Simple FSM

\usetikzlibrary{automata, positioning}

\begin{tikzpicture}[shorten >=1pt, node distance=3cm, on grid, auto]
  \node[state, initial] (S0) {On};
  \node[state, accepting] (S1) [right=of S0] {Off};

  \path[->]
  (S0) edge[bend left] node{press} (S1)
  (S1) edge[bend left] node{press} (S0);
\end{tikzpicture}