An extremely popular, low cost Rotary Paddle Level Probe.

When power is supplied to the R7z sensor a motor powers a revolving shaft to which a paddle is mounted. When the material level inside the container reaches the revolving paddle the rotation is halted and the motor rotates around the shaft and activates an isolated SPDT micro switch. This removes the power from the motor so that it stops rotating and an alarm signal is provided. When the material level falls below the paddle the motor resets and the micro switch restores the revolving action.