Abstract: A sensor to be activated by the ground for sensing a user's movements includes a conductor that contacts the user's skin or maintains a specific distance relative to the user's skin; an electronic device connected to the conductor and responding to changes of electrical charges carried by the user resulting from his movements, thereby generating corresponding electronic AC signals; a DC to AC converter ot convert said electronic Ac signals into DC voltage signals; and a voltage level changing meter having a voltage range and operative to generate a signal when the DC voltage signals are outside the preset range.