POV Light Wand

There are lots of designs on the web for persistence-of-vision LED displays, some with motors and some hand-operated. I happened to have a lot of green LEDs and some 18-core cable, so I made one up in the form of a hand-held wand. It has 16 LEDs soldered onto some square-pad prototyping board. The LEDs are driven through 220Ω resistors from a pair of 74LS373 latch chips. The resistors (which I got from the Bristol FreeCycle list) are on the upper left in the photo. The two chips underneath the blue wiring on the solderless breadboard are the 74LS373s, and the little green PCB module caries the Atmel ATmega8 microcontroller. The brown connectors on the far left of the breadboard go to the wand itself, via the 18-core cable. There are a few extraneous LEDs and transistors on the far side of the breadboard, where I was experimenting with PWM for controlling the brightness of some blue LEDs.

The software in the Atmel AVR chip is entirely written in assembler. The AVR is, like many microcontrollers, a Harvard Architecture machine, which means that it has separate memories for program and data. The program code is stored into the Flash memory by the AVRISP programming tool, while run-time data is stored into RAM by the processor itself. There's also some EEPROM on the chip, but this little program doesn't use it. The source code is here (ASCII text format, 8kbytes).

I rushed the software a bit to get it ready for a friend's birthday party; note the missing pixel on the bottom row! I fixed it shortly afterwards, and the photos taken at Dorkbot Bristol show the corrected version. Also, note the four unused LEDs at the top of the pattern. I have 16 LEDs, but I'm only using 12 of them for this pattern.