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Sensors, Actuators, and Robotics » Temp sensor LM34

January 10, 2010
by Iflyatwin
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Hi, I'm new to the Nerdkits and I have done and modified the temp sensor project. I got three sensors to work with one MCU and it all works fine but I need to have the sensors in different locations, with one location about 10 feet away and another at 50 feet. When I did that, the degrees f. were jumping from 50 to 600 for both remote sensors but not for the sensor by the mcu. I used shielded wire for this but did not grouded it. Does anyone knows how far the LM34 can be from the MCU, I would really appreciated. Israel

January 12, 2010
by BobaMosfet
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lflyatwin--

Think about reactance and impedance.

January 13, 2010
by mongo
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Try grounding the shield - but only at one end. Preferably at the processor end.

Mongo

January 13, 2010
by mrobbins
(NerdKits Staff)

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Hi Iflyatwin,

Also look at the LM34 datasheet, as on page 6 they talk about driving "heavy capacitive loads" (such as a long cable), and in Figure 4 on page 7, they recommend a particular circuit arrangement of a resistor and capacitor at the sensor end of the cable to help . (I would also recommend experimenting a similar arrangement at the microcontroller end.)

Also, having a bypass capacitor at the sensor end is important to prevent high-frequency noise from coupling from the power supply directly onto the signal you're trying to measure. They recommend a 0.1uF capacitor (just like the one we use with our microcontroller) on their datasheet.

Let us know if that helps!

Mike

January 13, 2010
by Iflyatwin
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BobaMosfet, Mongo and Mike thanks for your replies. I haven't had time to try any of your suggestions, I will try tomorrow and will let you know what happens

January 18, 2010
by Iflyatwin
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Hi guys, i tryed a 2k resister at the sensor end and grounded the shielded cable at the mcu end and it works great on the 10 feet cable. I'm going to try it on the 50 footer next and let you know. Thanks a bunch for your help. You guys are a great help.

January 18, 2010
by NK_EK
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Hi guys,

I did something similar a while back and simply used a piece of Cat 5e UTP cable. Mine was 100% stable and fairly accurate - not sure if 100% as I don't really have something else to compare it with :-(.

Thanks for the other ideas as well, will try those as well.

Cheers

Ernest

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