The device under test will be referred to as \textbf{appliance}. The requirements for the final \textbf{measuring device} are grouped to the three categories. Mandatory requirements are bound to be met at any cost. Some of the high importance requirements can be skipped or slightly modified, if unpredictable obstacles are found. However, they are all assumed to be completed for well being of the project. Optional requirements will be completed only if the resources allow it.
They are also divided to a hardware part and software part. Software is easier to change than hardware and requires hardware to be run on. Software is also limited by the resources provided by the hardware. Therefore, hardware needs to be logically completed first and are also highlighted in figures \ref{f:client_node} and \ref{f:serv_node}.
The first mandatory software requirement asks for a web server. It is entirely possible for every measuring device to contain its own web server. However, multiple points are requiring separate parts to work as a \textbf{system}. Two common system structures are \textit{centralised} and \textit{decentralised}.
Decentralised (peer-to-peer) systems are harder to build but are more fail-proof. Since fail-proofness is not mentioned in the requirements, centralised system might suffice.
Using centralised system means, that the measuring devices will use one separate accessory, from now called the \textbf{server node}, to do most of the work on the software side. The work includes, but is not limited to, receiving the measured data, storing them, hosting the web server with the \gls{gui} containing all necessary options and information, handling the \gls{usb} or communication with a \gls{cloud} and so on. The block diagram for a server node, depicting required blocks can be seen in the figure \ref{f:serv_node})
Where there are at least two nodes in a system, they have to communicate together in a particular way, known to both of them. The web server naturally operates over \gls{tcpip}. Therefore, same networking stack (the way of comunication), that is used for communication between the server node and user can be used to communicate to client nodes as well. \Gls{tcpip} hardware is ready to be used and is supporting a full-blown networking \gls{stack}, powering communication over today's networks.
The measuring devices, from now on called \textbf{client nodes}, will consist of blocks of the remaining hardware requirements. The resulting block diagram can be seen in the figure \ref{f:client_node})
For the \textbf{server node}, a complete working solution already exists, ready to be employed. The \textbf{GL.inet board}, described in more detail in the chapter \ref{s:glinet}, is greatly sufficient in all required aspects, and thus should be used for this purpose.
Luckily, a particular part of the required functionality for the client is already integrated as a \textbf{ESP-8266 module}, described in more detail in the chapter \ref{s:esp8266}. The module contains the TCP/IP stack, micro-controller (application processor) running the user program, \gls{wlan} and light indication, all in one piece, so this greatly simplifies the design process and allows for more focus on the actual measurement circuitry. The actual ESP-12E module should be used, because of the available certification, which allows it to be introduced on the market later. It was shown in the figure \ref{f:esp-12e}.
Talking about the measurement circuitry, the viable candidate is MAX78615 \cite{online:MAX78615} with the companion \gls{ic} MAX78700 \cite{online:MAX78700}. The couple \ref{f:schem_block} should be used, because it provides multiple ways of same voltage level communication with the processor, galvanic isolation via the pulse transformer for improved circuitry protection, great precision, accuracy and utility. The shunt resistor is utilised as a way of obtaining measurements, described in the sub-chapter \ref{ss:pmic}.
\caption{The proposed block diagram of a schematic of a \textit{client node} focusing on measuring part of the circuit}\label{f:schem_block}
\end{figure}
For the protection against fire a standard electric fuse or self-regenerative fuse should be used. The circuit protection against high voltage should be solved with an isolated DC-to-DC converter or with the linear transformer coupled with the linear voltage regulator. Since the former one is either expensive or hard to design, the choice should fall on the latter.
The remaining parts of the client node block diagram \ref{f:client_node} not yet mentioned are switching and sound indication. Either a mechanical relay or a semiconductor device, such as a thyristor or a \gls{ssr} isolated by an opto-coupler will do. Mechanical relays tend to be larger and produce sound noise, have slow response time, but have inbuilt separate isolation and are capable of switching higher currents without additional thermal issues than their semiconductor counterparts. The disadvantages of the mechanical relay are not relevant here, thus the choice is obvious.
