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I am using 2 DS18B20 temperature sensors connected to my raspi. They are connected to a 5 meter long cable together with a DHT11 humidity sensor, do they share plus and minus. The problem I have is both the sensors are returning really high values above 80°C. I heard they return 85°C when they fail, but that seems not to be the case here as they return like 83.6 for example. Is anyone experiencing similar problems?

Also the DHT11 sensors reports >90% humidity, could that be the problem?

EDIT: the cable I have has 4 wires. 2 are used for power and get shared between the 3 sensors. The 2 wires left are for the dht11 and the 1-wire interface. Also this does not happen all the time. Most of the time it worls great. I have another ds18b20 near the raspberry inside the case on a short cable which works without problems.

qwertz
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  • The DS18B20 are 1-wire sensors, the DHT11 is not 1-wire (it has a proprietary protocol). What connections have you made between these sensors and the Pi? Do the sensors work properly when on a shorter cable? – joan Jul 08 '15 at 21:36
  • @joan I edited the question – qwertz Jul 08 '15 at 21:39
  • I'm not convinced there can be a definitive answer to your question. I'd discount the DHT11 humidity as being a problem. You might have to experiment trying different pull-up resistor values for the DS18B20s. Perhaps try 4k or 3k3 instead of 4k7. – joan Jul 08 '15 at 21:59
  • I am already using a 2k5 and it seemed to work fine, till 2 days ago. – qwertz Jul 09 '15 at 07:51
  • 2k5 seems low. Some people put 4k7s on either end. Frankly it sounds like you have a signal conditioning problem, to do with the length of your cable, it's type, shielded, non-shielded, its capacitance, etc. etc. You might be better off asking on electronics stack exchange as I suspect the Pi is irrelevant to the question. – joan Jul 09 '15 at 08:09
  • @joan I read the longer the cable is the lower the resistor has to be. 2k5 seemed to be the sweetspot for me as they worked fine till some days ago. The cable is just a normal 4 pin cable. – qwertz Jul 09 '15 at 09:16
  • I don't understand electronics, I'm just going on what I've seen posted. – joan Jul 09 '15 at 09:31

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