PHOTOVORE // PCB Robot Project

01 // Overview

How it works

The photovore is a biologically inspired robot that chases light the way a moth chases a flame. Two light-dependent resistors mounted at the front act as "eyes," forming voltage dividers with 10k pulldown resistors. The ATmega328P reads both analog values, computes the brightness differential, and steers by adjusting individual motor speeds through an L298N dual H-bridge.

When more light falls on the left eye, the left motor slows down and the right motor speeds up, causing the robot to arc toward the light source. A configurable dead zone prevents jittery oscillation when both sensors read nearly equal values. Below a darkness threshold, the robot stops and waits.

The entire circuit runs on a custom-designed PCB powered by a 7.4V 2S LiPo, regulated down to 5V for the microcontroller while the motors run on the raw pack voltage for maximum torque.

◎

Differential sensing

Two LDRs in voltage divider configuration feed analog pins A0 and A1. Brightness difference drives proportional steering.

⚙

PWM motor control

L298N H-bridge with PWM-capable enable pins allows smooth, variable-speed control for each wheel independently.

⚡

Clean power design

AMS1117-5.0 regulator with dual decoupling caps. Separate motor and logic power rails prevent noise coupling.

⚙

Tunable behavior

Configurable turn gain, dead zone, speed limits, and dark threshold. Serial debug output for real-time tuning.

02 // Schematic

PCB circuit design

MCU Pin	Net	Connection	Function
A0	SENSE_L	LDR-L / R1 divider	Left light sensor input
A1	SENSE_R	LDR-R / R2 divider	Right light sensor input
D5 (PWM)	ENA	L298N Enable A	Left motor speed
D6	IN1	L298N Input 1	Left motor direction A
D9	IN2	L298N Input 2	Left motor direction B
D10 (PWM)	ENB	L298N Enable B	Right motor speed
D7	IN3	L298N Input 3	Right motor direction A
D8	IN4	L298N Input 4	Right motor direction B
D13	STATUS	LED + R3 220R	Status indicator

03 // Firmware

Control code

photovore_robot.ino

// Photovore Robot Firmware
// ATmega328P / Arduino Nano / Pro Mini

// Pin definitions
const int LDR_LEFT_PIN  = A0;
const int LDR_RIGHT_PIN = A1;

const int ENA_PIN = 5;    // Left motor speed (PWM)
const int IN1_PIN = 6;    // Left motor dir A
const int IN2_PIN = 9;    // Left motor dir B
const int ENB_PIN = 10;   // Right motor speed (PWM)
const int IN3_PIN = 7;    // Right motor dir A
const int IN4_PIN = 8;    // Right motor dir B
const int LED_PIN = 13;

// Tuning parameters
const int   BASE_SPEED     = 150;
const int   MAX_SPEED      = 255;
const int   MIN_SPEED      = 60;
const int   DEAD_ZONE      = 30;
const int   DARK_THRESHOLD = 100;
const float TURN_GAIN      = 0.8;

void setLeftMotor(int speed) {
  if (speed >= 0) {
    digitalWrite(IN1_PIN, HIGH);
    digitalWrite(IN2_PIN, LOW);
    analogWrite(ENA_PIN, constrain(speed, 0, MAX_SPEED));
  } else {
    digitalWrite(IN1_PIN, LOW);
    digitalWrite(IN2_PIN, HIGH);
    analogWrite(ENA_PIN, constrain(-speed, 0, MAX_SPEED));
  }
}

void setRightMotor(int speed) {
  if (speed >= 0) {
    digitalWrite(IN3_PIN, HIGH);
    digitalWrite(IN4_PIN, LOW);
    analogWrite(ENB_PIN, constrain(speed, 0, MAX_SPEED));
  } else {
    digitalWrite(IN3_PIN, LOW);
    digitalWrite(IN4_PIN, HIGH);
    analogWrite(ENB_PIN, constrain(-speed, 0, MAX_SPEED));
  }
}

void loop() {
  int leftVal  = analogRead(LDR_LEFT_PIN);
  int rightVal = analogRead(LDR_RIGHT_PIN);
  int diff = leftVal - rightVal;
  int avg  = (leftVal + rightVal) / 2;

  if (avg < DARK_THRESHOLD) {
    stopMotors();            // Too dark, stop
  } else if (abs(diff) < DEAD_ZONE) {
    setLeftMotor(BASE_SPEED);  // Go straight
    setRightMotor(BASE_SPEED);
  } else {
    int turn = (int)(abs(diff) * TURN_GAIN);
    int lSpd, rSpd;
    if (diff > 0) {
      lSpd = BASE_SPEED - turn;  // Turn left
      rSpd = BASE_SPEED + turn;
    } else {
      lSpd = BASE_SPEED + turn;  // Turn right
      rSpd = BASE_SPEED - turn;
    }
    setLeftMotor(constrain(lSpd, MIN_SPEED, MAX_SPEED));
    setRightMotor(constrain(rSpd, MIN_SPEED, MAX_SPEED));
  }
  delay(20);
}

04 // Bill of materials

Components

ATmega328P

Arduino Nano or Pro Mini module

AMS1117-5.0

5V LDO voltage regulator, SOT-223

L298N module

Dual H-bridge motor driver

2x LDR (5mm)

Light dependent resistors, CdS type

2x 10k resistors

Voltage divider pulldowns, 1/4W

1x 220R resistor

LED current limiter

100uF + 100nF caps

Decoupling, electrolytic + ceramic

2x DC geared motors

3-12V with wheels attached

7.4V 2S LiPo

Battery pack with JST connector

Toggle switch

SPST power switch

3mm LED

Red, status indicator

Screw terminals + headers

Motor connections, programming pins

PHOTOVORE ROBOT