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Topic: Troubleshooting Thermistor Readings (Read 656 times) previous topic - next topic



I'm having trouble with my project and I was hoping for some help.  I'm creating a project similar to this one: hruska.us/tempmon/.  There are some differences in my code/wiring for a different wifi shield, lcd, etc and formatting but it's pretty much just the same thing.

The problem I'm facing is that my temperatures are reading out incorrectly.  I've fiddled around a lot with the wiring to see if I was doing something stupid but it appears to me that it's more likely an issue with the voltage.  The current build out is on a breadboard and I'm wondering if it's possible that the breadboard is not isolating the circuits correctly.  I moved the thermistor to a different line and I started getting different readings.  Then I left removed the 5V power and left Pin 9 there and noticed it was still reading voltage.  I'm totally confused by how that can be.  I see this problem when both connected to the USB power, 12VDC power, and both (so I can read the serial port data).

I bought a multimeter after seeing this all so I can test the readings.

So here come the actual questions:
  • Is it possible that the breadboard is "leaking voltage" and causing this problem?
  • Should I be considering switching to 3.3V instead?
  • How would you approach this if you were in my shoes?



Apr 22, 2015, 01:34 pm Last Edit: Apr 22, 2015, 01:48 pm by Wawa
After some googling I found out that this is a food probe.
Do you have the datasheet with the thermistor value at room temperature.
Did you measure that resistance (out of circuit).
Does the value go up or down when you warm it with your hand.
The exact value of the pullup resistor and a stable supply are equally important.
I have seen 22k pullup on a page. A bit high to use without a (100n) cap to ground.



Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?

Can you please post a copy of your sketch, using code tags?
Please use code tags.. See section 7 http://forum.arduino.cc/index.php/topic,148850.0.html

Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....


Thanks for the replies.

I played around a bit and I found out a bit more info so I posted again here. http://forum.arduino.cc/index.php?msg=2198078

I am using a 22k resistor and last night I started playing around with a multimeter to measure the voltage at the circuit.

I have the thermistor datasheet and I confirmed my coefficients are correct.

The problem seems to be the voltage at the circuit. I'm always getting 1023 when I call analogRead() on any analog pin. The voltage is lower on the multimeter reading but not significantly so. It's reading out around 4.93V at the circuit and nearly exactly 5V if I check the source.

The value doesn't move, ie stays at 1023, when I hold it in my hand but I have not tried reading the voltage while doing so. I'll try that when I'm home.

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