the working principle is that when a photoelectric signal is generated to energize the motor winding l and when the current thereof is increased to reach the chopping threshold the transistor t 1 is cut off i e the chop off but no longer being connected and the transistor t 2 is connected at the same time since the current direction of the motor winding l is unchanged and the current valve of the motor winding l reaches its maximum the diode d 2 the motor winding l and the capacitor c form a circuit to charge the capacitor c wherein the polarities of the capacitor c are negative at the upper side and positive at the lower side when the current of the motor winding l is decreased to zero the capacitor c is fully charged at its maximum level the discharging current from the capacitor c passes through the transistor t 2 the motor winding l and the capacitor c to form the circuit the repeated cycle thereof will form the oscillation circuit with attenuation ability before reaching the preset lower threshold or the stored oscillation energy is exhausted or the subsequent photoelectric signal is generated the transistor t 2 is cut off and the oscillation circuit is cut off to complete one single control process during the control of the energy conversion unit and assuming that the voltage of the capacitor c is zero the energy is exchanged between the capacitor c and the motor winding l fig6 illustrates the waveform of the capacitor c when selecting reasonable parameters of the capacitor c the attenuation during oscillation process will be minimized in other words through the time required for the one single.