A very simple dc motor speed controller circuit can be designed using circuit that can be used to design a small drill project.
Q1, Q2 and U2 form a variable-frequency pulse width modulator, controlled by U3. Q4 forms part of a step-down power converter.
Speed regulation is accomplished by sensing the motor current with R17 and using it as positive feedback to compensate for motor resistance loss. The gain pot should be set to a point just below the point where the motor speed oscillates. After finding this point, you may want to change the value of R11 to get better speed control range.
Q3 limits motor current. D4 and C8 capture some of L1’s inductive kick to produce a loosely regulated -12 volts for U3. SCR1 acts like the usual flyback diode once C8 has stored the needed energy, preventing significant power losses.
This motor speed controller electronic project needs to be powered from a DC power supply from 12 volt up to 15 volt .
This speed regulator circuit is very simple an require common electronic components like 555 timer, operational amplifiers , transistors and passive components.