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Forum Index : Microcontroller and PC projects : Where should the pullup resistors be?

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Paul_L
Guru

Joined: 03/03/2016
Location: United States
Posts: 769
Posted: 05:13pm 14 Dec 2016
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I will be using 14 DS18B20 temperature sensors and 2 hall effect water flow sensors as data sources for my geothermal heat pump controller. The sensors will be up to 80 feet from the E100 board. The sensor wiring will go through breakout boards about 15 feet from the E100 board. All of the sensors will be powered by 5v, none will use phantom power.

I can conveniently place the pullup resistors on a breakout board next to the E100 or on the subsidiary breakout boards 15 feet from the E100. It would be inconvenient to locate the pullup resistors at or near the sensors which will be strapped to pipes, outside of the house, or possibly even buried in the ground.

Does anyone have any idea if the sensor performance will be degraded depending on the physical location of the pullup resistors?

Paul in NY
 
DaveC166
Regular Member

Joined: 13/09/2012
Location: New Zealand
Posts: 44
Posted: 10:05am 15 Dec 2016
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Hi Paul, back in 2010,I was having problems with DS18B20's giving false readings on 15 meters of cable attached to a Picaxe 28X2. I was advised on Picaxe forum to reduce the pullup resistor value and have the pullup resistor close to the picaxe. This mitigates cable capacitance.
The DS18b20's have been running ok with 3.0K ohm pullups attached to a UMITE for 3 years
 
Paul_L
Guru

Joined: 03/03/2016
Location: United States
Posts: 769
Posted: 04:29pm 16 Dec 2016
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Thanks Dave. I sort of figured that line capacitance might produce some pretty extreme exponential rise times but I wasn't sure. When the sensor pulls the line down its internal impedance should be much lower than the impedance of the pullup resistor so an exponential drop time would probably be less noticeable.
 
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