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There are many very good instructables on how to make interactive coffee tables with LED matrices, and I took inspiration and hints from some of them. This one is simple, cheap and most of all it is meant to stimulate creativity: with just two buttons, you can create animations on it!

We had an IKEA LACK table with a dent, bought a new one, leaving the old one up for reuse in a project. The top is 55x55x5cm, and it is hollow, with just a thin board on the top and bottom that is easy to cut through with a box cutter. The sides are more sturdy, made of ~1cm hardboard. It is filled with cardboard in a honeycomb structure, which is easily removed.

The local hardware store has 50x50cm plexiglass in various thicknesses, colors and transparency. The 4mm white-opaque is transparent enough, and the price reasonable (4.50EUR - still the biggest expense of the project!).

Instead of individually addressable LEDs, I use the readily available MAX7219 chip. The maximum output current according to the datasheet is 320mA total, so 5mA per LED. A bit below the nominal 20mA for 5mm white LEDs, but bright enough for this purpose.

Using only 2 buttons the user can make or modify animations. The Arduino has 1kB of EPROM memory, so that fits 128 images of 8x8 bits. Enough space to store 15 animations of 1-15 frames each.”

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