the interrupt request terminal irq it compares them to the time measured by the internal clock calculates the reciprocal of the time interval and then outputs the speed a relay driver 34 is connected to the output port of the microprocessor mpu when relay driver 34 supplies current to a solenoid 35 relay 36 is closed in the following description current passing through solenoid 35 causing relay 36 to close is referred to as 34 relay on 34 and a lack of current to solenoid 35 so that relay 36 is open is referred to as 34 relay off 34 a driving circuit 37 is connected to output ports op2 and op3 of the microprocessor mpu depending on the states of output ports op2 and op3 driving circuit 37 sets up certain field effect transistors as 34 on circuit 34 the driving circuit 37 also utilizes a voltage rising circuit to apply an external voltage of about 20 volts to the gate terminals of field effect transistors tr1 and tr2 the characteristics for driving circuit 37 are shown in table 1 table 1______________________________________characteristics for driving circuit 37op2 op3 tr1 tr2 references______________________________________high high off off stophigh low on off right rotatinglow high off on left rotatinglow low off off unused______________________________________ a pwm pulse width modulation circuit 38 is connected to the output ports op4 op11 of the microprocessor mpu depending on the output signal from output ports op4 op11 the pwm circuit 38 generates square wave pulses with spacing ratios.