Light-Seeking Robot

The finish line

What you will build

The robot turns toward a diffuse stationary light from three starting headings and stops within 30 cm in four of five trials.

Before you build

Materials, tools, and safety

Reuse-material cost: Usually under $5 with an existing kit. Supervision: Adult guidance recommended for wiring and cutting.

Build it

Step-by-step instructions

1. Step 1

   Build and align the rover

   Match wheels and confirm straight unpowered rolling.

   Keep the front deck clear.

2. Step 2

   Mount paired sensors

   Place sensors side by side with an opaque divider centered between them.

   Aim both level and forward.

3. Step 3

   Wire sensors and driver

   Use rated sensor voltage, H-bridge motor power, and common ground.

   Label left and right channels.

   Builder checkpoint: After wire sensors and driver, the first subassembly should stay aligned when handled gently.

4. Step 4

   Calibrate darkness

   Record each sensor under even room light and under the target at equal distance.

   Normalize mismatched ranges.

   Watch for: If this stage binds or drifts, inspect motor mismatch before adding more parts.

5. Step 5

   Test direction readings

   Rotate the robot through left, center, and right positions with motors off.

   Confirm the expected sign of error.

6. Step 6

   Run wheels raised

   Command slow left and right corrections from a moved flashlight.

   Verify steering direction.

   Builder checkpoint: After run wheels raised, operate the build slowly and confirm that steering toward higher light intensity begins without binding.

7. Step 7

   Add stop behavior

   Use average brightness or an ultrasonic limit to stop near the source.

   Start with a conservative threshold.

8. Step 8

   Run three-heading trials

   Begin left, right, and backward from the same radius.

   Record approach, final distance, and false turns.

   Builder checkpoint: At the final checkpoint, The robot turns toward a diffuse stationary light from three starting headings and stops within 30 cm in four of five trials.

See the engineering

Why it works

Input

left and right light-sensor readings

Output

steering toward higher light intensity

Motion

light gradient-to-differential wheel motion

Energy losses

ambient light, sensor mismatch, wheel slip, motor mismatch

const int leftLight=A0, rightLight=A1;
const int leftPwm=5, rightPwm=6, leftDir=7, rightDir=8;
const int baseSpeed=95, stopLevel=850;
void setup(){
  pinMode(leftPwm,OUTPUT); pinMode(rightPwm,OUTPUT);
  pinMode(leftDir,OUTPUT); pinMode(rightDir,OUTPUT);
  digitalWrite(leftDir,HIGH); digitalWrite(rightDir,HIGH);
}
void loop(){
  int left=analogRead(leftLight), right=analogRead(rightLight);
  int average=(left+right)/2;
  if(average>stopLevel){ analogWrite(leftPwm,0); analogWrite(rightPwm,0); return; }
  int correction=constrain((left-right)/3,-70,70);
  analogWrite(leftPwm,constrain(baseSpeed-correction,0,180));
  analogWrite(rightPwm,constrain(baseSpeed+correction,0,180));
  delay(15);
}

Make it behave

Test, troubleshoot, and tune

Keep experimenting

Try another version

Build together

Classroom and access options

Reflect on the design

1. How did sensor divider depth change the measured result?
2. Where did ambient light affect the build most strongly?
3. What evidence shows that differential light sensing explains the motion?
4. Which change would improve steering toward higher light intensity without creating a new problem?