Our thermostat has an annoying feature. If you press any button—but particularly the ‘OK’ button you have to press a lot to change anything—it reboots. Worse, it loses everything you’ve just entered. Last night it started doing this without my pressing anything. I first thought a dead battery might be to blame, but even with a steady 3V from my bench power supply it refused to work. I put it aside, and when it came back on ten minutes later, very carefully increased the temperature till it turned on. Here is a dud thermostat:
It looked a nightmare to take apart, but in fact they have helpfully told you where to lever it open:
and indeed it does just pop open, revealing a single board with everything soldered in place, and two battery clips:
…except the two battery clips, which are a push-contact, and have corroded:
Something about these clips was niggling, but I turned the soldering iron on to solder a little bit of wire to each one and have no more problems. It wouldn’t take. Not with a lot of scratching, which will normally make nickel behave itself, nor with a lot of heat and preheating. Bother. Yet it was magnetic: it had to be nickel or steel. And both of those have good to moderate solderability…
Wikipedia, of course, has an article on solderability. And it points out that some steels—like stainless—have poor solderability. That was what had been niggling: the clips are stainless steel (though they’ve corroded all the same). I have no acid flux, and whilst I could buy some tomorrow, it’s a bit much just for this job. So I cleaned both sides up thoroughly, bent them a bit and put it all back together. Whilst we’re at it, here’s a close up of the ICs used (the other side of the board is just the lcd, capacitance switches, and tracks):
I can’t imagine that ‘antenna’ works very well. If you had a large house, this thing would probably need to be in the kitchen. But anyhow, it now works, and doesn’t reboot no matter how hard I press the buttons. Now to see how long it lasts.