Reading temperature with Arduino and thermistor - strange behavior

Thread Starter

picotrain

Joined Apr 12, 2013
32
Hi,
I'm not sure if this should be in the Embedded section or Analog and Mixed Signal section. If it's in the wrong sections, mods please move.

I'm reading the temp using a thermistor in a voltage divider. Connect that to an Arduino ADC input and I read the room temp ~61F. All is good.

Then I put the voltage divider output through a unity gain opamp and now the room temp reads ~167F.

This was a previously working circuit that I had put aside during semester. Now semester's over I have returned to it and can't figure out the problem.

I thought maybe the opamp had received a static shock, so I soldered a new one to the breakout board, but the result was the same.

I have disassembled the circuit and reassembled it a few times, tried using a different Arduino, different breadboard, always the same problem.

I have attached a photo of the schematic being used. Where am I going wrong?
 

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MrChips

Joined Oct 2, 2009
23,080
Do you have a voltmeter or DMM of some sort?
Measure the DC voltage at the output of the voltage divider and compare with the output of the opamp. Voltages should be the same.
 

Thread Starter

picotrain

Joined Apr 12, 2013
32
Do you have a voltmeter or DMM of some sort?
Measure the DC voltage at the output of the voltage divider and compare with the output of the opamp. Voltages should be the same.
If I disconnect the opamp from the thermistor, I read 0.28V from the voltage divider.

When I connect the MCP6001R opamp, I've got 1.2V on the non-inverting input and 1.8 on the output. So thats pretty weird because I would expect the input voltage to be the same as before (0.28V)

I tried using a 741 instead, using the standard pinout and get 0.28V at the non inverting input, but get 1.98V at the output.

I've triple checked pinouts and rewired both opamps several times. Can't figure what I'm doing wrong.
 

crutschow

Joined Mar 14, 2008
26,769
A 741 won't work with a single 5V supply.

There must be something wrong with your breadboard connections because the schematic you show using the MCP6001R should work.
Double check all the wiring.
Are you counting the pins on the op amp from the top of the package as shown in Albert's post?
 

Thread Starter

picotrain

Joined Apr 12, 2013
32
A 741 won't work with a single 5V supply.

There must be something wrong with your breadboard connections because the schematic you show using the MCP6001R should work.
Double check all the wiring.
Are you counting the pins on the op amp from the top of the package as shown in Albert's post?
I found the problem. When I changed the MCP6001R on the breakout board, one of the pins on the opamp was slightly bent up and not contacting the pad. Resoldered it and its all happy again!

Wow, that was an adventure!

Thank you everyone for your help.
 
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