LaTeX Forum

\documentclass{scrartcl} 
\usepackage[utf8]{inputenc} 
\usepackage{tikz} 
\setlength\fboxsep{-0.1pt} %nur zur Demo
\setlength\fboxrule{0.1pt}
\begin{document} 
\begin{tikzpicture} 
\draw[fill=gray](0,0) rectangle (2,2); 
\node[below,circle,inner sep=0pt,minimum size=0pt,fill=white]at (1,1){\fbox{$\oplus$}};
\draw[red,fill=white] (1,1.2) circle(0.14) +(-0.14,0) -- +(0.14,0) +(0,-0.14) -- +(0,0.14);
\end{tikzpicture}
\end{document} 

\documentclass{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{amsmath, amssymb, amsfonts, amsthm}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows,snakes,automata,backgrounds,matrix,decorations.pathmorphing}

\begin{document}
\begin{tikzpicture}
\draw[fill=gray](0,0) -- (0,2)-- (2,2)--(2,0)--(0,0);
\node[below,circle,inner sep=-.5pt,fill=white]at (1,1){$\oplus$};
\end{tikzpicture}
\end{document}

\begin{tikzpicture}
\draw[fill=gray](0,0) -- (0,2)-- (2,2)--(2,0)--(0,0);
\node[below,circle,inner sep=-.5pt,fill=white]at (1,1){$\oplus$};
\end{tikzpicture}