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Author Topic: Weird reading with LM335  (Read 633 times)
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Hello,

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
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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.
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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)..
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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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