Using Transistors and Driving Motors - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 76

Operating an H-bridge Circuit
Next up, you write a program to control the motor's direction and speed using
the potentiometer and the H-bridge. Setting the potentiometer in a middle range
stops the motor, setting the potentiometer in a range above the middle increases
the speed forward, and setting the potentiometer in a range below the middle
increases the speed backward. This is another perfect opportunity to employ
functions in your Arduino program. You can write a function to stop the motor,
one to cause it spin forward at a set speed, and one to cause it to spin backward
at a set speed. Ensure that you correctly disable the H-bridge at the beginning of
the function before changing the motor mode; doing so reduces the probability
that you will make a mistake and accidentally short out the H-bridge.
Following the logic diagram from Figure 4-6, you can quickly figure out how
you need to control the pins to achieve the desired results:
■ To stop current low through the device, set the enable pin low.
■ To set the switches for rotation in one direction, set one high, the other low.
■ To set switches for rotation in the opposite direction, swap which is high
and which is low.
■ To cause the motor to stop immediately, set both switches low.
NOTE Alwaysdisablethecurrentflowbeforechangingthestateoftheswitches
toensurethatamomentaryshortcannotbecreatedastheswitchesflip.
First, you should devise the functions that safely execute the previously
described motions. Create a new Arduino sketch and start by writing your
new functions:
//Motor goes forward at given rate (from 0-255)
void forward (int rate)
{
digitalWrite(EN, LOW);
digitalWrite(MC1, HIGH);
digitalWrite(MC2, LOW);
analogWrite(EN, rate);
}
//Motor goes backward at given rate (from 0-255)
void reverse (int rate)
{
digitalWrite(EN, LOW);
digitalWrite(MC1, LOW);
digitalWrite(MC2, HIGH);
analogWrite(EN, rate);