the amount of the current supplied from the charging circuit 14 to the capacitor 12 can be appropriately adjusted by means of the variable resistor 20 to change the on off duty cycle at the output circuit 3 and thereby to change the rotation speed of the rotor the variable resistor 20 may be replaced with a fixed resistor of which resistance is fixed to a value in accordance with a desired rotation speed of the rotor when the rotor is locked due to an external cause the rotation detection circuit 5 does not generate an output pulse which causes the capacitor 12 to be additionally charged through the charging discharging circuit 7 as is described above when the potential of the capacitor 12 rises to reach a second predetermined potential v off the output of the first comparator circuit 10 becomes high then the cut off reset circuit 11 is turned on through the or circuit 18 to turn the output circuit 3 off when the capacitor 12 is further charged to reach a potential v h the inversion detection transistor 8 is turned on which sets the charging discharging circuit 7 to the state for discharging the capacitor 12 through the first flip flop circuit 9 as a result the potential of the capacitor 12 starts to drop when the potential drops to a third predetermined potential v on which is lower than the first potential v s the output of the first comparator circuit 10 becomes low resulting in that the pulse circuit 17 generates a reset pulse which resets the second flip flop circuit 16 this operation is repeated until the external cause is removed fig3 b 3c 3d and 3e show waveforms output from the second flip flop circuit 16 the first.