting for the vinyl cutter:

But still three pads connected to the same pin gave similar values. Plan B is to make each pin connect to single pad and make 3x3 grids.

18-resolution board can still be used as a test board to send message to the magnet board.

testTouchConnectMag

Test to communicate with the magnet board. Success!

testTouchConnectMag Code

Finally I figuired out the touch board is actually working. It was the way of interaction matters. In order to generate different signals, the pads on both layers needs to be touched at the same time.

ITO film

ITO stands for indium tin oxide. It is transparent and electrically conductive, which fits my project perfectly. Matti helped me order the ITO film.

Next is to test the power and speed value to engrave and cut the ITO.

If the engraving is too light, it won’t cut off the continuity between pads. If the engraving is too much, it will leave visible marks.

ito_engrave_test

Engrave:
Power Speed Thickness Outcome
100 20 1mm too deep
100 60 1mm too deep
100 5 0.05mm too light
100 10 0.1mm OK
100 8 0.1mm too light
Cut
Power Speed Frequency Outcome
100 8 500 x
100 20 500 ok

Here is what I got.

ito_pads

Since ITO is not able to be soldered, I cut out FR-4 board and used clips to clamp them together.

clamp

Test with the board.

I fixed ITO layers on top of a piece of acrylic using tapes as below.

ito_on_acrylic.jpg

Up to this point, i realized that i didn’t have to cut them out. Engraving instead would give same result and easier to bond two layers together.

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