"LED-Freund" makes use of the persistence of vision effect to write short words or pictures right into the air.
When the button on the side of the device is pressed, the five LEDs on the front will blink in a pre-programmed sequence. When the device is shaken from left to right in wide movements, an image 5 by 40-60 pixels in size appears, thanks to the low temporal resolution of the human eye.
All this is controlled by an ATtiny85, which was programmed with the image before. Simple graphics and words of up to 7 letters length work best. Utilizing a classic tilt sensor the ATtiny works out the direction in which it is going, so that it doesn't write backwards on the way back.