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TOPIC: Wireless Temperature and Humidity Sensor

Re: Wireless Temperature and Humidity Sensor 9 years 8 months ago #15217

  • andyo
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Re: Wireless Temperature and Humidity Sensor 8 years 9 months ago #5542

WOW - thats a GREAT finish!

I'm no expert with charging NiMH batteries - is it safe to float charge 2.4V pack with 4V (well, 3.5V after the diode drop)?

If that is the case - then my world of isolated power supplies just got a whole lot easier! (Also, what is the general rule of thumb with float charging NiMH's? - Vdiff, current etc)

Re: Wireless Temperature and Humidity Sensor 8 years 9 months ago #5543

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Cheers. The solar charging thing was a bit of an experiment for me. The panels are rated at 4V 200mA but I found that they only provide anything like that when they're in direct, strong sunlight.

As far as I could find out from my research, NiMH cells should be okay if they're constantly charged at rate which 1/10th of their capacity (i.e. a 1000mAH battery can safely be charged long-term at 100mA). My cells are 2500mAH so they should be able to handle a constant charge current of 250mA.

My first design used a regulator to limit the charging current but I ommitted that from the final design. With the panels I'm using the cells only see a charging current over 1/10th capacity for a couple of hours per day. On a normal, clear day the panels generate approx. 3.5V so the cells only see around 3V after the diode drop.

As I said - this is a bit of an experiment. The sensor's been out in the garden for a couple of weeks now and so far, so good. The nice thing about getting the PIC to measure and send the solar panel and battery voltages is that I can monitor how the charging is going - looking at how much the battery voltage drops overnight, I reckon I probably could have used only one solar panel as the batteries don't lose much juice and they quickly return to full capacity.

Another side effect of getting the solar panel voltage readings is that they work as a crude "cloud cover" measurement - when you graph the readings over time it's easy to spot the cloudy days when the panels don't generate quite as much juice.

Hope this helps.

Andy.

Re: Wireless Temperature and Humidity Sensor 8 years 9 months ago #5544

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One thing I should mention - the circuit for the power supply shows a 68uH inductor. It probably should more like 22uH but I only had a 68uH one to hand so I thought I'd give it a go and it seems to work okay.

I built the power supply as a separate board (you can see it top-left in the picture of the assembled remote sensor). That was so I could easily change it if I found the charging circuit needed refinement or there were any other problems.

My final goal is to build a remote weather station so this project is a bit of a test-bed for that. Everything might not be absolutely spot-on but hopefully there'll be some lessons learned which I can apply in the future....

Re: Wireless Temperature and Humidity Sensor 8 years 9 months ago #5545

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Hi AndyO,
Just would like to say your project looks great. I am looking at monitoring some internal temp and humidity sensors.
I would love to see your final circuit. I was wondering if you could post it so I could have a look please.

Thanks
Mark

Re: Wireless Temperature and Humidity Sensor 8 years 9 months ago #5546

The circuit used for the remote sensor can be found here: http://digital-diy.com/file-browser/doc ... ensor.html

Hope this helps. Let me know if you need any more info.
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