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-   -   temperature sensor to 10 bit ADC (http://www.homebrewtalk.com/f170/temperature-sensor-10-bit-adc-210346/)

cheese 12-07-2010 03:28 PM

temperature sensor to 10 bit ADC
 
Hi,

I'm trying to automate my brewery and using this board.

It's usb powered and has a 10 bit adc input among other IO's.


Which temperature sensor do you think will fit this board?


I was thinking of LM35/34 but wasn't sure if the usb voltage supply is stable enough for it.



Thanks,
Royi

Quaffer 12-07-2010 04:33 PM

Quote:

Originally Posted by cheese (Post 2458571)
I was thinking of LM35/34 but wasn't sure if the usb voltage supply is stable enough for it.

That should work. It is not clear, however, what the input voltage range of the ADC is. You may have less than 10 bit resolution for your temperature range.

I would try it with the 5V source from the board. If it is too noisy or have drift you will need to provide a more stable supply. Try doing other things with the computer and see if the temperature reading fluctuates. Generally, computer power supplies (which this is) tend to be rather noisy when used for data acquisition. Since the board receives 5V from the USB port there is no headroom to regulate the 5V output on the board. There appears to be a regulator in the bottom left of the board, but it must be for a lower voltage used internally only. The LM34/35 temp sensors require at least 5V supply.

jkarp 12-07-2010 07:23 PM

LM35 supports down to 4V. In my experience, these aren't that sensitive to Vs. Certainly nothing a cap or two couldn't handle. You could also average out any noisy readings in your code.

Quaffer 12-07-2010 10:20 PM

Quote:

Originally Posted by jkarp (Post 2459281)
LM35 supports down to 4V.

That's weird. I just now realized that LM35 can indeed go as low as 4V while LM34 needs 5V. I guess it has to do with the numerical difference of Fahrenheit vs. Celsius.

cheese 12-08-2010 07:08 AM

Thanks, Guys

I'll try it and update.

National suggests to add capacitor and/or resistor in some cases , Do you think I need that?

cheese 12-13-2010 05:21 AM

I've connected the LM35 to Vdd, Gnd and one of the ADC inputs.
It got very hot in a few secs and then the board automatically turned off.

tried that with 2 sensors, same thing.

Any suggestions?

jkarp 12-13-2010 11:46 AM

Can you sketch out your circuit?

Quaffer 12-13-2010 03:33 PM

Yes, a sketch would really help. In the mean time check that you don't have the VDD and GND pins swapped. That would make it heat up quick.

cheese 12-13-2010 06:57 PM

pretty strait forward ,
I double checked the pins, but now I'm that positive...
http://www.homebrewtalk.com/gallery/...edium/Lm35.JPG

Quaffer 12-14-2010 06:22 PM

Quote:

Originally Posted by cheese (Post 2473862)
pretty strait forward ,
I double checked the pins, but now I'm that positive...
http://www.homebrewtalk.com/gallery/...edium/Lm35.JPG

Your sketch looks OK. It is so simple it is hard to see what can go wrong. Well, there is one thing that still could be wrong, but you already checked the pinout. Do you understand that the pinout drawing is the view from the bottom of the sensor? If you think it is the top view then Vs+ and GND pins will indeed be reversed. Look at the data sheet again, compare to your part. That's the only thing I can think of now.


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