Duane Benson

Avoid the trap: confirm the polarity on LEDs.

PCB designers face a lot of challenges, but sometimes the simplest things cause the most common fails. I was caught by one of my own favorite “simple” traps last week: the dreaded LED footprint mess.

I designed a board based on the Microchip PIC32 that has a number of RGB LEDs on it; it’s a ChipKIT Arduino-compatible board. I used RGB LED part number LTST-C19HE1WT from Lite-On. The datasheet is easy to find, and the footprint information is right up front, just the way we like it.

Almost all is well, but I somehow missed taking my own advice, and I didn’t double-check the footprint. The footprint I used is more or less 180˚ off from this one (FIGURE 1). The common anode is still on pin 4, but the numbering is different. It’s got pin one in the same place; then pin two is in the lower left. Pin 3 is on the same place, and pin 4 is on the upper right. That’s the conventional pin numbering order.

Figure 1. Never assume the pin numbering per the datasheet is correct.

Fortunately, the fix won’t require any mod wires. If I rotate the LEDs 180˚, the anode will be in the right spot. All I’ll need to do is adjust my software for the correct R, G and B pin locations.

Why waste parts? We love parts on reels. Who doesn’t? But reels aren’t always practical, and it’s not just about cost. Cost is, of course, important, but there may be other factors to consider.

Say, for example, you need 18 180Ω, 5% 0805 resistors for the LTST-C19HE1WT LEDs. You could buy a small strip of 25 from a distributor for $0.32. That gives the 18 you need, plus a few spares just in case.

Alternately, you could buy a custom quantity reel. On the reel, you’ll probably want more parts to keep the strip long enough for the feeder. Let’s go with 250 parts for $1.39. The distributor might charge around $7 extra to create a custom reel, so that’s a total of $8.39. Still peanuts.

For a third choice, you could buy a full reel of 5,000 for $10.64. Still peanuts. If you’re going to need the same part for a lot of designs, this might make sense. But, there’s more than just cost to consider. You need to store and ship it. Shipping two dozen reels gets expensive. Storing and inventorying several dozen reels can become a hassle too.

The beauty of DigiKey, Mouser and other places that sell cut strips is they essentially become your parts warehouse. You pay the 32 cents and never have to worry about whether the part is in your inventory, how many are in inventory, digging it out of wherever you stuffed the reel when you last needed it, and so on.

If you do buy and store the whole reel, ask first whether to ship it to the EMS. You might be able to cut a strip with the number needed, plus about 10% for that “just in case” (50% extra for tiny 0201 parts).

Duane Benson is marketing manager at Screaming Circuits (screamingcircuits.com); This email address is being protected from spambots. You need JavaScript enabled to view it..

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