LM35 Temperature sensor

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M.Yasser
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LM35 Temperature sensor

Post by M.Yasser »

Hi,
I’m trying to connect three LM35 temperature sensors to my USB DrDAQ external ports (Ext1, Ext2 and Ext3). These sensors are powered from +5v out of DrDAQ. The readings of the sensors when connected to DrDAQ are wrong (24 C becomes 120 C), when I disconnect the output of the sensor from DrDAQ the readings are accurate (measured with DVM and compared to medical air temperature sensor).
The problem I think is with the internal 100k pull-up resistor, which is pulling the o/p of the LM35 high. What should I do to get an accurate measurement?
Regards.

ziko
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Re: LM35 Temperature sensor

Post by ziko »

Hi try using the scope input instead of the external input, it should work like this.

Regards
Ziko

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Pete
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Re: LM35 Temperature sensor

Post by Pete »

Alternatively you could build a buffer into your LM35 circuits as in http://www.picotech.com/support/topic7671.html or you could use thermistors (which will create a potential divider with the internal 100K). Alternatively you could use three DrDAQ temperature sensors, http://www.drdaq.com/sensors.html.

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M.Yasser
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Re: LM35 Temperature sensor

Post by M.Yasser »

Helo Ziko, helo Pete,
Thanks for the useful replies.
ziko wrote:try using the scope input instead of the external input, it should work like this.
This is a reasonable solution since the input impedance of the scope is hi. Unfortunately DrDAQ has only one channel and I need to measure three Lm35 simultaneously.
Pete wrote:you could build a buffer into your LM35 circuits as in http://www.picotech.com/support/topic7671.html.
Pete, this is a very nice solution. I saw this post after I created this thread and I liked it.
Here what I’ve done:
I had some telephone junctions lying around, modified them to extend the external ports (I had to grind the RJ11 to fit theFCC68 4/4 connectors).
Image

Image

Image

I used an LM324 (quad OPAMP) which was also in my reach and connected each one of the Lm35 as shown below:
Image
I used 2.2K, 3.3K, 10K resistors for auto scaling. The output of Lm35 is 10mV/C so I used a scaling equation to convert the o/p of the Lm35 from voltage to temperature (i.e. X*100). The o/p of the OP AMP contains an offset of 1mV, in terms of temperature 0.1C. So I added this to the scaling equation to compensate for the difference between the o/p of the sensor and the o/p of the OP AMP, the scaling equation become ((X-1)*100).
Regards.

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