Christmas Lights - SMT Soldering

Last night was a night of working on small things: my wife continued her efforts on a counted cross-stitch stocking for me and I soldered some very small transistors to break-out boards.

Specifically, I soldered two IRLML2502s to a 10-pin SOT23 break-out board. I knew these transistors were small and I had figured out a way to squeeze two of them onto the break-out boards. It took me about an hour to make five the modules (one of which you see below) and I haven't electrically verified that all the connections are made properly so there may be a little bit of rework.

So, yeah, small. The horizontal dimension of one of the transistors is between 2.67 and 3.05 mm, the vertical, counting the leads, is between 2.1 and 2.5 mm.

My original plan was to solder these directly to 0.1" perf board that makes up most of the custom Arduino shield I'm making. I think that would still work with a little bit of fudging but when I realized I could get two transistors on one break-out board, I decided the extra hassle was worth having the transistors easily replaceable.

Because, oddly, there is no nominal current rating for these transistors, only an absolute maximum rating (4.2A continuous at 25'C, 3.4A at 70'C). I won't be needing more than 1A per channel so I feel safe using them but it feels a little bit like dangerous or rebellious engineering to use them not knowing what a normal current rating is. How lucky do you feel? How hot will they get? Will it work or will it end in the magic smoke being released? Having them on a module means I can easily replace a pair if something goes wrong.

You might notice that the second pin in from the upper right is missing. This is intentional and a result of my experiences on other projects. One of the most common problems I face with symmetrical connections is trying to figure out which way I should plug things in. Plugging in the wrong way can blow things up but I rarely want to take the time to go back to a schematic to figureit out. By leaving the pin empty I can put a dab of hot glue over the corresponding hole in the connector and effectively prevent myself from being able to plug these in backwards.

Oh, and in case you missed the title, I'm working on some decorative LED Christmas lights using the ever popular RGB LED strips. More to come, hopefully very soon.