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Gadget Freak Case #253: Peltier Thermopile Cloud Sensor

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78RPM
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Platinum
Fascinating. Can improve accuracy
78RPM   4/18/2014 3:18:24 PM
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This is a fascinating project that could be a good science fair experiment measuring the effect on cloud temperature between summer and winter of carbon emissions using long-term readings. One could hook the output to an Arduino or Raspberry Pi and keep a log of readings on an SD card. One could also calibrate an analog panel meter using a custom decal (e.g., DecalPRO) with direct voltage or intensity units.

One problem with using two batteries for positive and negative supplies is that they get imbalanced over time. A better solution would be to use a rail splitter buffer opamp to create a virtual ground exactly half way between positive and negative points.  TI offers a variety of these. The outputs of this, +,-, and VGND provides the supply voltages to your circuit.

armorris
User Rank
Platinum
Re: Fascinating. Can improve accuracy
armorris   4/18/2014 4:54:15 PM
The author did a nice machining job on the mechanical parts, and the documentation was good, but as 78RPM pointed out, the circuit relies too heavily on the positive and the negative voltages being exactly the same. As 78RPM mentioned, a "rail splitter" circuit  would solve that problem. Then, you could just run the circuit on one 9 volt battery. A rail splitter would consist of a second opamp and two identical-value resistors. The output of that additional opamp would be a virtual ground. Even if the two resistors (say 10K) are not identical, their ratio would not change and the error could be reliably nulled out.

You could make those resistors 100K and use an MCP607 ultra-low current dual opamp, which draws less than 25uA per opamp, and make a 9 volt battery last a really long time.

78RPM
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Platinum
Re: Fascinating. Can improve accuracy
78RPM   4/18/2014 9:13:24 PM
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Good idea, armorris. We could use devices using far less current and much lower voltage.  Your comments inspire me to suggest that we could use an FPGA which could draw a couple of nanoamps in sleep mode while providing all the op amps we need and all the peripheral eeprom or SD card data storage we might require when active.  We could store a graph of data over a long time period using very little power.

Astro-Eric
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Blogger
Better at the Mechanical
Astro-Eric   4/19/2014 12:29:06 PM
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Thanks for taking a look at my project and for the comments on the rail splitter.  That's a new device for me.  I knew the electronics were not optimal, but I'm much better at the mechanical end of these projects.  So your comments and ideas are definitely appreciated.

I like the datalogging idea.  I do a lot of deep sky astrophotography.  As you can imagine, the best results are on very transparent nights.  I mainly rely on GOES satellite images, on the water vapor channels.  But that's always up for a little interpretation.  I was looking at this as one of the options for measuring the transparency and logging these, when I do my deep sky imaging.

 

78RPM
User Rank
Platinum
Re: Better at the Mechanical
78RPM   4/19/2014 11:17:25 PM
That's a terrific hobby, Eric.  You must live in an area away from light pollution as I do.  If you want to see an example of a rail splitter used in a power supply, take a look at my Gadget Freak #240

http://www.designnews.com/author.asp?section_id=1362&doc_id=263087

 

Astro-Eric
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Blogger
Re: Better at the Mechanical
Astro-Eric   4/20/2014 10:03:28 AM
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Thanks for the link, 78RPM.  I've been tinkering with electronics for most of my life.  Admittedly, I'm much better at the mechanical side of things.  But how on Earth did I not know about this Rail Splitter/virtual ground technique...???  You have no idea how many bipolar power supplies I've built over the years.  I'll definitely file this one away for future projects.

bobjengr
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Platinum
PELTIER THERMOPILE
bobjengr   4/21/2014 7:59:18 PM
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Eric, this had to be a fascinating project and one that obviously gave a great deal of satisfaction when completed.  I would love to see university classes teach this type of thinking to student AND provide the hands-on experience building the device would provide.  It would make a great semester project if the theory were presented along with the assembly.  Great post. 

mrdon
User Rank
Gold
Re: PELTIER THERMOPILE
mrdon   4/22/2014 9:33:05 AM
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bobjengr

I agree. This cloud sensing device would make a great addition to Earth Science and Physics classes. I like the compactness of the sensor and the fact the design can easily be packaged to accommodate the user's application. This project reminds of the work Forrest Mims is conducting on atmospheric HAZE using homebrew sensing circuits. Great post and project!

armorris
User Rank
Platinum
Re: Better at the Mechanical
armorris   4/22/2014 9:59:00 AM
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I see that 78RPM has used a rail-splitter component. I didn't know they existed. I always used an opamp set at unity gain and connected between two identical resistors connected to the power supply rails.

 

I have been designing electronic circuits all of my life, and even I learned something new from this blog.

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