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while learning how to blink a led with Arduino (!), i read that if the frequency is too high, you won’t see the blinking anymore, although it’s still there. The way to see it is to move the board very quickly, and due to persistance of vision, the eye can catch a series of dots.
I found this cool and thought of using this property to display slightly more advanced patterns, like geometric shapes and letters. There are many such devices, attached to a motor, that does the job pretty well. But I wanted something very light, that you could take anywhere.
I thought of a stick with a bunch of LEDs that you could shake to display something. One of the challenge was the frequency : to make the word visible, it has to appear every time at the same position. So the idea was to have a sensor detecting if you are moving the stick on the left or on the right, and display the characters in one direction or another. The other challenge was the Arduino board, that is too big to carry and shake. So I went for its mini version, the ATtiny 85, that can be operated with a simple button cell. The problem is that there are only 5 input/output ports on this chip, and I wanted 8 LEDs + 2 input ports for the left/right sensor. So I incorporated a Shift Register chip to take care of the 8 LEDs while using only 3 ports.”

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