LM35 temp sensor + op amp problem

Thread Starter

Peytonator

Joined Jun 30, 2008
105
Hi,

I've attached a pic showing a basic LM35 temperature sensor + 741 op amp. The op amp is set up as a buffer for now but I will change it to an amplifier once I get the buffer to work.

I put 9V on the input (blue-green wires).. yes, the polarity is correct. I get about 250mV at room temperature on the output of the sensor (middle pin). Yet, on the output of the op amp (pin 6) I get a unchanging 1.92V.

Why is this?

I have tried:
  • changing the op amp
  • changing the sensor
  • changing the breadboard
and I get the same result. Yet, if I put a voltage divider on the input I get the correct identical voltage on the output of the op amp.

Datasheets:
741
LM35

Thanks :)
 

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SgtWookie

Joined Jul 17, 2007
22,210
the 741 is not good for that application.

Use an opamp that is rated for single supply. If you have nothing else available, an LM324 could be used; wire the unused channels as voltage followers (- input to output, + input to gnd).

TL07x, TL08x, LF35x opamps will NOT work for this with just a single supply.
 

Thread Starter

Peytonator

Joined Jun 30, 2008
105
For the sake of future projects, what other amplifier could be used? I know one could possibly use a single MOSFET, but is there something easier? LM324 just seems a waste, since there are 3 unused op amps...
 

wayneh

Joined Sep 9, 2010
16,159
Look at the LM358.
+1
These are everywhere, easy to find and cheap. They're slow, so don't expect to use them for hi-end audio or such, but it's fine for temperature. With the LM35, you'll be able to measure down to almost 0°C, zero volts out, with a single supply. Of course you can go colder with a split supply.
 

bountyhunter

Joined Sep 7, 2009
2,512
+1
These are everywhere, easy to find and cheap. They're slow, so don't expect to use them for hi-end audio or such, but it's fine for temperature. With the LM35, you'll be able to measure down to almost 0°C, zero volts out, with a single supply. Of course you can go colder with a split supply.
Good op amp but input offset error voltage is something like 8 mV which is an error term against the output of the LM35.
 

wayneh

Joined Sep 9, 2010
16,159
Since the OP is playing with an op-amp, I assume he'll figure out an offset and scale configuration - an instrument amp. If he just wants accurate temperature, he could read the LM35 directly with a DMM.
 

Thread Starter

Peytonator

Joined Jun 30, 2008
105
Thanks for the advice everyone :) For now it's not so important to get 100% accuracy... just working on a proof of concept design. Even if there is a significant offset, it'll probably be simple enough to eliminate it in software (assuming it remains constant)...
 
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