My friend Bear was on the BM cleanup crew this year, and he came upon a seriously tricked-out bike that had been abandoned, locked to a bike rack at a big sound camp. He removed the electronics for me to study. There are 8 strands of analog (not individually-addressed) 12v RGB LEDs, like this one.

So 8 * 3 (RGB) connections (each of which sinks) is a quick tip here. Of course there’s an Arduino.

Then it gets more interesting for me. The Arduino sends a serial signal to a series of 3 74HC595N shift registers, thereby splitting the serial signal into 24 (ahem, 8 * 3) parallel 5v outputs.

But the LEDs are 12v, and will need to sink a lot of current. Here’s where the lost inventor does something I would not have thought of: they used 3 ULN2803AN ICs to sink a lot of current (again 24 signals, but at 12v!), switched by the Arduino at 3.3v.

Note that the ULN2803ANs are stacked, one on top of another, putting them in parallel so each output can sink up to 1A. Good thinking, and careful soldering.

There’s a lot for me to learn with this teardown. Using the ULN2803AN as an array of Darlington transistors is pretty cool. I’ve used single Darlingtons before, and it seems that arrays of transistors and resistors is something I’m learning about right now (more on this in a later post).

Even more, the inventor has srsly used perf board before. They organized the chips and the headers to minimize the wiring in smart ways that I’m going to keep studying.

It sure is great to have a lab to study stuff like this! Thx to Bear for spotting this in the dust and schlepping it home for me.