Use of \tikzexternalize - compilation

Question

I want to improve compiling speed of my latex thesis. I have read that using \tikzexternalize, only when there is a change in the tikz codes they are compiled.
The problem is that I can no use \tikzexternalize properly. This is my preamble:
\usepackage{circuitikz}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric, arrows, shadows, calc,positioning}
\tikzexternalize
And this is not working at all. I received next error:
Undefined control sequence. \tikzexternalize
If it is useful, I am using MikTex with TextStudio and TextLive with Textstudio also under Ubuntu. It doesn't work in any of these system.
Hope you can help me.

Answer 1

Didn't you forget to add
\usetikzlibrary{external}
? I just found it here : tex.stackexchange

Answer 2

I have done it.
Now I don't have any error messages, but no figures are saved into TikzFigures folder. This is my preamble:
\usepackage{circuitikz}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric, arrows, shadows, calc,positioning,external}
\tikzexternalize[mode=graphics if exists,
figure list=true,
prefix=TikzFigures/]
This is how I add my tikz figures. First example is by using circuitikz.
\begin{figure}[h]
\centering
\tikzsetnextfilename{nonInverter}
\begin{circuitikz}[scale=1]\draw
(4,.5) node [op amp](opamp) {} % Amplificador Operacional
(0,0) node [ ground]{} to (0,1)
to [R=$Sensor A201$,-*] (opamp.-)
(opamp.out)|- (5,2.5) to [R=$R_{F}$](2.8,2.5) to (opamp.-)
(opamp.+) to [short,-o] (2.8,-1) node [right] {$V_{+}$}
(opamp.out) to [short, *-o] (6,.5) node[right] {$V_{ADC}$}
;\end{circuitikz}
\end{figure}
Next one, is by using the common tikz package:
\begin{figure}[h]
\begin{center}
\tikzsetnextfilename{masterFlowChart}
\begin{tikzpicture}[node distance = 2cm, auto]
% Colocamos los nodos
\node [cloud] (init) {Initialize System};
\node [block, below of=init] (connection) {Connection to the master};
\node [block, below of=connection] (wait) {Wait for master events};
\node [block, below of=wait] (measure) {Measure and send to master};
% invisible node helpful later
\node[right of=wait,scale=0.05](inv){};
% Colocamos las relaciones entre nodos
\path [line] (init) -- (connection);
\path [line] (connection) edge[loop left] node {No Network}();
\path [line] (connection) -- (wait);
\path[-,draw] (wait) -| node{} (inv.north);
\path[line]{} (inv.north) |- node[above]{Reconnect} (connection);
\path [line] (wait) edge[loop left] node {Sleep}();
\path [line] (wait) -- (measure);
\path [line] (measure) -- (wait);
\end{tikzpicture}
\end{center}
\label{fig:slave_flowchart}
\end{figure}
And last one is by using sequencediagram package, that if I am not wrong is also based on tikz.
\begin{figure}[h]
\begin{center}
\tikzsetnextfilename{sequenceDiagramaECnsole10}
\begin{sequencediagram}
\newthread{pc}{PC}
\newinst[2]{master}{Master}
\newinst[2]{slave}{Slave}{}
\node[below right of=master,font=\scriptsize](master_state){Idle};
\node[below right of=slave,font=\scriptsize](slave_state){Sleep};
\begin{sdblock}{Run Loop}{}
\begin{call}{pc}{Start Measurement}{master}{Master Data}
\begin{call}{master}{Data Request}{slave}{Slave Data}
\end{call}
\end{call}
\end{sdblock}
\node[below of=master_state,font=\scriptsize, node distance= 4cm]{Idle};
\node[below of=slave_state,font=\scriptsize, node distance= 4cm]{Sleep};
\end{sequencediagram}
\end{center}
\label{fig:ecnsole_sequence_diagram}
\end{figure}