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.
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
Share