Page 90 - The Unofficial Guide to Lego Mindstorms Robots

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arbitrarily. Next, main configure s input 2 for a light sensor. Finally, it starts the lightWatcher task.

Using Two Light Sensors

In this section, I'll present an NQC program that wor ks with the two light sensor version of Trusty. As you may recall,
programming this robot in RCX Code was cumbersome.

The programming is a lot cleaner in NQC. It's pretty straightforward to translate Table 3-1 into source code. The basic strategy
is to use a state variable to repr esent the four states of the robot, represented by the four lines of Table 3-1. Then one task
examines the sensors and updates the state variable. Another task examines the state variable and sets the motors appropriately.

The four possible states are r epresented by constant values. A fifth value, INDETERMINATE, is used when one or both of the
light sensor values is not in the d ark or light range:

#define BOTH_ON 3
#define LEFT_ON 1
#define RIGHT_ON 2
#define BOTH_OFF 0
#define INDETERMINATE 255

The main task simply tests the value of the state va riable and sets the motors accordingly. No action is taken for
BOTH_OFF and INDETERMINATE:

while (true) {
if (state == BOTH_ON)
OnFwd(OUT_A + OUT_C);
else if (state == LEFT_ON) {
Off(OUT_A);
OnFwd(OUT_C);
}
else if (state == RIGHT_ON) {
Off(OUT_C);
OnFwd(OUT_A) ;
}
}

A separate task, watcher, exam ines the light sensor values and sets the state variable. Here is the entire source code for
the two sensor version of Trusty:

int state;

// "ON" re fers to whether the light
// sensor is on the line. If it is,
// the light sensor is seeing black.
#define BOTH_ON 3
#define LEFT_ON 1