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Controlling DC motors is at the heart of many robotic projects. Servo motors are sexy but DC motors are cheap and a lot more useful to control wheel based robots. In this article I’ll display a “robot shield” circuit that allows you to use cheap motors to drive your robot.

Custom robot shield

Wiring

This shield has been built on top of the Adafruit Prototype Shield. Using the prototype shield makes it much simpler to develop a custom Arduino shield. You can get one from Adafruit here.

The assembly is powered by a 6V battery connected to the Vin pin of the Arduino and to the voltage regulator input pin. The voltage regulator output is connected to pin 9 (Vcc2) of the chip to power the motors. The Arduino 5V pin is connected to pin 16 of the controller to power the internal logic. Make sure to use a proper heat sink on the voltage regulator and on the chip (pins 4,5,12,13) if you plan to power the robot with a higher voltage source.

The L293 current driver chip amplifies the input signal received on pins 2, 7, 10 and 15 to outputs pins 3, 6, 11 and 14. This allows us to take a low power signal from our circuit and transform it into a higher power signal for the motors. Using these 4 input/output the L293 can drive two motors forward and backward.

Motor #1 is connected to pins 3 and 6 and motor #2 is connected to pins 11 and 14. To drive motor #1 you apply 5V on pin #2 and 0V on pin #7. To drive the same motor in opposite directions you apply 5V on pin #7 and 0V on pin #2. You can use the same technique to drive motor #2 with pins 10 and 15.

Speed can be controlled by sending a logical pulse to the L293 “enable input” pins. Pin #1 drives the speed of motor #1 and pin #9 of motor #2. The best way to do this is by using the Arduino PWM outputs on pins 9, 10 and 11. In this example, the Arduino pin #10 is connected to the L293 pin #1 and the Arduino pin #11 is connected to the L293 pin #9.

Speed And Direction

There are two methods to control the direction of a wheeled robot. One is to build a direction mechanism similar to cars where the wheels are oriented in the direction you want the robot to go. This method involves complex mechanisms and requires more space for the robot to maneuver.

The alternate method is to drive left and right wheels in opposite directions allowing the robot to make a sharp 360 degrees turn. This method requires no additional mechanism and is much simpler to implement by using a micro-controller. However, this method assumes that both motors are running at the same speed, which is often not the case with cheap motors.

This code shows how to control direction using the L293 chip with an Arduino board.

...

void backward()
{
  digitalWrite(MOTOR_1A, LOW);
  digitalWrite(MOTOR_1B, HIGH);
  digitalWrite(MOTOR_2A, LOW);
  digitalWrite(MOTOR_2B, HIGH);
  checkDistance = 1;
}

void forward()
{
  digitalWrite(MOTOR_1A, HIGH);
  digitalWrite(MOTOR_1B, LOW);
  digitalWrite(MOTOR_2A, HIGH);
  digitalWrite(MOTOR_2B, LOW);
  checkDistance = 0;
}

void right()
{
  digitalWrite(MOTOR_1A, LOW);
  digitalWrite(MOTOR_1B, HIGH);
  digitalWrite(MOTOR_2A, HIGH);
  digitalWrite(MOTOR_2B, LOW);
  checkDistance = 0;
}

void left()
{
  digitalWrite(MOTOR_1A, HIGH);
  digitalWrite(MOTOR_1B, LOW);
  digitalWrite(MOTOR_2A, LOW);
  digitalWrite(MOTOR_2B, HIGH);
  checkDistance = 0;
}

void stop()
{
  digitalWrite(MOTOR_1A, LOW);
  digitalWrite(MOTOR_1B, LOW);
  digitalWrite(MOTOR_2A, LOW);
  digitalWrite(MOTOR_2B, LOW);
  checkDistance = 0;
}

...

For example, the Tamiya double gearbox includes two low cost DC motors. The gearbox is amazing for small robotic projects but heading straight with these motors can be much more difficult than expected.

This is where the L293 chip comes to the rescue. This chip can control both the direction and the speed of the motors. To control the speed of the motors you connect the two PWM digital outputs of the Arduino micro-controller to the enable input pins 1 and 9 of the chip. As explained in the datasheet: “When an enable input is high, the associated drivers are enabled and their outputs are active and in phase with their inputs”.

You are now able to adjust the motor speeds by changing the PWM output value of the enable input of the faster motor. PWM outputs range from 0 to 255 so, to slow down a motor by a ratio of 10% you should set the associated PWM output to 230. Trial an error is the only way to get that done and make your robot head in a straight line.

void setup()
{
...

  pinMode(10, OUTPUT); // To L293 pin #1
  pinMode(11, OUTPUT); // To L293 pin #9
  analogWrite(10, 185); // Faster motor at 72% of its full speed
  analogWrite(11, 255); // Slower motor at 100% of its full speed

...
}

What’s Next

In a future article we will see how to use the shield to drive a simple Robot controlled by an infrared remote control. This project will pack together many tutorials published on this blog. If you have worked with the L293 chip to control motors, let us know in the comment section below.