Thursday, Feb 07, 2013

Hotrodding a ShiftBrite

I needed a way to drive a number of color LEDs for my Ambarometer project, so I turned to Garrett Mace's Shiftbrites, which are very capable color LED drivers. Each ShiftBright is addressable, and allows 30-bit color control of its single Piranha RGB LED. Since I'm a fan of diffuse lighting, I wanted to drive several LEDs from the same ShiftBright to spread out the light a little.

It turns out this is an easy modification. Because the ShiftBrite's driver provides a constant current sink, it will drive a several LEDs in series, provided the supply voltage does not exceed the Allegro A6281 specs.

LED strips are a cheap and widely available source of RGB LEDs. Most have LEDs in a serial-parallel arrangement, where every group of 3 LEDs are in series and share current limiting resistors. The strips are also designed to be trimmed on 3-LED boundaries, which is super-handy if you want a cheap source of bright, series-connected LEDs.

Both the RGB LED strip and the ShiftBrite LED are common-cathode: each of the three colors share a common positive terminal. This is perfect: you can hook up a 3-LED segment of RGB LED strip directly in place of the ShiftBrite's single LED. Because of the constant-current driver, you can even keep the current limiting resistors and it will still work perfectly. (You can also short them out if you are like me and feel that wasting power in the dropping resistors is suboptimal.) Though you might be tempted to increase the length of LED strip, consider that all the LEDS will have to share the fixed drive current. So if you hook up, say, a 6-LED string, each one will get only half the current and will be only half as bright.

This photo shows the very straightforward wiring: the common cathode (+12V) is orange, the B(lue) LED anode is on the blue wire, the R(ed) is on the green wire (sorry!) and the G(reen) (LED pin 1) is connected with the yellow wire. The ShiftBrite is oriented such that LED pin 2, the common cathode is at the bottom left. (Here's a higher resolution image if you want to see more detail.)

ShiftBrite hotrodded to drive LED strip

The hardest part of this mod was unsoldering the Piranha LED as it's designed to dissipate heat, plus the copper traces on the ShiftBrite board are very small. But I managed to retrofit eight of them for the Ambarometer . Maybe we can convince Garrett (@macegr) to add a 4-pin header to his next version! (The ShiftBrights shown in this picture are the original V1.0 which look a little different from the updated version 2.0 now available.)

Update: These drivers may be, sadly, obsolete now that WS2812 ("neopixel") and APA105 LEDs are cheap and widely available, but you can't hook them up to strip like the above

Tagged: HW hacks LEDs



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