The pulse signal of circuit. When is adjusted to get the duty cycle too much there is a maximum of 100%.(signal has the most positive pulse.) Makes the motor it will spin the fastest. If the percentage of the duty cycle reduced, the speed it will reduced as well. The pulse output signal from IC1/2 will enter to Q1(2N3906), Q2(2SC1061) that are connected to the Darlington amplifier circuit. To extend the current pulse up. Then to drive to the the motor, connected to the output terminals of the circuit.
The Q2(H1061) acts through a direct current to the the motor. The selection will need to look in accordance with the current use of the motor. I choose to be more current, because when the motor start to draw more the current from the power supply than the normal rotation. Or not the the motor is loaded. This circuit is suitable for the DC motor with a power that is not much. Is small enough that the voltage and current not exceeding 12 volts 2 amps. Which reduces power loss to the the motor and the circuits.