Its already some time ago that I wrote my previous post in this series (Old meets new, the 1-Wire Weather Station on the SPARK Core. (part 6)).
In the meantime I did a lot of tests using the solar pannel and different types of batteries and discharge schedules monitored by a bitscope/Raspberry Pi combination ( http://www.element14.com/community/roadTestReviews/1986 ).
Now its time to wrap up and mount everything together in a neat package which can be placed in the backyard. In order to do so a relaible WiFi connection is needed over this longer than usual distance. The SPARK CORE I'm using is equipped with a u.FL connector to which you can connect an external antenna with more gain than the standard chip antenna on the PCB. First tests on my desk were done with a simple wire antenna with a length of 1/4 of the wavelength (3 cm). For my backyard experiments I decided to build a BiQuad antenna. This type of antenna is very popular, lots of information can be found on the internet. My design is inspired by: De ultieme biquad Wifi antenne bouw pagina where you also can find drawings of the exact lengths and distances.
Building the WiFi BiQuad antenna
Here is a picture of the material I used. A piece of coper clad as base and reflector. On the backside I will mount the electronics. I had some silvered coper wire laying around. This will be used for the radiator, furthermore some ptfe isolated coax cable and a piece of brass tube.
In the center of the plate a 3mm hole is drilled, and the brass tube is placed at 16 mm and soldered on both sides.
A pice of coax cable is soldered to the end of the brass tube.
A pice of silvered copper wire is cut at 244 mm, and 8 pieces of 31 mm are marked.
Then the radiator is bended.
And soldered to the brass tube on one side and to the kernel of the coax on the other side.
With this step the antenna is already finished. Simple isn't it?
In order to connect the antenna to the SPARK CORE, a u.FL connector had to be connected to the other side of the coax cable.
Due to the small size of this connector and the 'still' large diameter of the coax this was not a simple task. I took a simple WiFi antenna (66089-2406 - ANAREN - ANTENNA, 66089, 24C, 2400MHZ, U.FL | Farnell element14), cut the wire and soldered that to the kernel of the coax. And a small piece of wire connects the coax shield to the u.FL connector shield. Some heat shrink tubes act as isolator. I realize that this is not a good 50 ohm connection, but I don't see a better option. As you will see later in this blog the antenna functions very good, so I don't think this connection is a big problem.
Mounting it all together
The electronics are mounted on the other side of the coper board, resulting in a neat compact unit.
The Richtek EVB_RT7275GQW Evaluation Board is soldered using some pieces of coper wire.
I decided to use the breadboard for the SPARK CORE, so that I easily can make modifications to the circuit.
A RJ-11 connector connects the Weather station to the SPARK CORE.
Isolated coper wires and a piece of foam are used to mount the LiPo battery.
The input and output of the RichTek board are connected to to analog inputs, in order to monitor the Solar panel and power supply voltage.
Voltage dividers converts te max 21V input of the solar pannel to 3.3V which is the max input of the SPARK CORE.
The front and backside of the final unit:
Some test from my backyard at about 20m from the WiFi router showed very good results, The signal strength of the SPARK CORE was about 10 dB stronger than the signal from my smartphone. When you need real long distances you can combine the BiQuad antenna with a satellite parabole dish. joeman has a nice video on this (The specified item was not found.).
The project is now almost ready, in my next and final post on this project I will put everything together and update the software.
Stay tuned.
Do not hesitate when you have questions.
