linear shift feedback register

2 Articles

LED Christmas Tree Is Perfect Holiday Build

December 24, 2017 by 4 Comments

Soon the most wonderful time of the year will be upon us. Families all over the globe will gather together to exchange gifts, eat good food and enjoy some quality time with each other. For many, it will be the first time they’ve seen each other since the last holiday season. For us hackers –  this translates to a time we get to talk about ourselves and show off a little about what we do. Been taking it easy this year? Have no hacks to talk about? Well, it’s not too late! Break out the soldering iron and whip up the perfect conversation starter – an LED Christmas tree!

[Gumix] took a handful of those flickering LEDs and a step down DC-DC converter to make his simple but elegant tree. No microcontroller here… no code is running. As soon as power is applied, the flickering LEDs do all the work to create a visual delight.

Flickering LEDs have been the focus of a few hackers. They’re basically LEDs designed to flicker like a real candle. [cpldcpu] hooked a scope to one and guessed that a linear shift-register was responsible for the randomness behind the flickering, which would be confirmed several months later.

Be sure to check out [Gumix] LED tree and the video demonstration below.

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Reverse Engineering A Candle Flicker LED

December 16, 2013 by 55 Comments

Candle flicker LEDs are a one part replacement for a real candle. They contain both a yellow LED and a control chip that modulates the light to create a candle effect. [Cpldcpu] took a deep look into reverse engineering one of these LEDs.

To analyze the circuit, which is potted into the LED itself, a shunt sense resistor was connected to the LED. By connecting this resistor to a logic analyzer, the control signal could be observed.

This control signal looked like pulse width modulation, with some randomness to the duty cycle. [Cpldcpu] determined that a linear feedback shift register was most likely used to generate a pseudeorandom bitstream, and some shaping was applied to make the LED look more like a real candle.

It turns out a blinking LED can be quite complex, and this takes a deep look into it by analyzing the signal. [Cpldcpu] took the lessons learned and wrote an implementation of the algorithm for AVR.