Designing my own line follower

Hello All,

So I’ve been designing my own line follower for a specific project that I have. The 3pi is supposed to go around the track and if it detects a black line that is horizontal on the line path then it is supposed to stop at a box at the end of the track.

Ex) if the 3pi sees 2 horizontal black lines then it stops at box 2.

So I have created some code that uses specific sensors instead of all of them from the line_following example that should allow me to detect the line.

My problem is that when it gets to a specific part of my code then it quits, meaning everything is wiped from the display of the bot and it remains on without doing anything else.

If someone could help me out as to why it is doing that as well as maybe to suggest some code for me to implement then I would appreciate that!

It follows the line, slowly, but it quits when it detects the horizontal black line.

The Code


// The 3pi include file must be at the beginning of any program that
// uses the Pololu AVR library and 3pi.
#include <pololu/3pi.h>
#include <math.h>
#include <stdlib.h> //used for the abs function
#include <stdio.h>
#include <avr/pgmspace.h>
const char welcome[] PROGMEM = ">g32>>c32"; //sound used for debugging...
#include <stdbool.h>

void rotate_cw(); //function to turn clock-wise...
void rotate_ccw();
void display_battery(); //displays battery voltage...

int main()
	bool seen_whiteline = 1; //automatically set to it has detected "whiteness"
	int dark_threshold = 1000; //black line be seen with more than this
	//int light_threshold = 300; //white line should be seen with less than this...
	int l_counter = 0; //counter for the line...
	//int sq_counter = 0; //counter for the square...
	pololu_3pi_init(2000); //initialize the max value on the 3pi @ 2000...
	lcd_init_printf();	// required if we want to use printf()
	unsigned int sensor_values[5]; //initialize the array for the sensors...
		read_line_sensors(sensor_values,IR_EMITTERS_ON); //function to turn onthe IR sensors...
		bool sees_blackline = 0; //has not yet detected black...
		int leftsum = sensor_values[0] + sensor_values[1]; //variables for sensors 0 and 1...
		int rightsum = sensor_values[3] + sensor_values[4]; //variables for sensors 3 and 4...
		printf("%d\n",abs(leftsum-rightsum)); //display the value for the computation of their difference...
		delay_ms(1000); //delay the reading
		clear(); //clear the reading from the display
		int threshold = 500; //if above the threshold rotate...because you would be off the line...
		//otherwise if threshold is met or under met then keep moving forward...
		if(((leftsum-rightsum) > 0) && (abs(leftsum-rightsum) > threshold)) 
		//check if the computation of the difference is greater than zero and if it's greater than the threshold...
			//turn counter-clockwise if sensor 0 and 1 see to much black...
		else if(((rightsum - leftsum) > 0) && ((abs(rightsum - leftsum)) > threshold))
			//turn clockwise if sensor 3 and 4 see to much black...
		else if(((sensor_values[0] > dark_threshold) && (sensor_values[1] > dark_threshold) && (sensor_values[2] > dark_threshold) && (sensor_values[3] > dark_threshold) && (sensor_values[4] > dark_threshold)) && (!sees_blackline && seen_whiteline))
			sees_blackline = 1;
			seen_whiteline = 0; //hasn't seen a new white line...
			l_counter += 1;
				//robot gets stuck over here...convert to an if statement instead???
				while((((sensor_values[0] > dark_threshold) && (sensor_values[1] > dark_threshold)) || ((sensor_values[3] > dark_threshold) && (sensor_values[4] > dark_threshold))) && (sees_blackline && !seen_whiteline))
				//(while sensors 0 & 1 OR sensors 3 & 4 see black) AND the robot has not yet detected a new white line...execute...
				//reset the booleans back...
				seen_whiteline = 1; //it saw the white line...

void rotate_cw()
void rotate_ccw()

void display_battery()
		int bat = read_battery_millivolts();

		print("Press B");

Thank You


I looked over your code briefly and noticed that it does not look like it is trying to follow a line (e.g. it just sets the motor speeds directly). It also has a lot of delays in it, which could cause it to miss a line altogether. If you are trying to follow a line and detect another line that is perpendicular to it, you might look at the how the “Maze Solving” example handles it.

It is hard to determine why the 3pi might be stopping like you described. Have you checked that your batteries are fully-charged or fresh? From the comments in your code, it sounds like you think it is getting stuck in the while loop; however, the while loop is commanding the motors to spin, which does not match the behavior your described of it stopping. You might try simplifying your code to try to determine if part of it is causing the problem.


Hello Brandon,

You say that my code does not look like it is trying to follow a line. However, the 3pi would keep rotating towards the line until a sees a specific value (the threshold), once it sees the threshold then it continues on straight until it veers off a little, then it continues rotating until it see the threshold again. Yes it’s a crude method of accomplishing a line follower but I need access certain sensors with my project and the line_follower examples would not help me in that way as they use all of the sensors.

But I will take a look at the maze solving example. If this is one code where it can access certain sensors and make decisions based on that then that would be helpful.