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Problem with reading the LM355z temperature sensor

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woody1189

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Is anyone familiar with the LM335z Temperature sensor? I am having trouble getting my Handyboard to interpret the readings correctly. I'm not sure if the schematic online is correct and could use any help whatsoever.

Schematic-http://makezine.com/images/07/sensor.gif
U1 =Op-amp chip OPA 4241PA
 

blueroomelectronics

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The LM335 is a Kelvin Temperature sensor, 0.01v per degree Kelvin so expect just under 3V for room temperature. I don't know what a Handyboard is so I can't help you with that.

I've hooked the LM35 directly to a PIC A/D pin and they read fine.
 

woody1189

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Re: NEED HELP

the handyboard is a microcontroller designed by M.I.T. , would you say the lm35 is more accurate than the lm335 or will i not notice a difference?
 

blueroomelectronics

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It's pretty much the same part, One is Kelvin the other Centigrade. There are quite a few varieties the LM34 is Fahrenheit.
I've used an LM35 (or LM34 will work too) on my newest kit prototype. Funny it's also called a Cricket.
 

woody1189

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Re: NEED HELP

how should i go about connecting the lm35 to a microprocessor's analog input?
 

blueroomelectronics

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Directly to an analog I/O pin, RA0 is often the first analog pin on many PICs.
If you need accuracy you'll need to provide a reference voltage. I've used the TL431 (a 2.5v shunt reference) on RA3 (the VREF+ pin) The TL431 should have a 2.7K resistor between +5 and it's K and ADJ pins (the Anode goes to ground).
With it hooked up and configured properly the PICs A/D will have a 10bit (1024 steps) conversion between gnd (0V) and VREF+ (2.5V). The LM35 (0C is the lowest value it can read in this mode) will output at 23C (0.23V) and the A/D conversion result will be 0.23/(2.5/1024) = 94 (it chops off the remainder)
You'll need to multiply this result by 4.096 to get the value in C
I find it's easier to stick with integer math and do this in my kit.
94 * 125 / 512 (512 is extremely easy to divide on a computer (throw away the LSB and shift right))
A slightly more elegant approach is to round up the result using the LSB. I'll be posting project details and schematic on my site soon.

PS here's a schematic of a typical weather station with a PIC.
http://www.circuitcellar.com/library/print/1101/Wasinger136/f1.htm
 

jayavardhan

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your circuit is perfect. To get better output dont use those may channels of opamp. Even with Single channel u can achieve it.

Try with only one channel and with unity feedback.

u should connect the i/p resistors to the opamp.

no problem if u use LM335 or LM35 or any other try with this u will suceed.
 

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