1-Wire USB on a Long Ethernet Cable

Units on 1-Wire bus must be connected in a daisy chain, which means you’ll have a lot of different connection points along the bus cable, in my case an ordinary Ethernet cable. This has been a problem for me earlier since my cable was placed outdoors for air and soil temperature measurements, and soil moisture measurements. I actually need a connection point to the 1-Wire bus at each unit I connect to the cable, because of the daisy chain topology. In order to make these connections somewhat weatherproof I chose to solder any cable joints together and cover it with heat shrink. This has worked well and my old outdoor system collected data for almost a year, before I took it down again when I had to move to another house.

But the soldered joints also gave me problems, because it was hard to make adjustments afterwards, like moving a unit, because the cables would have a fixed length, or taking a unit indoors for repair. This can of course be solved by investing in high quality outdoor enclosures for each unit or cable joint but since this weather data collection project is a hobby there not any immediate return on this relatively large investment. However, if you are getting paid to build this, you should be aware that the system would have a much longer lifetime if you mount the units and joints in a weather proof enclosure. But let’s stick with the hobby type of project for a while.

I wanted to make it easier to modify the physical setup of the system, and one of the easiest ways to connect wires is by using terminal strips:

(There’s a DS18S20 1-Wire IC wrapped in heat shrink to the left in the picture.)

The 1-Wire bus uses only 2 wires (?…) for communication and power transfer. (I guess the name is referring to the communication part.)

A small test showed that the bus worked just fine through the terminal strip (and 17 m ~ 56 ft of Ethernet cable), which makes my life a whole lot easier, as I’m now able to modify the physical setup in many ways, without going outside with my soldering iron in the middle of the snow to replace a unit – brrr… Just bring a screwdriver.

For soil temperature measurements I have glued a DS18S20 temperature sensor onto an aluminium sheet. Note the two cables due to the daisy chain topology:

I assume that the metal sheet will give a good average temperature as is has some mass and therefore filters out high frequency temperature changes. The large area helps averaging the temperature too.

This is all of the components in the 1-Wire system so far:

  • To the left: DS18S20 temperature sensor, for air temperature measurements
  • Aluminium sheet: DS18S20 temperature sensor, for soil temperature measurements
  • In the middle: DS2760 PCB, for soil moisture measurements
  • To the right: Watermark soil moisture probe

The DS2760 PCB has been installed in my previous garden, in a cheap plastic enclosure, wrapped in a plastic bag, but the PCB survived for several months in all kinds of weather. It still looks new with no corrosion at all. The plastic enclosure has changed color due to sunlight:

I’ll also be using terminal strips for this unit and see what happens regarding corrosion.

The soil moisture sensor part of this system is a bit special because the circuit board needs 12 volts to function, which I’m injecting into the Ethernet cable using the brown wire pair, next to the 1-Wire bus carried by the blue wire pair. The extra voltage is generated by a generic mains adapter, protected by a fuse (red and black wires in the picture below):

I have mounted the sensor for air temperature measurements on a stick and covered the sensor with a small cap with tin foil on it to reflect the sun:

I also need a barrier between the soil and the soil temperature sensor (hopefully this is not a biodegradable plastic bag ;-) ):

The soil moisture sensor circuit is tied to a stick and covered with a plastic bag to provide some protection from the weather:

Again, if you want a more reliable setup you should use a weather proof enclosure, but I haven’t experienced any problems so far, although it would be an obvious upgrade to make in the future.

The NSLU2 is placed indoors to protect it against the weather, but I have to get the signals and power outside to the sensors, so I have to go through the window. Since this is a rented house I don’t want to drill large holes in the window frame, so I have removed the extra four wires in the Ethernet cable to be able to close the window without cutting any wires accidentally. Using two different sizes of heat shrink I’m able to make a pretty discreet transition from the indoor to the outdoor environment:

There was already a not so neat looking satellite coax cable going through a hole in the window frame, so adding an Ethernet cable is not big deal, although it’s beginning to look messy:

Rather discreet looking 1-Wire cable outside (the two cables in the top are existing coax cable for satellite):

I have dug a 25 cm (10″) deep hole in the ground for the Watermark soil moisture sensor and the temperature sensor on the aluminium sheet, and once I have calibrated the soil moisture sensor the hole will be filled with soil again and the system will collect data over the winter:

The air temperature sensor is also in place and ready to collect data:

Now I’m going to work on the OWFS software setup on my Debian NSLU2.

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