Thermistor (temperature)

A thermistor is a resistor that changes in value according to it’s heat. In actual fact all resistors change in value as they heat up or cool down, but thermistors are manufactured to show a large resistance change.

By placing the thermistor as the top or bottom resistor in a potential divider circuit varying temperatures can be turned into varying voltages which a PICAXE can read.

Thermistor response is not linear, and so the readings will not change in exactly the same way as with a potentiometer. In general there is a larger resistance change at lower temperatures.

Because of this non-linear response and the need to calibrate for a particular sensor chosen it is often easier and more convenient to use a digital temperature sensor such as the DS18B20.

Related Commands:

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Schematic

PCB

Reading a thermistor

The following program will read the thermistor value and display it as the contents of the 'b1' variable.

Code Example:
main:	readadc C.0, b1		; read the value
	debug			; show the value read
	pause 1000		; wait a short while
	goto main		; repeat
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Temperature level indicator

This program will read the thermistor value and, depending on that value, will light one of three LEDs to indicate what range it is in.

Code Example:
main:	readadc C.0, b1		; read the value
	if b1 < 100 then light1 ; range 0-100   = 1
	if b1 < 145 then light2 ; range 100-145 = 2
	if b1 < 175 then light3 ; range 145-175 = 3
	goto main

light1:	high B.1		; switch on  LED 1
	low  B.2		; switch off LED 2
	low  B.3		; switch off LED 3
	goto main		; loop

light2:	low  B.1		; switch off LED 1
	high B.2		; switch on  LED 2
	low  B.3		; switch off LED 3
	goto main		; loop

light3:	low  B.1		; switch off LED 1
	low  B.2		; switch off LED 2
	high B.3		; switch on  LED 3
	goto main		; loop
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Create Module

A thermistor (and 10k pull down resistor) may be connected to the generic terminal block create module.

Bill of Materials

DescriptionCodeQty
Miniature disc thermistor SEN005 1 Buy Now
10k resistor (pack 100) RES10K 1 Buy Now

Simulation

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Comments

  • Posted by romarlo

    Please tell me how can i calibrate the thermistor

  • Posted by earl91

    I saw on Pixsys a ntc thermistor. Is there any difference between a ntc thermistor and an ordinary thermistor?

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