Usual, we just enter the voltage and DC current source to the DC motor, Makes motor can be rotated. If will modify or regulate the speed in rotation of the motor. Wewill need to enter the voltage look into pulses or periodically. Which will makes the average voltage at output is different. By the pulse voltage is high frequency will has the average voltage than the low frequency pulse. The same with the pulse has over duty cycle value, it will has the average pressure much than low duty cycle value.
The working principle.
The average voltage will be supplied to the DC motor. To control the motor rotation is slow or fast. This circuit uses the above principle. By makes the pulse generator circuit or astable multivibrator Which consists of IC1 / 1 and IC1 / 2 inverter gate with C1, R1, R2, VR1 and diode D1, D2. The signal generator has the duty cycle value that can adjustable or can rotate with VR1 there.
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.