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I have built a networked temperature sensor by connecting several DS18B20 temperature sensors to my Arduino. I've tried to visualize the setup with this schematic (sorry if this doesn't make any sense, I'm not really experienced with electronics):

schematic

simulate this circuit – Schematic created using CircuitLab

I've distributed the DS18B20 sensors in multiple rooms in our office and used existing unused Ethernet cables to connect them. The pins of the patch cables are used as follows:

8P8C plug

  1. n/c
  2. n/c
  3. n/c
  4. 1-wire bus
  5. GND
  6. 5V
  7. n/c
  8. n/c

This wiring is probably non-standard even for 1-wire, but as I'm using an altered ISDN hub (I've removed the resistors) I'm limited to 4 pins and so I made up my own wiring. The DS18B20 sensors are directly connected to these pins of the plug, without any additional components. On the Arduino side, there's a small circuit board with the pull-up resistor that connects to the 1-wire hub.

My question now is this: what could happen if someone accidentally connects an Ethernet device to the 1-wire bus, or vice versa, a temperature sensor to the Ethernet bus? Could the 1-wire bus or the temperature sensor damage a PC or a switch?

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The voltages on Ethernet (ranging from -2.5V to +2.5V) are not compatible with the 5V the DS18B20 uses. The easiest way to prevent damage is to use different pins on the modular plug you're using for the sensors. Standard 10/100 Ethernet cabling (including POE, Power over Ethernet) uses pins 1, 2, 3 and 6. That leaves pins 4, 5, 7 and 8 free for hooking up your sensors.

If you can steer clear from cross-over cables and or gigabit capable equipment you should be fine for your project.

http://en.wikipedia.org/wiki/Ethernet_over_twisted_pair

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    \$\begingroup\$ Practically all our equipment is Gigabit-capable, so that probably won't help much. \$\endgroup\$ – Martin von Wittich Aug 4 '14 at 10:38
  • \$\begingroup\$ Correct. I guess the only thing left would be to use a similar technique (voltage differentials and bias tee's to offset the power and signal) and magnetics for hooking up your sensors but that would make the design (relative to the size of just a single to-92) quite a bit larger. Maybe go wireless with RFM12B modules? :-) \$\endgroup\$ – captcha Aug 4 '14 at 22:55
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With this mapping, the bus will be connected to GND through the coupling transformer between 4 and 5.

If you use 3 and 4 instead, you should be safe, because these lines are not connected.

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