Spring 2021 · ECE 3724, Mississippi State University

Line Follower Robot, from 3D Model to PCB

A two-sensor line follower built around a Microchip dsPIC33EP128GP502 on a board I designed for ECE 3724. I modelled the chassis in SolidWorks, 3D printed it, and generated the motor PWM from timer interrupts.

dsPIC33 / PIC24
PCB design (KiCad)
Timer interrupts
L293D
SolidWorks
MPLAB X

Watch the demo Original write-up on Hackster

Assembled line follower: black custom PCB with the dsPIC33 and L293D, two yellow gear motors, and two TCRT5000 sensor boards on a grey 3D-printed nose — The finished robot: custom PCB, 3D-printed chassis, two gear motors and two TCRT5000 sensor modules.

$44.86Total parts cost

1 kHzSoftware PWM

30 %Motor duty cycle

2IR line sensors

At a glance

MCU: Microchip dsPIC33EP128GP502 (28-pin DIP), from the course kit
Firmware: C in MPLAB X, using the pic24_all.h support library
Motor driver: L293D dual H-bridge, two DC gear motors on a 9 V supply
Sensors: Two TCRT5000 IR reflective modules, read as digital inputs
Board: Custom PCB designed in KiCad, with an LM2937 regulator, FTDI serial header, reset button and status LED
Chassis: Modelled in SolidWorks, 3D printed

How it steers

The two sensors sit side by side at the front, one on each side of the line. Each module outputs 3.3 V over white and 0 V over black, so every pass of the main loop is a two-bit lookup:

Sensor A	Sensor B	Action
White	White	Line is between the sensors: drive forward
White	Black	Turn left
Black	White	Turn right
Black	Black	Reverse (placeholder that still drives forward)

A turn stops one wheel and keeps the other one driving.

Motor PWM from two timers

The GP variant of the dsPIC33 has no motor-control PWM module, so I generated PWM in software. Each motor's L293D enable pin is toggled by its own timer interrupt: Timer2 for the left motor and Timer3 for the right. The ISR alternates the timer period between 300 µs (enable high) and 700 µs (enable low), which gives a 1 kHz PWM at 30 % duty and runs the motors from the 9 V supply at reduced speed.

line_follower.c (excerpt)

//create PWM for left motor
void createPWM_Left(void){
    //700+300 = 1000us = 1ms results in 1KHz PWM with 30% duty cycle.
    if (MOTOR_1_EN == 1) {
        PR2 = usToU16Ticks(700, getTimerPrescale(T2CONbits)) - 1;
        MOTOR_1_EN = 0;
    } else {
        PR2 = usToU16Ticks(300, getTimerPrescale(T2CONbits)) - 1;
        MOTOR_1_EN = 1;
    }
}

// Interrupt Service Routine for Timer2 which is for LEFT motor
void _ISR _T2Interrupt(void) {
   createPWM_Left();
  _T2IF = 0;
}

The full source, including the sensor and timer setup, is in the original code download.

Board and chassis

The board carries the regulator, the dsPIC33, the L293D, an FTDI header for serial, a reset button and screw terminals for the motors. Select an image to enlarge it.

Schematic: power management, MCU and LED, FTDI-to-TTL connector, sensor headers and L293D motor driver — Schematic: power, MCU, FTDI-to-TTL connector, sensors and motor driver.

3D render of the PCB from above — 3D render, top

3D render of the PCB at an angle — 3D render, angled

Bare black PCB with the Designed by Ajaya Dahal for ECE 3724 silkscreen — Bare PCB

Assembled PCB with the dsPIC33, L293D, regulator, capacitors and screw terminals — Assembled PCB

SolidWorks model of the robot with the board, gear motors and wheels — SolidWorks model of the chassis, board and wheels.

Bill of materials

Part	Notes	Cost
Custom PCB	Designed for this project	$15.90
Gear motors	DC gear motors	$11.99
L293D motor driver	Dual H-bridge	$7.99
TCRT5000 IR sensors	Reflective sensor modules	$8.98
dsPIC33EP128GP502 and support parts	From the course kit	$0.00
Chassis	3D printed	$0.00
Total		$44.86

Demo

Final project video. The YouTube player loads only when you press play.

What I would change

Read the sensors as analog values instead of thresholded bits.
Replace the if/else steering with a proportional (then PI) controller that sets each motor's duty cycle from the left/right sensor difference.
Move from two sensors to a five-sensor IR array.