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Author Topic: Your latest purchase (January 27, 2011 to August 19, 2011)  (Read 37904 times)
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Seattle, WA
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The AAG weather station is their upgrade to the original Dallas Semiconductor 1-wire station from the 90s and for $79 you get quite a box-full of stuff. I'm spending the summer at my property near the Gulf coast where strangely enough it is considerably cooler than my home in Dallas. It doesn't have a weather station though so I'm about to fix that. 

Are you blogging your experiences with the weather station?  I'd be interested in reading about your approach.  Are you using the 5 bazillion LEDs in the station to scare/annoy the neighbors?  How does the device compare with the original Dallas model?  Mine has held up quite well to the soggy Seattle weather without any apparent leakage, do you think you'll have to resort to RTV as much as you did with the original? 
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Dallas, Texas
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Are you blogging your experiences with the weather station?  I'd be interested in reading about your approach.

I'll start a new thread on the projects forum.

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Are you using the 5 bazillion LEDs in the station to scare/annoy the neighbors?
This is Texas, so my neighbors are not that close. Still, I turned the darn things off. The cat thought they were pretty cool. smiley

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How does the device compare with the original Dallas model?

Well, it's considerably different. The original didn't have any built-in intelligence for starters. This one appears to have an Atmel processor. The original had magnets and reed switches, where as this one has magnets and hall effects. There was no ambient light sensor or supply voltage monitor on the original and of course, none of those LEDS. The original was pure 1-wire and a lot more complicated to talk to. Better documentation though.

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Mine has held up quite well to the soggy Seattle weather without any apparent leakage, do you think you'll have to resort to RTV as much as you did with the original?

It appears to be a lot tighter than originally. I will probably run a bead of clear RTV around it before I put it on a pole, though. It would be a shame to damage all those LEDS.  smiley-grin
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11050-1017-NDARDUINO UNO BOARD28.05000$28.05
3401-1809-NDSWITCH TACT SHORT MANUAL SEALED0.81000$2.43
1BC2301-NDTHERMISTOR NTC 10K OHM LEADED0.35000$0.35
1SBH341AS-NDHOLDER BATT W/COVR 4AA ON/OFF SW1.39000$1.39
3296-15340-1-NDIC 4BIT 1OF2 MUX/DEMUX 16-SOIC0.66000$1.98
1492-1321-NDLED PURPLE CLEAR 5MM ROUND1.46000$1.46
2490-3772-NDCAP CER 9.1PF 50V C0G RADIAL0.43000$0.86
1ATTINY13A-PU-NDIC MCU AVR 1K FLASH 20MHZ 8PDIP1.13000$1.13
1ATTINY4313-PU-NDIC MCU AVR 4K FLASH 20PDIP2.80000$2.80
252N3904FS-NDIC TRANS NPN SS GP 200MA TO-920.18000$4.50
252N3906FS-NDIC TRANS PNP SS GP 200MA TO-920.19520$4.88
2004.7KQBK-NDRES 4.7K OHM 1/4W 5% CARBON FILM0.02420$4.84
510.0KXBK-NDRES 10.0K OHM 1/4W 1% METAL FILM0.12600$0.63
1DS18B20+-NDIC THERM MICROLAN PROG-RES TO-925.99000$5.99
1DS18S20+CT-NDIC THERM MICROLAN HI-PREC TO-925.14000$5.14
1495-2142-NDTHERMISTOR NTC 10K OHM 1% RAD2.08000$2.08
1615-1090-NDTHERMISTOR ASSEMBLY 10K OHM8.18000$8.18

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Left Coast, CA (USA)
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THERMISTOR ASSEMBLY 10K OHM 8.18000 $8.18

WTF, $8 for a thermistor? is it gold plated? Or come with some calibration papers signed off by A. Einstein himself? Must be the 'assembly' part that drives up the price?

Lefty
 
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THERMISTOR ASSEMBLY 10K OHM 8.18000 $8.18
WTF, $8 for a thermistor? is it gold plated? Or come with some calibration papers signed off by A. Einstein himself? Must be the 'assembly' part that drives up the price?

 smiley-grin  3 inch stainless steel sheath.  At least I hope that's the one I put in the basket.  The ones with a pedigree start about $100 (and, no, I won't be needing one of THOSE).
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My temperature sensor of choice is still the platium 100ohm RTD sensors:

http://www.ebay.com/itm/0-2-Class-Platinum-Resistance-Thermometers-PT100-/120762820978?pt=LH_DefaultDomain_0&hash=item1c1e066972

Use to be somewhat expensive and complex to interface, but with modern analog op-amps it's easy. there are even specific RTD interface IC's that work great, or just use as a variable resistor in a simple Wheatstone bridge circuit.


Lefty
 
« Last Edit: August 19, 2011, 02:58:38 am by retrolefty » Logged

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Use to be somewhat expensive and complex to interface, but with modern analog op-amps it's easy. there are even specific RTD interface IC's that work great, or just use as a variable resistor in a simple Wheatstone bridge circuit.
My curiosity kicked in here as I've considered these platinum resistors tricky to interface.

If you build an interface from discrete parts (such as a Wheatstone-bridge plus op-amp) would you not require a constant current source to always keep excitation at the specified level (say 1mA or so)? Then if the constant current source is used to power the bridge, the variable resistance leg (the one with the RTD) would see a variable current. Likewise if you power the bridge from a constant voltage source, excitation current will still be variable and thus contribute to error (through self-heating).

If we ignore self-heating (assuming this will be insignificant across a modest temperature range) we loose out on range/accuracy which I assume is the motivation for wanting this part in the first place.

Another option might be to use a constant current source with the RTD in a simple resistive circuit, but then we miss out on the common mode rejection we get from the bridge.

Still (assuming we find a solution to the above) we have to build the op-amp interface with its own challenges, feed the output to the microcontroller ADC input and finally work out the math and possibly a challenging calibration scheme to get results near to the RTD’s potential for range and accuracy.

… Or, is there an easy trick to getting this right (other than buying an interface for it)?

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Still (assuming we find a solution to the above) we have to build the op-amp interface with its own challenges, feed the output to the microcontroller ADC input and finally work out the math and possibly a challenging calibration scheme to get results near to the RTD’s potential for range and accuracy.

… Or, is there an easy trick to getting this right (other than buying an interface for it)?

Due to the low impedance value of the standard PT100 RTDs (100 ohms) they almost always need to be wired up in a Wheatstone bridge circuit so as to keep the current level through the RTD at a low enough value so that self-heating does not effect the reading of the true temperature. How much a RTD sensor is effected by self-heating is related to the platinum wire gauge size used in the specific RTD, and the price of platinum being what it is, the standard industrial size RTDs tended to be somewhat pricey, esp compared to thermocouple sensors. So we tended to use TCs for not so demanding process temp points and the RTD sensors for the PID temperature control loops.

 A simple circuit could just drive the top of the bridge with a well regulated constant voltage, say 5vdc. The upper two fixed resistors might be say 1k ohms, and the fixed lower resister would be 100ohms to match the PT100 sensor at 0c temp. So at 0c the current though the RTD would be around 4.5ma and the cross bridge voltage would measure 0vdc differential. At 100c the bridge diff voltage would measure around 161 mv diff (math may be a little off, was rounding off) and the differential input instrument op-amp would have to be set up for a gain of 31 or so to have a measurement voltage output range = 0-5vdc = 0c to 100c.

 We worked with tons of PT100 RTD sensors at the refinery I worked at. The industrial RTD are almost always a 3 wire version where there are two wires attached to one side of the RTD, and sometimes 4 wire versions with two wires on both sides of the RTD wired back to the 'control house' for measurement. That way a constant current could be sent through one of the double wire end and the other wire of the double wire would read a voltage drop proportional to the wire length run from the RTD sensor to the input bridge circuit that could be hundreds or even a thousand feet away, effectively 'compensating' for the resistance of the lead wire. That's why the special RTD interface chips are so useful as some are designed to support 3 and 4 wired RTD to perform the lead wire resistance compensation, as well as measuring the sensors change of resistance with temp.

As a related side note I have been playing around with a 16 bit I2C ADC chip that has programmable gains and can be run in differential input mode. So it can be set to read a 16 bit +/- .256 vdc differential full scale voltage, which in many cases would be able to read a Wheatstone bridge circuit without the need on an external differential op-amp, how cool is that? It's a great little ADC chip:

  http://www.ti.com/lit/ds/symlink/ads1115.pdf

Lefty
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Use to be somewhat expensive and complex to interface, but with modern analog op-amps it's easy. there are even specific RTD interface IC's that work great, or just use as a variable resistor in a simple Wheatstone bridge circuit.

… Or, is there an easy trick to getting this right (other than buying an interface for it)?



I addressed the problem in a self-calibrating pressure transducer I designed with this chip:
http://cds.linear.com/docs/Datasheet/2411f.pdf

If you take a look at the data sheet in the applications section, you'll see an RTD example. This chip has both differential inputs and differential reference inputs. I used a reference voltage chip as the power supply for the part and was able to simplify the design somewhat. The big disadvantage of this part for hobbyists is the package it comes in. You really need a breakout board or you can buy a development kit board but it's $50.
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We have passed 511 replies (2^9-1) so should someone just start a new thread and let this one go?
Something I got for free:

A FORTRAN IV Primer by E. I. Organick published in 1966
I Book on C 4th ed by Kelley and Pohl 1998  <- is this book any good?
Pascal plus Data Structures by Dale and Lilly 1985
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I Book on C 4th ed by Kelley and Pohl 1998  <- is this book any good
Don't know about that one, but their "A Book on C" is excellent.
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We have passed 511 replies (2^9-1) so should someone just start a new thread and let this one go?
heh, why? Will only make it more confusing if someone is missing stuff to do, and want to get ideas. Plus new might not even know that there are multiple threads with same topic.
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We have passed 511 replies (2^9-1) so should someone just start a new thread and let this one go?
heh, why? Will only make it more confusing if someone is missing stuff to do, and want to get ideas. Plus new might not even know that there are multiple threads with same topic.

If you really wanted to do this, the best way would be on the "last post" here a link to the new thread, and as the first post on the new thread, a link to the old thread (whether to the last or first message on the old thread, I don't know). smiley
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Closing this one, new starts here: http://arduino.cc/forum/index.php/topic,69933.0.html
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