LM135 - Precision Temperature Sensor

Forum 2005-2010 (read only) Hardware Interfacing
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Hi all, I have been trying to play around with a LM135. But i cant seem to get it to work. I have a LM35 and have been using it as a base to comare to.

From what i understood from the data sheet, i connected the "-" to ground, "+" to 5V and the ADJ to an analog ping. before trying to convert anything from kelvin to celsius i tried to just print the output and it doesn't seem right. I just get a constant 3.4 or around about there.

So i thought that i may have wired it up wrong. but just kept getting odd results.

Anyone used one of these? Any tips on what i could be doing wrong?

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i connected the "-" to ground, "+" to 5V and the ADJ to an analog ping.

That's not what the data sheet says.

  • to ground, + to a resistor and the analogue input, other end of resistor to 5V. The adjust can be left floating or connected to the wiper of a 10K pot who's ends are connected across the device.
    Top diagram page 6.
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Thanks for the reply Grumpy_Mike. I think i understand it now.

I threw it all together in fritzing and came up with this. The one on the left is a Calibrated Sensor going to analog ping 0 and the one to the right is a Basic Temperature sensor going to analog pin 1.

I haven't tested this yet, but i do have 2 questions. 1) How do i know what resistor i will need for R1? and 2) if you only need the + and - pins what the hell is the ADJ pin there for? :-/

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  1. How do i know what resistor i will need for R1?

You calculate it. The sensor sucks 10mV per degree K. So you work out what voltage you want to see at a spot temperature. Suppose you want it to read 1V at 0C (that's about 273K)

Therefore the sensor drops 273 * 10 = 2730mV or 2.730mV across it. If you look at the data sheet it says that it operates from 400uA to 5mA. So say you want to run it at 2mA, you want 2mA through a resistor to have a voltage drop of 5 - 2.73 = 2.27V. Ohms law gives you a value of 2.27 / 0.002 = 1135R (ohms). So choose the closest value of 1K.

  1. if you only need the + and - pins what the hell is the ADJ pin there for?

To adjust the absolute value of the voltage out and so calibrate it. Please read the data sheet.

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