Stan’s Obligatory Blog

Happy Thanksgiving

2/7/2014

More electronics hacking

Filed under: — stan @ 7:06 pm

I recently found out I had some stupendous number of ‘reward points’ on my credit card. I had a look at what I could get with them, and I got this. It’s a little weather station, complete with an outside temperature sensor. So I set it up, and it works well. The only downside turned out to be that the outside sensor eats batteries. A pair of good AA batteries would only last about two weeks in it, which seems pretty pathetic for something that you’re supposed to be able to just set up and leave, like a clock. So I started thinking about what I could do about that. Since I’ve done things like this before, I figured I could rig something up.

One possibility might be to just put more or bigger batteries in it, so that there’s just more power behind it, and they wouldn’t have to be changed as often. But then I remembered that I’d found the remains of a little solar-powered flashing warning light after the wind storm in 2011, and it had two AA-sized batteries and a little solar panel to recharge them. So I thought I might be able to rig something up to have those batteries run the outside sensor, and set up the solar panel to recharge them every day.

The instructions for the weather station unit say specifically not to use rechargeable batteries in it. I think that’s probably because it’s a relatively high-current use, and rechargeables just don’t hold as much as regular batteries. But I did some tests, and I found that the unit could run just fine on a pair of NiMH rechargeable batteries. It just wouldn’t last very long. So it was time to rig up a charger.

I had to make some little wooden fake batteries to go in the battery compartment of the sensor, since it seemed that it would not run if I just hooked wires up to the battery compartment contacts. It seems to want some physical pressure on the contacts to work. So I made some fake batteries and I just ran wires out of it to an external battery pack.

Once that was working, I got the little solar panel and a diode and hooked the whole thing up with alligator clips, and I set it up on the window sill. I got it to work there, and even inside the window, I got a good 3.6V off the solar panel. So it charged the batteries whenever the sun was shining. I left it running like that for about a week. After that, I moved it all outside and set it up there with the batteries and and wires inside a cardboard box, and the solar panel on top. It ran fine for a week, so then I made it slightly more permanent. I put the sensor back on its bracket under the garage roof overhang, and mounted the external battery pack underneath it. I ran a wire across to the south side of the overhang on the garage roof, and I put the solar panel there. And what do you know? It works.

It’s been running on the NiMH rechargeables and solar panel for over a month now, and it’s worked flawlessly. On the other hand, I noticed something else annoying. I’d thought that the indoor unit was just a passive receiver for the signal from the outside sensor, but apparently they must talk back and forth. And because of this, the inside unit eats a pair of batteries about every 2-3 weeks, too. This is annoying, both because of the expense of the batteries, and also the fact that both units need to be powered off and reset to change the batteries, even if only one is being changed. It’s a Pain in the Ass, and it also will cost on the order of $20 a year for batteries alone for just the inside unit. So I went looking and found a little 3V plug-in power supply for $7.99 online. I don’t know what it’s supposed to power, but that doesn’t matter. I just clipped off the plug at the end of the wire, soldered some wires onto the contacts inside the battery compartment, and hooked it all together. And it works.

So now I’m set. The inside unit runs off regular house current, and the outside unit is running on batteries, but they are getting recharged by the sun every day. And it was a fun little project.

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