I got one of those magic wand clocks as a gift last Christmas, the kind that have a wand made from PCB with eight or so LEDs on it that waves back and forth with the LEDs flashing in such a way that you see text “floating in mid air”, a technique called persistence of vision (POV). It was pretty cool, but I was kind of disappointed when I powered it up only to find that it was mis-calibrated so that the left and right strokes of the wand were out of sync rendering the time unreadable. Solution – bypass the internal LED control circuitry and wire in a keyboard for custom message display!
To do this I used a propeller microcontroller (from Parallax) and a few basic tools. All the required circuitry to interface with the LEDs and control the wand movement was already on the clock’s PCB, I just had to run wires from the propeller to the bases of the LED-controlling transistors and run a line from the wand-detector so that I could sync. After about an hour of playing with Spin code (the unique high-level language for the propeller) I managed to get a stable field of pixels out of the hacked clock:
the pixel-field hovering an inch above the table
the pixel field looking straight at it
a view of the whole set-up, running in the Stevens’ Darkness and Death Laboratory
where Kyle and I worked until the early hours trying to the font to be readable
Then it just took a few more hours to design an 8×4 pixel font on graph paper (done by Kyle), enter the hex into a massive table, and program text scrolling into the propeller chip. Adding keyboard support was a no brainer, since the propeller works – just import the keyboard Spin object! Here is the result:
video of “Hello, World!” on the hacked device
You may download and use my code if you wish, from here (you want the .spin files).
This project was quite entertaining! Now the clock sits in my room and acts as a free-for-all message board for anyone who walks in – they just type on the attached keyboard and their message is added to the scroll!
After using the propeller with this project I would very highly recommend anyone with an interest in microcontrollers (or electronics in general) check it out! It is very powerful, well designed, and most importantly, the hobbiest/hacker community surrounding it is extraordinary!
Other POV links of note:
* U.S. Government hack of a similar clock (uses real-time linux distro)
* Spoke POV (for bicycles)
* MAKE magazine’s POV toy (easy)
* POV analog and digital clocks (lots of photos)
* High Resolution POV (impressive)
Hack of the Whenever I Get Around to It 