Breadboard Bits - Rainbow Sequencer

The rainbow sequencer in action. Kingbright, in their infinate wisdom, decided to use a non-standard pitch on their otherwise wonderful RGB LED, making it impossible to use on breadboard. Impossible, that is, unless you have mountains of minigrabbers.

The rainbow sequencer was an idea inspired by a friend. He had used a timer to sequence an RGB led in a basic count pattern from his HDD activity signal on his computer. I thought it was a good idea, but it could have been so much more.

The idea that came to me was a simple N-hot roll register, driving a set of transistors driving resistors, to make a proper rainbow sequence. Essentially stepping through the visible colors.

Another picture of it. Minigrabbers are also visible from the oscilloscope and clock oscillator, and gator clips are visible from the power supply.

I implemented it as a two-in-six roll register in a 74LS374, and it mostly worked the first time I powered it up, with a sporadic glitch on one transition. After I added a debounce cap, the glitch went away. A perfectly sensible condition actually, considering the problem was a sporadic sag during that particular switching event, which caused the chip to pull against the pair of 1K resistors that are used to cause two registers to power up in a set state.

Adding extra stages to the shift register would improve the smoothness of the fading. A five-in-fifteen would be great, and would only require two 74LS374's.