And … (drum roll, please) … it works.

I put in a couple of batteries, programmed the MCU and turned it on. It works.

I’m always surprised when something I design works on the first try. This being such a simple design, I probably shouldn’t be surprised, though. I should at least give myself a little more credit.

The unpopulated lands on the board in the photo are supposed to be unpopulated. I left a few things out because they aren’t needed for what I’m doing with this piece now and leaving them off keeps the cost down.

So, what did I learn from the process?

• If you have a lot of different parts laying around, it’s pretty easy to grab the wrong one.
• I ran into some variability in the “zero rotation” position in the CAD library land patterns.
• The whole process is pretty easy, but start to finish, there are quiet a few steps.
• It’s a nerve-racking wait after sending off a box of parts.
• Good communications between designer and assembler are very important.
• Clear documentation from the designer is very important.
• This was a WHOLE LOT easier than hand soldering all the SMT parts (I’ve done that before).

That’s a good set of educational results. Next time, I think it will be easier.

Note the large diode polarity indicators on either side of the long row of LEDs and by LED D25. D1, the Schottky on the upper right has the same polarity indicator, but it’s in between the pads, under the part. In case you’re interested, I have a 3V supply. The LEDs drop 1.8V and I’ve got a 150 ohm resistor for each. That gives me a theoretical 8 mA per port for a total maximum of 176 mA with all 22 lit up. That’s within spec in the -40C to 85C temperature range but too much when above 85C. I’m not sticking this in an engine compartment or anything, so no worries there.

Duane Benson
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http://blog.screamingcircuits.com/