A battery-less multi-rotation encoder including detection coils with the Barkhausen effect includes a rotation detection mechanism and a signal processing circuit. The detection coils generate voltage pulses with different positive and negative signs, and transmit them to the signal processing circuit, and the signal processing circuit includes a controller and an adder. The controller can set states of the detection coils to be High or Low and to maintain them at High or Low, based on the positive and negative signs of the respective voltage pulses and no voltage pulse being generated therefrom. The controller is configured to store the states of the respective detection coils in a memory. The adder can update a number of rotations according to the changes in the states of the respective detection coils. The signal processing circuit can determine the rotational angle of a rotational shaft within about 1/4 rotation unit.