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Topic: Weird reading with LM335 (Read 1 time) previous topic - next topic



I'm building a small system with 2 thermosistor (LM335) and a PC fan.  I'll put 1 thermosistor outside and 1 inside.  When the outside temperature is greater then the inside temp, I'll start the fan.

Everything was ok when I used both LM335 on the breadboard, but since 1 of them is outside (at the end of 10 feets of phone wire), the reading is jumping up and down with no reason.  I was expecting to have a different reading because of the resistance of the wire but I don't understand why the reading are like this (in Celcius) : 16.23, 22.17, 25.12, 18.24, 24.43... when the outside temperature is 22.

Thanks for your help


You probably pick up electrical noise with your cable. You may try a shielded cable or better switch to DS18B20 one wire sensors, they measure the signal locally and transfer the value digitally.


This sensor is very sensitive to the voltage and you may have a voltage drop on the line.

Most calculations assume a steady 5V and the change in voltage is proportional to that full range. If the voltage is 4.5 or 5.5 a very large error can be induced in the calculated value.

Have you checked the voiltage at the remote sensor? You may find adding a small capacitor across the power supply near the remote sensor will help (stores charge and helps to even out V fluctuations)..
Arduino Libraries https://github.com/MajicDesigns?tab=Repositories
Parola for Arduino https://github.com/MajicDesigns/Parola
Arduino++ blog https://arduinoplusplus.wordpress.com


Thanks for the help.  I've tried some of your suggestion but finally went for the DS18B20.  It works on the first try. :smiley-mr-green:

Thanks again.

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