Test and Measurement Forum

LM35 Temperature sensor

Post any suggestions about sensors to be used with the DrDAQ here

LM35 Temperature sensor

Postby M.Yasser » Sun Aug 19, 2012 6:14 am

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.
User avatar
M.Yasser
Newbie
 
Posts: 0
Joined: Sat Oct 15, 2011 7:01 pm

Re: LM35 Temperature sensor

Postby ziko » Tue Aug 21, 2012 10:06 am

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

Regards
Ziko

Technical Specialist
ziko
Zen Master
Zen Master
 
Posts: 1705
Joined: Fri Dec 01, 2006 10:03 am
Location: St Neots

Re: LM35 Temperature sensor

Postby Pete » Wed Aug 22, 2012 9:14 am

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.
Pete
Site Admin
Site Admin
 
Posts: 12
Joined: Tue Feb 08, 2011 4:04 pm

Re: LM35 Temperature sensor

Postby M.Yasser » Thu Aug 23, 2012 8:51 am

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.
User avatar
M.Yasser
Newbie
 
Posts: 0
Joined: Sat Oct 15, 2011 7:01 pm


Return to Sensors

Who is online

Users browsing this forum: No registered users and 0 guests