Go Down

Topic: DS18B20 problem bad reading with two or more sensors (Read 1 time) previous topic - next topic

murdocx

Hello,

I have a problem reading temperatures from two or more DS18B20 sensors. Iam using dallastemperature and onewire libraries.
If I connect only one sensor it works fine, but if I connect a second sensor, it is reading about 0.80~0.90 more degrees. And if I connect a third sensor, the first and second now works fine, but third sensor is reading about 0.80~0.90 more degrees.
Separately, all sensors works fine, problem comes when I connect two or more, then the last sensor is getting bad reading.

I expect your help. Thanks

robtillaart

Did you use the necessary resistors? (see datasheet)
What topology are you using?

Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

murdocx

Im using this scheme



I only have a 4k7 resistor, like image shows.

Nick_Pyner

I suppose you are trying to save on wire with that setup. For the sake of the exercise, you might try wiring them more conventionally - 5v to 5v, signal to signal, and gnd to gnd. You still only use 1 4k7.

There may also be a problem of cable length, compounded when you add more DS18B20, or bad home connection. I have DS18B20s on 3m leads, but I have seen them with 5m.

michinyon

It is hard to see how suboptimal wiring would cause a digital sensor to return the wrong result.
The communication would either work,  or not work.  Hard to see how it would tamper
with the result by adding 0.8 degrees to it.

Go Up