Category Archives: Motor Controllers

Simple stepper motor controller

The circuit shown above can be used to control a unipolar stepper motor which has FOUR coils (I’ve swiped it off an old fax machine). The above circuit can be for a motor current of up to about 500mA per winding with suitable heat sinks for the SL100. For higher currents power transistors like 2N3055 can be used as darlington pair along with SL100. The diodes are used to protect the transistor from transients.

Activating sequence:
| Inputs |Coils Energised|
|————-+ |
| D0 D1 | |
| 0 0 | A,B |
| 0 1 | B,C |
| 1 0 | C,D |
| 1 1 | D,A |
To reverse the motor just reverse the above sequence viz. 11,10,01,00.

Alternately a 2bit UP/DOWN counter can also be used to control the direction , and a 555 multi-vibrator can be used to control the speed.

Discrete component motor direction controller

This circuit can control a small DC motor, like the one in a tape recorder. When both the points A & B are "HIGH" Q1 and Q2 are in saturation. Hence the bases of Q3 to Q6 are grounded. Hence Q3,Q5 are OFF and Q4,Q6 are ON . The voltages at both the motor terminals is the same and hence the motor is OFF. Similarly when both A and B are "LOW" the motor is OFF.

When A is HIGH and B is LOW, Q1 saturates, Q2 is OFF. The bases of Q3 and Q4 are grounded and that of Q4 and Q5 are HIGH. Hence Q4 and Q5 conduct making the right terminal of the motor more positive than the left and the motor is ON. When A is LOW and B is HIGH, the left terminal of the motor is more positive than the right and the motor rotates in the reverse direction. I could have used only the SL/SK100s, but the ones I used had a very low HFE ~70 and they would enter the active region for 3V(2.9V was what I got from the computer for a HIGH), so I had to use the BC148s. You can ditch the BC148 if you have a SL/SK100 with a decent value of HFE (like 150). The diodes protect the transistors from surge produced due to the sudden reversal of the motor. The approx. cost of the circuit without the motor is around Rs.40.

Note: You can change the supply voltage depending on the motor; only thing is that it should be a 2 or 3V more than the rated motor voltage (up to a max. of 35V).

DC Motor-Driver H-Bridge Circuit


Physical motion of some form helps differentiate a robot from a computer. It would be nice if a motor could be attached directly to a chip that controlled the movement. But, most chips can’t pass enough current or voltage to spin a motor. Also, motors tend to be electrically noisy (spikes) and can slam power back into the control lines when the motor direction or speed is changed.

Specialized circuits (motor drivers) have been developed to supply motors with power and to isolate the other ICs from electrical problems. These circuits can be designed such that they can be completely separate boards, reusable from project to project.

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