Persistence of Vision
Persistence of Vision refers to an effect by which a moving lightsource remains visible to the eye for a short timespan despite the fact that it has already moved to a different location. This effect can be used to create a device which seemingly displays a text in the air while being moved.
This handheld device makes use of the effect to display text.
I ran into Ladyadas interesting website the other day (see here), attracted by several projects published within. One of those was a rather simple to build portable POV device, based on an AVR processor chip. I had in mind an application for it which would be to hail a taxi in Singapore.
I have therefore taken the original hardware and schematics of the project (see here) but have modified it slightly by adding one additional switch between pin 11 of the AVR chip and ground and one 10K resistor between the same pin 11 and pin 20. Then I have written a different program to run the device which allows me to have 8 different texts and permits the usage of bold and inverse attributes.
The 8 possible texts can be configured in the source code in this section:
;The following are the 8 texts (0 terminated) that can be selected
;by push button.Fonts can be controlled by adding control characters
;A 0xF1 character activates bold font, 0xF2 inverted font, 0xF3
;bold and inverted and 0xF0 returns to regular font.text1:
.db "TAXI needed!",0text2:
.db "5$ EXTRA",0text3:
.. etc ...You might get a sense how difficult it can get to hail a taxi over here! ;-)
The code can then be uploaded to the chip using the programming hardware mentioned in the original website (or through the STK500 board).
As can be seen from the source code snippet, adding control characters into the text can be used to trigger bold and inverted fonts.
Then, the desired text can easily be selectected by powering the device on while pressing the button at the same time. The messages can be chosen by further presses of the button while watching the LEDs. The topmost LED corresponds to message 8 while the one at the bottom corresponds to message 1.
How to download?
Documentation, source code and binaries have been published under the GNU GPL