- My barometer was not working.
- We were in the eye of a hurricane.
|Broken Barometer Badness|
The first part of the process was figuring out how the hell I was going to do this. Everything else on the station was working well, so I figured it was just a dead sensor. But exactly what sensor are we talking about? I figured this much out a while ago and documented it in this post.
The part is a Measurement Specialties 5534CM. Applications include "Weather control systems" according to its product page (man, I'd love to be able to control the weather). Here is its product page, here is where you can buy it, and here is some Arduino code to drive it!
The cool thing about this part is that it has a digital interface rather than an analog one I had originally guessed at. It also is factory calibrated. Davis was smart and kept things simple: the amount of A/D they have to do in the console is exactly zero. But there seems to be a price to pay for this simplicity: $27.27 for a single piece, to be exact. Good thing they are cheaper by the 1,000.But there was a problem. This picture was taken in my old console that has since failed me. I got a new one and U84 above is not populated. U85 is.
|What is U85?|
|You can, but...|
|The Price is Definitely Right|
So I had the What part of the equation covered. Equally important is the How. How do you get the old part off and the new one on? Any time I had ever tried to take a multi-leaded part off a board like this with just a soldering iron and desoldering wick (a fine copper braid that soaks up solder like a sponge), it always ended in heartache. You never get all of the solder, and you can't get enough heat around all the part at once to keep the solder molten to have the whole thing come off. In the end, I would end up just wrecking what I was trying to remove.
That's why I got me a hot air rework station. Which one? This one.
Dave Jones Has Many Fine Traits. Brevity is not One of Them
I got the What. I got the How. The Who is me. The Where is here. The When is now. I got a two week Christmas holiday. Time to get busy. Open the case. Set up on something that could take an accidental blast of heat (in this case, my stove), and put some aluminum foil around the surrounding parts to keep the heat off them to prevent their inadvertant removal.
|Preparing for Battle|
The next part of the operation did not go smoothly. At all. The plan was to use my soldering iron to solder the new part into place. I took the new sensor out of its package, applied a bit of flux to the circuit board with a flux pen (always use flux when soldering), and placed the part on the board. The first problem I had was the part did not want to line up well on the board: the part had some solder on each of its eight pads on its underside. The circuit board also had a bit of solder on the eight copper pads receiving the part. Placing the part on the board invariably caused the part to slip out of place. My first attempts to solder the part in place were met with total failure: the part would be misaligned, I'd get a solder bridge, I'd get a missing connection... and all of my attempts had the part seemingly sitting on top of the board rather than being fastened to it.
A good night's sleep made the answer clear. As so many instruction manuals say, installation is the reverse of removal. I was using my soldering iron to attach the new part when I should really have been using the hot air station. I had been leery to try this because I thought the heat might damage the new part. But thinking about it some more, these boards are usually built using some kind of wave soldering machine that basically heats the whole board with all parts attached in an oven brought to temperatures above that of the solder's melting point. That is basically what the hot air station does.
So, I laid down the aluminum foil in the area again and hand placed the part as best I could after having put a little liquid flux in the area. I set the fan speed of the air gun about halfway and again set the temperature to 340°F. I slowly lowered the nozzle of the gun directly over the part, trying hard not to blow the part off to one side.
What happened next was beautiful. The solder around the part melted and the resulting surface tension moved the part perfectly into position. The part also visibly pulled itself toward the circuit board and was no longer just kind of sitting on top of it. Huzzah!
I let the area cool down a bit and then used a Q-tip and some isopropyl alchohol to clean the residual flux away. I put it all together and...
|Victory Is Mine!!!|
And by the way, Merry Christmas and a Happy New Year from all of us here at Mad Scientist Labs.