1 Wire Station * Updated 26th March 2008

The 1 wire sensors, other than the temperature sensor are now out of use as a fault developed in the wiring, so this page will now be dedicated to 1 wire setup information, etc.

This system is more experimental than accurate, although the temperature and rainfall is usually very reliable.

> If your looking at this page to find out more about 1 wire (and possibly build your own) then you should read on, as i explain how it works.

Should i buy a pre built weather station?

> This is a question i get allot of emails about, now we run a Davis VP2 weather station. This comes all pre built, tested and ready to go, but has a high price tag of around £600 in this country. If you want a quick setup and very reliable, accurate data then it would probably best to go with a 'pre built' or packaged solution. If you take a look at the ukweathershop.co.uk web site (where we bought our Davis from) you can see a whole range of ready to go weather stations at varying cost, a word of warning though - Like most things, you really get what you pay for.

Compared to most pre built stations, the 1 wire system has a few benefits over them. Firstly cost, each sensor is around £20-30, and unlike pre built stations you can run as many sensors as you like of many different measuring capabilities.

If you would like a challenge of the build and don't really need weather sensors up and running the day they arrive then read on.

What can the sensors record?

> There are many 1 wire sensors that you can buy, these are digital semiconductors and require a computer to view what they are reading, which can measure varying things. Now we only looked at the ones which measured weather variables but you can get others. You can get , Temperature sensors, Barometer Sensors, Rain sensors, Leaf wetness Sensors, Lightning detectors, Humidity sensors, Solar sensors, soil moisture, just to name a few.

So what could you use this for ? Well maybe you want to measure the temperature in your greenhouse, and maybe the garden too, rainfall also. With 1 wire you can attach many of the same or different sensors over a distance in nearly any application. Another example could be having a temperature sensor in each room in our house to measure differences. However in the case i will show you later on i am only interested in measuring the weather. So lets look at what exactly 1 wire is, i also recommend that if you seriously want to build your own 1 wire system to buy the 1 wire book from the Hobby boards web site, i have this and it helps out a tonne when things go a little wrong.

The 1 Wire BUS

1 Wire is basically an easy way of getting a whole array of sensors to talk to a host (the host in our case is the PC - more on this later). This technology was built by Dallas Semiconductors in the states (its now owned by Maxim IC ), where by you use 2 wires - this is like a powered device that has a positive and negative. The first wire goes from the host to the sensor input , the second comes out of the sensor and back to the host, Simple enough.

The componants for each module look like this (this is a non prebuilt temperature sensor) from hobbyboards:

Here you can see the little PCB with the temperature sensor (this is a transistor), block connector and 2 rj45 terminals. You simply daisychain from one sensor module to the next, so if this sensor was on the end of the line it would only have 1 cable going into it, if you later decided to add another sensor next to it then you would run out of the second port to the next sensor and so on. If you were mounting this in a sheltered spot then it would be fine as it is, but if your planning on putting it outside to measure air temp then you really need to put it in a infra red radition shield (more on this later).

*** All sensor kits (bar the wind sensor) from hobbyboards have RJ45 terminals as you can see above, however all the modules from AAG have RJ11 terminals (RJ11 are the small cube like telephone ends, RJ45 are the larger rectangular fittings used on network cables).

You may also see the small terminal block. This is used when you dont wish to use RJ45 ends. You simply strip the 2 core ends on your cat5e that you are using and secure them in using the small grub screw (just like in electrical wiring). This would mean that you wouldn't have to use any rj45 ends, which saves you crimping and time but if your not so confident around wiring then it would be best to stick with using the plug and play RJ45 system.

You can use as many sensors as you like on the 1 wire circuit. Ofcourse there is a limit based on what you are using and cable distance, but for local sensors it is usally something like 20 temperature sensors on the same line. If you are thinking about using the Barometer, this needs its own power source as it is classed as a high powered device. What you have to remember is that the 1 wire line carries DC 5v along the data line out of the PC, so each sensor uses this power to work. In the case of the barometer 5v is not enough for it to function, so you must supply 12v to it either on the spare cores in your cat 5 wiring or maybe just attach a 12v dc transformer to it. Another option would be to use a power injector, you can find these on the hobby board site, it uses the principle of sending the extra power down the free cores in your cable.

I'll show you a diagram of our system:

The red wire is the data line, this goes from the host to each sensor in a daisy chain. When you get to the last sensor you go from its output back to the host to complete the circuit, just like any electrical circuit. >> When you start your 1 wire system you will need to buy an adapter which either connects on your serial port or USB. This is the device known as the host, on it is a standard telephone fitting (RJ-11).

Now a word of caution - This caught me out the first time, on the serial or usb adapter there is a RJ-11 fitting, now if you buy your sensors from Hobby boards or AAG electronica the fittings on the sensors are RJ-45 (network cable style connectors) but for the wind sensor it has RJ-11 as it is older (they moved over to using rj45 for the other) sensors. So you will need to make a cable coming from the host to the first device RJ-11 one end and RJ45 the other, that is unless you go to the Wind sensor first in which case it would be RJ-11 both ends from the host to the wind sensor, then RJ-11 out of the wind sensor to the next sensor with an RJ-45 end (hope that make sense ;-) )

What data can you collect?

> There are many 1 wire sensors to choose from, take a look at AAG electronica or Hobby Boards for instance. Now when you buy these different sensors they come in a small white pvc box, why? Well firstly each sensor comes onboard a small PCB about 5cm by 5cm, on this board you will have 2 sockets as i said earlier RJ-45, one is in, the other is out. The out will either go straight back to the host or to the next sensor. The wind sensor comes usually in bits so you have to build it but this is fairly easy and all the screws etc are supplied, this sensor has the 2 RJ-11 input/output sockets so here you will have to adapt the cable, second reason is for protection although they are not waterproof so you will have to put them in some sort of container (apart from the Rain Gauge and Wind sensor, obviously).

If you take a look at the hobbyboards web site there is a lot of helpful information about the varying sensors and how to set them up, you can either buy as a kit and solder yourself or have them pre built and tested. You can think of each sensor as a module, and you just simply plug each module to the other.

How it works and wiring

> Each sensor has a long code stamp such as '420008014CD96B10' , this is our main temperature sensor. Each sensor you put on your system will have its own unique code, slightly different from the others so you know which sensor is providing the data - Otherwise it would get a little confusing. Now you will need software on your PC to view the 1 wire data, this can be a little complicated but i use Weather Display, its just a case of telling it you have 1 wire then copy and pasting each of the detected codes (sensors) into the different boxes for rain and temp etc. Unlike other stations the 1 wire system talks to the sensors in complete real time, the more sensors you have it may take a little longer to 'Poll' but we are only talking about 5 seconds for 10-20 sensors. If you only have 1-5 sensors on your circuit then it brings back a reading from each sensor in about 2 seconds, now when you consider that some of those pre built stations take 2 minutes or even longer to return their data you can see why this system is so appealing.

The rainfall sensor you can buy from Hobbyboards is a converted Rainwise Gauge that contains a 1 wire sensor - This is a counter. As the rain gauge tips it sends a pulse to the counter and is counted as 1 (or 0.2mm) it then sends this on to the computer.

Now this is where the 1 wire system lacks, most pre built systems have a console with records the data and displays it. However the 1 wire system only works when your PC is on, and you can only log data if you have a computer on 24/7.

In terms of wiring, you can either use standard 4 core telephone cable (you only need use the 2 middle wires) or use category 5 twisted cable (which you can get cut to length at your local maplins). The picture to the right shows cat 5 shielded cable, i would strongly suggest you use this instead of phone cable as it provides a much smoother operation.

It may look complicated at first sight but its relatively simple. I decided to use the 'blue' twisted pair. As i explained earlier the solid blue will become the 'data line', the white with blue stripe will become the return.

If your sensors are close together then it may just be as easy to use pre-manufactured cat 5 cables that you can by to connect all together, but remember that you need a converter cable from the pc with a phone style RJ-11 one end, and network standard RJ-45 the other. If you are going to cut your own lengths of cable then you will need a RJ-11 crimper and a RJ-45 crimper tool (again if you go into maplins, you can find these and the cable). You will also need to buy the connectors, might be worth getting quite a few of each of the style connectors just in case a few go wrong.

The wiring is fairly simple, whatever connector you are using (RJ11 / RJ45) you need to have all the ends matching up. If your going to firstly connect the pc (host) to the wind instrument then you can go and buy any standard telephone cable with the rj11 connectors (they are square not rectangular) at length and simple connect the host to it. Now if you decide that you want to go from the wind instrument to a temp sensor, then buy a length of cat 5, put a RJ-11 the one end and RJ-45 the other, so here's the wiring concept to help you (click for bigger view):

So as you can see you keep the pair you are using in the middle and orientated in the same way on every connector you use, these are the only 2 cables you have to use, all the other pairs can be cut off. I always use the rule when looking down at the connector where the prong is on the top, keep the dark blue to the left then the white/blue striped on the right next to it. Always make sure they are in the middle and just connect one new device at a time to make sure the cables working.

How it works and wiring

> So now the next option would be to order the sensors you want, i bought mine from the hobby-boards.com web site, they arrived here in the uk with 2 weeks of ordering and very helpful staff too. Your other alternative is the AAG electronica but that choice is really up to you. This picture shows the devices i ordered after they arrived:

You can see the wind sensor (which has an on board temp sensor but i don't use it as it gets a little too hot in the sun), the rain gauge, temp sensor (in its case) the host adapter (brown connector in middle) and the solar sensor in its packet. As you can see the rain gauge comes with cable already so this can connect into another sensor at the end of the line.

That's just about it now, the rest is up to you. One point i would make is that this is a more DIY approach, but say for instance a shield for the temp sensor, you can either make one from plant pots or buy a ready built one from ukweathershop.co.uk (look for davis radiation shield). Also place the anemometer as high as possible to get a good reading, and the temp sensor in complete shade if possible at 1.2 meters. If you need help or advice about setting your system up feel free to email me and i'll happily help you along.

Few other points about my setup

> The purpose of the wind sensor at such a low level gives more of an indication of speeds moving through the garden, it also shows just why we have our main davis anemometer at 4 metres above the roof line in 'clear air'.

>The Temperature sensor (in the shield) also has a fan located just above it that draws air in through the plates towards the bottom then up through the middle chamber, this ensures that we are recording true air temperature and not a infrared heat signature (all objects around the sensor give off heat when in direct sunlight so we need to protect it). A standard 12v computer fan is used and power supplied down a spare twisted pair on the 1 wire cat5 cable.