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I am looking at using a DHT11 for logging my temp and humidity in a few boxes i have.

I need to be able to run these sensors several feet away (50ft ish) I would like to use Ethernet cable to do this. I know there is power loss when you start talking feet so im planing on using a 12v to 5v 5A Buck Converter to power them rather then powering them off the arduino.

Before i start down this road i have 3 questions that i can not find the answers on.

  1. Can i get data off these sensors at 50 feet away?
  2. I would like to run 10 per arduino and save to the sd card every 3 hours is there a way to do this all at one time or is it best to do one at a time?
  3. Is there a better sensor to use other then the DHT11 that is still around the same price range
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  • ask only one question please – Juraj Jan 16 at 19:21
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    This article: home-automation-community.com/… claims the the DHT22, a slightly upgraded version of the DHT11, is able to handle wires of up to 30M when powered with 5V. I don't think power is the issue, as much as the ability of the signal line to carry the signal over that length of wire. – Duncan C Jan 16 at 22:40
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  1. DHT11 sensor uses a "single-Wire" protocol with open drain IOs. so it's fine for cable to be long. use a lower value Pull-up resistor for Data line of DHT11. the datasheet says a 5K pull-up resistor is enough for up to 20m wire length.
  2. the process of reading a sensor can't be paralleled. you have to read them one at a time, store them on a local variable and save that to SD card.
  3. DHT11 is a very cheap, not-so-accurate sensor (up to 2 degrees). it's a little slow too (at least 1 second for measurement). there are analog sensors for both temperature or humidity which are cheaper. but need proper circuit and calibration for using at that range.
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This article claims the the DHT22, a slightly upgraded version of the DHT11, is able to handle wires of up to 30M when powered with 5V. I don't think power is the issue, as much as the ability of the signal line to carry the signal over that length of wire. Just use a 5V Arduino board and you should be ok. (It looks like each one only needs about 1.5 mA, so you would not need a separate power supply. The only real issue would be voltage drop from the resistance on the long wires.)

I believe the DHT11 and DHT22 both need one of the UART pins on the Arduino in order to work. I don't know of any Arduino boards that have 10 UART lines, so you'd likely need a digital multiplexer to switch between sensors. Something like this: https://www.sparkfun.com/products/9056 (or use the bare CD74HC4067 chip, since the breakout board is really just the chip mounted to a board with through holes in it.)

Using a multiplexer might reduce the maximum wire distance you can use - you'd likely have to experiment.

These DHT sensors take about 2 seconds for a reading, if memory serves, so for ten of them it would take about 20 seconds to read them all.

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