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I recently bought a LM35 Linear Temperature Sensor off littlebirdelectronics.com (as well as other Arduino stuff) and everything all worked fine when I had the temperature sensor attached up to the Arduino using just short wires (10cms). It was reporting a stable 20 degrees and was nice and happy.

However after I setup the system fully, the LM35 was connected to the exact same PINs but using a 4 metre shielded cable. Using the same code, same PINs and everything the reported temperature now jumps all over the place. It will be -31, 30, 1, 15 degrees all within the space of 10 seconds!

I'm assuming the problem is that the 5V power line is interfering with the analogue data line, thus messing with the results.

My question is, am I doing something wrong, or is this the accepted performance of the LM35 temp sensor? (ie it will only work at small distances)

Also, if I purchase this 18B20 Temperature Sensor instead, will I end up with the same results or should the fact that it's digital allow it to transmit over longer distances?

Thanks!

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    \$\begingroup\$ How do you know it's the 5V power line? Have you put a scope on it and seen noise? In my limited experience I have had issues with small cables running long distances, either because of the voltage drop or interference. Sometimes the shield is grounded and sometimes it's not. I don't have any expertise in this area, but I'm sure someone else here will have tips for you. \$\endgroup\$ – Dave Jun 21 '11 at 2:59
  • \$\begingroup\$ Well I don't know it's due to interference, that's why I'm asking here to see if this is the case or if it's due to something else. All I know is it worked with wires of 10cm and now goes mental on a wire of 4 metres :( \$\endgroup\$ – TTT Jun 21 '11 at 4:45
  • \$\begingroup\$ Try a different cable, or different lengths, just to see what happens. Is your 4m cable homemade or did it come from a cable manufacturer? \$\endgroup\$ – Dave Jun 21 '11 at 5:14
  • \$\begingroup\$ It came from Jaycar. It's a four wire cable with each individual wire having copper shielding wrapped around it, then all four wires wrapped in plastic. Plus all 4 wires are twisted around each other as well. I can't think of any way to make it less susceptible to interference! \$\endgroup\$ – TTT Jun 21 '11 at 5:24
  • \$\begingroup\$ Are you using LM35 or LM25??? \$\endgroup\$ – Johan Jun 21 '11 at 9:51
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Have you 'scoped the output? The LM35 is susceptible to EMI (which you have addressed with shielded cable) and load capacitance - which may be an issue. If there is instability due to load capacitance (>50pF) you can isolate this capacitance with a series resistor. If external EMI is still an issue, an RC damper can be added. Both effects are addressed on p7 (figs 3 & 4) of the datasheeet.

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  • \$\begingroup\$ RC damper did the trick for me. \$\endgroup\$ – Nikola Malešević Oct 9 '17 at 0:13
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Make sure that you have the 5V out at the sensor so they have not dropped to 3-4V on the way.

Then make sure that you have the 1uF cap out with the sensor. Fig 4 in the datasheet, when I used that last time I ignored the resistor but do some tests and see if you need it.

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Another point is that the LM35 needs a cap accross its power leads at the end of the 4 meter cable. 8 meters of cable in series with the power supply will add some impedance, which could cause the LM35 to do strange things, including oscillate.

Put a 1uF or so ceramic cap as close as possible accross the power leads at the LM35 and see how that changes things.

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The LM35 can drive a maximum of 50 pF output capacitance. Your cable is probably giving you 50-80 pF / foot capacitance. This will not work.

As suggested you can try to isolate it with a series resistor, but this will reduce your accuracy as it will be more susceptible to noise and external interference.

A better method is to use a 4-20 mA current signalling for remote transducers. All you need at the other end is a precision 250 ohm resistor to convert to 1-5 Vdc.

you might be able to find an IC that does this otherwise you could probably achieve the same result using the LM35 and an opamp and few extra components.

Using a 10 mV signal over four meters will never work!!! no matter how much you shield it :)

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