Soldering problems

In Newton’s measurement of g, I said “In preparation for this, I had bought a “photo interrupter” from Sparkfun and a breakout board to mount it. (Actually, I ordered 2, which was a good thing, since one of them did not work—Sparkfun is sending me a replacement).”

I think I owe Sparkfun a couple of bucks for that replacement, because I now no longer believe that the part was faulty.

The replacement part arrived yesterday, so this morning I unsoldered the part that I believed was faulty and put in the new one.  Getting the holes clear enough to insert the new part was a bit difficult with just a soldering iron and a solder sucker, but I eventually managed to do it.  Since I already had the good photogate set up for the physics lab, I did not get a chance to test the spare until after lab was over.  It didn’t work either!

Now, I’m willing to believe in one random part failing, but two in a row seemed unlikely.  That lead me to suspect problems with either the soldering or with the breakout board.I had already checked thoroughly for shorts (I always do that before powering up a board), and I knew there were none.

I had noticed when taking pictures of the photogate that the IR LED is clearly visible on the camera’s LCD display (strangely, it comes out looking blue, not red), so I looked at the IR diode through the camera—not lit up!  I double checked with the good part and it lit up very visibly.

I then checked the bad board for open circuits.  I quickly found that the resistor, which should be connected on one side to the ground plane was not connected to the ground pin of the header.  I re-examined all the solder joints, and one of the ones on the resistor looked a little bit less than perfect, so I reflowed the solder joints on the resistor.  Still nothing.

In desperation, I tried reflowing the solder joints on the header, although they all looked good.  Success!  It seems that the solder to the ground pad, though looking like a perfect connection, was not connecting. Now the second photogate is working just as well as the first, and I’m feeling very sheepish about having trusted visual inspection of a solder joint—I should know better than to do that.  I certainly should have done a better job of debugging before complaining to SparkFun, who were very nice about replacing the part, no questions asked.

So what can I do?  I feel I owe Sparkfun for the $1.95 part they sent me, but I’m not sure that the effort to get them the money wouldn’t cost them so much in labor costs for handling something unusual that they would lose money on my attempt to pay them.  About all I can do is encourage others to do business with them, since they seem to have real superb customer service.

If anyone does get Sparkfun’s photogate and breakout board, look at the easy Lego mounting I have in More on pendulums, which was easy to set up and worked very well. And check your solder joints carefully!