18B20 and cable length

[Scampy]

Guys, need some suggestions.

I'm using two sensors in a project, and AM2302 and DS18B20. I made up sets of sensors approx 3m when testing and everything worked well. However I have a need to place one set of sensors on the othre side of the room, with a cable length of around 11m. When wired up the AM2302 sensor works fine, but the DS18B20 isn't read.

Thinking it might of been a bad crip connection on the plug I took one of the working 3m cables and tested that which worked fine, so I knew that both ends (the RJ11 and DS18b20) were fine, so I cut the cable in half and connected the RJ11 to one end of the long cable run and the 18b20 to the other, having used a multi-meter to prove the continuity of the long cable first.

is there any limit on the cable run when using a DS18B20? the pull up resistor on the signal line is 10K, would this need changing, if so higher or lower ?

[MichelJasmin]

I did something like this and it worked. I had a 10µF tantalum capacitor close to the DS18B20. I don't remember exactly but the 10k pull up was close to the sensor.

[Scampy]

Thanks for the advice. I've done some reading and it seems that lowering the resistor value to something like 2K, and placing that near the DS18b20 rather than the PIC should work. I'll try this later as the device is currenty controlling the heater in an incubator, amd don't want to risk the eggs getting too cold whilst I try and mod the PCB etc

[Charlie]

10K is too high. I have successfully run 50' with a 2.2 K driving directly from the pin of the PIC (resistor at the PIC end of the wire). There are also circuits that use transistors to more aggressively pull up for long range or multiple devices on the bus.

[towlerg]

FWIW the data sheet says 4k7

George

[Scampy]

Charlie , George, thanks for the inputs. Once the eggs have hatched (approx 4 weeks time) I'll be able to change the resistor values without any impact. I'll stick 2K2 in to all of the 4 inputs so it won't matter which input I use for the long run sensor

[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