10k thermistor .. reading backwards?

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello guys,

I seem to be stumped a bit on a seemingly simple problem ... my thermistors are reading backwards. For example, if I add heat, the printed value decreases ....

anyone out there have this happen to them?
any suggestions?
thanks in advance!

I've been working with a couple pieces of code .. one real simple for debugging the sensor .. the other I found online and is a bit too long to post but both have the same problem...

int T1 = 0;    
int ledPin = 13;   

      

void setup() {
  pinMode(ledPin, OUTPUT);  
  Serial.begin(9600);
}

void loop() {
  val = analogRead(T1);     
  digitalWrite(ledPin, HIGH);
  delay(val);
  digitalWrite(ledPin,LOW);  
  Serial.println(val,DEC);
  
  }
2

There are both positive and negative temperature coefficient thermistors. You probably have the opposite version of what you expect.

3

A Negative Temperature Coefficient (NTC) Thermistor will decrease in resistance as temperature increases.
A Positive Temperature Coefficient (PTC) Thermistor will increase in resistance as temperature increases.

Also, how do you have this wired up? If it is hooked up like so:

v+
|
thermistor
|
analog pin
|
10K resistor
|
GND

And you have an NTC Thermistor, as the temperature increases, the resistance of the Thermistor will decrease and the voltage will increase.
If in this case you have a PTC Thermistor, the resistance will increase as the temperature does and the voltage will drop.

If the circuit is wired as follows

V+
|
10K resistor
|
analog pin
|
Thermistor
|
GND

The circuit will behave as follows:
NTC: the resistance will drop as temp rises, the voltage will decrease
PTC: the resistance will increase as temp rises, the voltage will increase

From your description, I'm assuming you are using something similar to circuit 1 and a PTC Thermistor
or
a circuit similar to circuit 2 and an NTC Thermistor.