  To measure oscillation of a simple pendulum was probably a first idea coming
to mind after appearance of smartphones with small but powerful acceleration
sensors~: Simply attach the telephone to a playground swing or hang it on two
string as the pendulum bob and record the data. But immediately the problem
becomes not so trivial. To deeply investigate on it or to make the phase
diagram of the movement require complex calculations that are far beyond the
capability of the youngest students. In this article we propose another way to
study the pendulum by putting the sensors at the axis of rotation. This method
gives immediately the figures very close to ones in textbooks.
