Temp Sensors

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wolfpackmars2
 
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Joined: Thu Mar 29, 2012 3:23 pm

Temp Sensors

Post by wolfpackmars2 »

Project overview
I'm attempting to create a temperature logger utilizing up to 10 temperature sensors. I settled on the DS18B20s due to the ease with which many sensors can be used on a single bus. Through my research, I am under the impression that there is a general consensus that these sensors can be difficult to work with and can be problematic. Most of the problems seem to be centered around parasitic power mode. While I couldn't find specific examples of problems (or solutions to those problems), I decided it would be best to use dedicated power as the resources are available and there is no compelling reason not to. I'm using an Uno with a DataLogger shield.

Now, my problems.

I soldered my first sensor directly to the data logger shield. The sensor works, but I noticed it would drift high over time. I started by using the Arduino 5V power source, and after further research, there was information that the built in 5V supply is noisy and can cause problems. So, I looked into using the 3V3 provided by the DataLogger, as this appears to be more stable and has more capacity available. I connected a second DS18B20 to a 20ft Cat3 cable and hooked it up. After about 50 reads, the sensor disappears. It also seemed to read a little high and drift (though not as bad as the on-board sensor). Since I'm not too sure about the authenticity of the sensors (eBay special) :p - I decided to connect my remaining 8 sensors to my breadboard and connect them to the One-Wire network on my DataLogger shield. With these 8 sensors connected, the one sensor on the Cat3 wire is not seen at all. The soldered sensor still drifts high. The 8 sensors in the breadboard seem to provide consistently good readings, well within their margin of error, and the values reported seem to be correct. If these are knockoff chips, then they are pretty darned good knock offs. However, it's odd that my two soldered sensors are behaving erratically while the non-soldered sensors appear to be just fine.

I'm sure I have several problems here. My thoughts:

1) I'm burning up the sensors when soldering. Maybe 650Degrees is too much heat :p This is what I am accustomed to, but maybe for commercial electronics, this is too much. Also, maybe these temp sensors are more sensitive than usual.

2) Not ruling out that they could be non-genuine chips, while pretty good functional replicas, they could be using substandard materials in their manufacturing.

3) 3.3v - while enough to power the DS18B20, is pretty close to the lower limit 3V supply power for these chips. The 20feet cat 3 could provide enough resistance to drop the voltage to just at or below the 3V minimum. Reading the specs on the 3v3 power supply, I'm certain it can handle the load and provide stable current/voltage - but there may still be enough of a voltage drop between the board and the sensor at the end of the wire.

4) Since the one sensor is still drifting, I don't think noise on the power line was the cause of my bad temperature reads. I may go back to the Arduino 5V supply. Unfortunate, because I have that beautiful 3V3 supply which is underutilized...

I think I will try connected a sensor to 20feet of cable without soldering it and see what happens. I will also try lowering the temp on my soldering iron. Anyone have have any additional advice/things to try? Is there a way I can use the 3V3 supply and still have my sensors on 20feet of wire? Is there a way to get a more reliable 5V power supply from the arduino?

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zener
 
Posts: 4567
Joined: Sat Feb 21, 2009 2:38 am

Re: Temp Sensors

Post by zener »

Well I just left you a long answer but the system looged me off and erased everything I wrote. Sorry!

wolfpackmars2
 
Posts: 4
Joined: Thu Mar 29, 2012 3:23 pm

Re: Temp Sensors

Post by wolfpackmars2 »

I really hate when that happens.

I have learned to Ctrl+C certain posts online before submitting. So frustrating I know :p

A for effort tho! :)

wolfpackmars2
 
Posts: 4
Joined: Thu Mar 29, 2012 3:23 pm

Re: Temp Sensors

Post by wolfpackmars2 »

Turns out the problem with the wired sensor was I over crimped the DQ line at the connector, and the wire broke. :p

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