18B20 and cable length

[Scampy]

FWIW the data sheet says 4k7

George

George, thanks for the advice. I made the mistake and installe dthe wrong value resistors. 4K7 is indeed the correct value, but I had to wait until today to try it out as I was using the habistat controller to run the reptile incubator and had to wait for the eggs to hatch. Now that's over I have changed the resistor for a 4K7 and the sensor is working fine over the cable length and the controller has been running fine all day

[Charlie]

It's great that you've got it going, Malcolm. The datasheet talks about 5K as sort of a maximum value, optimal for a single device but there are all sorts of conditions where you'd want it smaller. If you are running in parasitic mode with a long lead, if you have multiple 1-wire devices on the same bus, if you are running at lower voltages (say 3.3 V), you will find you need to lower that value. The datasheet has an example circuit for effectively lowering the value to near zero while the device takes it's reading. Bottom line is that there isn't a single best value.
So if you have 1 device on the bus, running at 5V, not in parasitic mode, 4.7K is perfect. If you have a different configuration, and you have troubles, try lowering the resistor to 2.2 K. Larger values like 10 K are unlikely to ever work, as you discovered.
There's a lot of discussion online about these devices including a couple application notes that talk about these exceptions.

[Scampy]

Thanks for the additional info Charlie. Yes I'm running at 5v and each sensor is on it's own pin on the PIC.

The controller was installed at the weekend. It sits on a stack of three vivariums, with the fouth being a large 160cm x 50cm x 50xm the other side of the room. The cabling for heater, light and two sensors are routed in trunking over the door and along the skirting board to the unit



I've updated the schematic and PCB drawings to use 4K7 resistors... Where I got 10K value from I don't know !