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Topic: 1 wire temp sensors (Read 4233 times) previous topic - next topic

troypearce

Hello all,

Working on my greenhouse controller with 4 1 wire temp sensors in parasitic power.  All were working for several weeks and a few days ago all of the sensors show -196 deg F.  Is this a common sysmptom.  I have them all on the same bus wires with the longest run about 35 feet from the arduino.  Using a 4.7k ohm pullup resistor.

Any Ideas

Thanks

Troy Pearce

ArduKu

what happens if you unplug all the sensors from the board, does it report the same? Sounds like they're not talking to the board and some element of your code is showing them up as -196 deg.

troypearce

hello thanks for the reply,
pulled the bus wire and the output changed to 32deg f  (0c).... I cannot find any math altering the data for temp other than that from the library.   Also this was working O( no changes to software in weeks)

interesting that it changed from -196 to 32 by unhooking the data wire to the bus???

Thanks
Troy

igor86

If all the sensors share the same bus, could it be a bus contingency/timing problem? i.e. two or more sensors are trying to drive the bus at the same time.
HTH
-Igor

Rusty in Texas

Troy,
From the numbers yer supplying I'd say that you have a short to +V.
You are also truncating your temperatures instead of rounding too. :-)
Are you supplying +v to your sensors or using parasitic power and
just supplying ground and data?

    -Rusty-

troypearce

Rusty thanks for replying....
the 4 - 1 wire temp sensors are in parasitic power.....
a short makes since as I have not changed any of the programming and they were working for several weeks.

I am new to electronics and don't have a lot of experiance troubleshooting this.... what would be a good next step?

Thanks

Troy


ArduKu

Hi Troy

I'd recommend perhaps starting afresh with your connections and replacing them one by one to see if there's a problem with a particular connection.

I'm actually midway through a project using the DS18B20 one wire temp sensors, but I'm using a power rail too. I really don't know much about these units yet, but using parasitic power with 4 sensors on the bus and cable elngths up to 35 feet..... I dunno, it seems that may be pushing it in terms of stability.

If you don't want to pull anythign apart, you could add loads of serial output debug code to see at what stage the problem is being introduced.

troypearce

yes I saw a great deal about the stability issues in the forums but figured if it was working let it keep working.  How would I add for a powered setup.... really cant find any pictures or schematics for non parasitic.

do I just add a 5 v power supply to the ground and +5v with a 4.7k (some say smaller with long lengths like1.0k) resister between +5v and signal (is this on every sensor or just at the bus?).  I tried that with my spare 1 wires but could not get it to work.

Thanks

Troy

billcramer07

If you are using parasite power mode the pullup resistor should be 2.2k. Make sure the two outside pins of the 1820s are connected to gnd.

MarkT

Long cable runs and parasite power may be pushing it.  The bus really needs pull ups at both ends if a long run (to suppress ringing and reflections) which really means running Vcc as well, in which case you might as well not use parasite power.  The datasheet I found doesn't seem to have information about cable run length issues alas.
[ I won't respond to messages, use the forum please ]

troypearce

thanks for the help but .... I am not sure what this means..... I think I should change from parasitic to non parasitic but how..... I have a 1 amp 5 volt wall wart.... so do i just wire it into the v and neg part of the buss with a resistor between the signal wire and the 5v wire (at both ends)?   I really want to learn this stuff as well(helps me tackle other problems down the road) any links that may help? 

again any help you can lend is appreciated as I am sorta stuck.... multiple searches have not come up with much about non parasitic power for 1 wire.

Thanks

Troy

billcramer07

If you will send me a private message with your email I will send you all the info you need.

jd.steinberger

I kind of agree with the "short" theory.  I had a similar problem on an engine fuel temperature sensor, that was giving me bizarre values, and it was a short.
Just curious, being that this is in a greenhouse, is your circuitry in an air tight container with one of those dehydrant bags inside?  I used to work in a greenhouse and I know how high the humidity is in there.
I wonder if condensation could have caused the short?
Strange to me that it ran fine for a few weeks.

troypearce

Well, i pulled my buss apart and tested each 1 wire separately.... one of the 4was bad..... put it all back together with a new to replace the bad.... and it works....

What I would like to do is figure out how to run the units in non parasitic power.... i think i may be pushing the limits of parasitic mode.

if you have any links that show pictures or schematics i would appreciate it.... tried to connect in what i thought was logical way with a 5v wall wart.... but did not work.

Thanks

Troy

billcramer07

I just sent you a link thru your email as a starting point. Sorry it took so long.


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