Josh Pieper - Microcontroller Projects

In February of 2007 I spent a little while prototyping a board that turns any bicycle wheel into a moving display billboard using a single string of LEDs. The idea was to do something that would be very visible, turn itself on and off automatically, have enough processing power to dynamically generate bike computer style displays, and have enough memory to store full resolution bitmaps.

The first revision was a hand built board using a single ATTiny26 micro, 14 LEDs, a resistor array, and a magnetic reed switch.

Next I drew up a board using PCB and had it printed. The printed board differed in a couple of ways from the original hand-built prototype:

• More LEDs: 48 LEDs total, 24 on each side of the board. This gives higher resolution patterns, and they can be seen from both sides of the bike. The LEDs are controlled from a series of 74x373 ICs connected to an 8 bit bus on the microcontroller. As opposed to shift registers, all 48 LEDs can be updated in 20-30 microcontroller cycles, leaving plenty of time to spare to figure out what should go there.
• No Resistors: Turns out the I/V curve of the particular 74x family IC I had maxed out at around 20mA with two fully charged AA or AAA batteries with no series resistor. Yeah!
• Bigger Micro: While the ATTiny26 is very versatile, the ATTiny861 is a strict superset including 8k of program memory. All the better to store full resolution bitmap patterns in.

And some pictures of the final product: (some have been flipped to account for my lack of a proper bicycle stand)