Our own [Mike Szczys] recently sat down and put together a great tutorial on building a Larson Scanner. The ubiquitous circuit is usually one of the first few projects on a budding hackers list of things to build, since they are just so darn fun.
Simple versions of the scanner sweep back and forth lighting the LEDs without any sort of transition between them. The configuration most familiar to us all as featured in Knight Rider and Battlestar Galactica are a bit more complex, and have a fading trail of light that follows behind the leading edge of the sweep. [Mike] notes that this fading is traditionally accomplished through the use of capacitors, which cause the light to gradually fade as the animation sweeps across the LED array. He decided to take a different route with his circuit, relying on PWM control of the LEDs instead.
Mike put together a simple circuit using an ATmega168, a handful of resistors, and of course, an array of LEDs. Utilizing interrupts and PWM, he was able to accurately recreate the iconic light sweep without the use of any capacitors. One big benefit to his design aside from the lower component count is the fact that he can easily adjust the speed of the sweep as well as the fading properties with a few small code tweaks.
Be sure to check out his blog at some point, where he shares his code, some circuit diagrams, and plenty more details on how his scanner was built. In the meantime, take a look at the video below to see the result of [Mike’s] work.