📖 Lesson 3.4 – Threshold Value Logic

🎯 Learning Objectives

After completing this lesson, students will be able to:

✅ Understand what a threshold value is

✅ Understand why threshold values are important

✅ Calibrate an LDR sensor properly

✅ Select suitable threshold values for different environments

✅ Improve the performance of automatic lighting systems

✅ Avoid false triggering problems

1. Introduction

In the previous lesson, we used this condition:

if(ldrValue < 400)
{
   digitalWrite(13, HIGH);
}
else
{
   digitalWrite(13, LOW);
}

But why did we choose:

400

Why not:

200

or

800

The answer is:

Threshold Value

A threshold value is the decision point that tells Arduino when to perform an action.

2. What is a Threshold Value?

A threshold value is a predefined sensor reading used to make decisions.

Think of it as a boundary line.

Example:

Threshold = 400

If:

LDR < 400

Darkness detected

↓

LED ON

If:

LDR > 400

Light detected

↓

LED OFF

3. Real-Life Example

Imagine a school examination.

Passing Marks:

40

If student scores:

45

Pass

If student scores:

35

Fail

The value:

40

acts as a threshold.

Similarly, Arduino uses threshold values for decision making.

4. Why Do We Need Threshold Values?

Sensors continuously produce changing values.

Example:

615
620
618
610
605

Arduino needs a point where it decides:

Bright

or

Dark

That decision point is called the threshold value.

5. Understanding LDR Readings

Every environment produces different readings.

Example:

Bright Sunlight

900 – 1023

Room Light

500 – 800

Evening

250 – 500

Darkness

0 – 250

These values vary from place to place.

6. Selecting a Threshold Value

Suppose your readings are:

A reasonable threshold would be:

400

Because:

Above 400

Bright Environment

Below 400

Dark Environment

7. Testing Sensor Values

Before selecting a threshold:

Always measure actual values.

Upload this program:

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  Serial.println(analogRead(A0));

  delay(200);
}

Observe readings in:

• Morning
• Afternoon
• Evening
• Night

8. Creating a Reading Table

Students should create a table like this:

This helps determine the correct threshold.

9. Example Threshold Selection

Suppose:

A suitable threshold:

400

This cleanly separates bright and dark conditions.

10. What Happens if Threshold is Too Low?

Example:

if(ldrValue < 100)

Problem:

Only complete darkness activates the LED.

Evening light may not trigger the system.

Result:

Poor performance.

11. What Happens if Threshold is Too High?

Example:

if(ldrValue < 900)

Problem:

LED may remain ON almost all the time.

Even normal daylight may trigger the LED.

Result:

False activation.

12. Finding the Best Threshold

A good threshold:

✅ Detects darkness correctly

✅ Ignores normal daylight

✅ Avoids false triggering

✅ Provides stable operation

Example

Darkness Reading:

150

Room Light:

700

Choose:

400

Middle value.

13. Updating the Program

Example:

int threshold = 400;

Program:

int threshold = 400;
int ldrValue;

void setup()
{
  pinMode(13, OUTPUT);

  Serial.begin(9600);
}

void loop()
{
  ldrValue = analogRead(A0);

  if(ldrValue < threshold)
  {
    digitalWrite(13, HIGH);
  }
  else
  {
    digitalWrite(13, LOW);
  }
}

14. Making Threshold Adjustable

Instead of hardcoding:

400

Use:

int threshold = 400;

Advantages:

• Easier calibration
• Easier modifications
• Better code readability

15. Understanding Sensor Calibration

Calibration means adjusting the system to work correctly in a specific environment.

Example:

Classroom

Threshold:

450

Outdoor Garden

Threshold:

600

Street Light

Threshold:

500

Different environments require different calibration values.

16. Real-World Example

Automatic Street Light System:

Day:

LDR = 850

LED OFF

Evening:

LDR = 380

LED ON

Night:

LDR = 120

LED ON

This entire decision is based on threshold logic.

17. Industrial Applications

Threshold logic is used in:

Automatic Street Lights

Water Tank Systems

Fire Detection Systems

Gas Leakage Detection

Temperature Monitoring

Smart Agriculture

Industrial Automation

18. Common Beginner Mistakes

Mistake 1

Using random threshold values.

Always measure sensor readings first.

Mistake 2

Copying threshold values from the internet.

Every environment is different.

Mistake 3

Not recalibrating after changing location.

Mistake 4

Ignoring sensor testing.

Always verify readings using Serial Monitor.

19. Best Practices

✅ Observe sensor readings first

✅ Create a reading table

✅ Select middle-range threshold values

✅ Test in different lighting conditions

✅ Recalibrate when necessary

📊 Summary

In this lesson, we learned:

✅ What a threshold value is

✅ Why threshold values are important

✅ How Arduino uses threshold logic

✅ Sensor calibration

✅ Selecting the correct threshold value

Threshold values are the foundation of automation because they allow Arduino to make intelligent decisions based on sensor readings.

📖 Key Terms

Threshold Value

A reference value used for decision making.

Calibration

Adjusting sensor settings for proper operation.

Analog Reading

Sensor value between 0 and 1023.

Automation Logic

Decision-making process used in automatic systems.

False Triggering

Incorrect activation of a system.

🎯 Quiz

1. What is a threshold value?

A. Power Supply

B. Reference value used for decisions ✅

C. Sensor Type

D. Programming Language

2. Why do we use threshold values?

A. To power sensors

B. To upload programs

C. To separate different conditions ✅

D. To increase voltage

3. What should be done before selecting a threshold?

A. Buy a new sensor

B. Measure actual readings ✅

C. Change Arduino

D. Replace LED

4. What happens if the threshold is too high?

A. Sensor stops working

B. False triggering may occur ✅

C. Arduino resets

D. LED burns

5. Calibration means:

A. Deleting code

B. Adjusting the system for correct operation ✅

C. Installing software

D. Charging batteries

🏠 Assignment

Task 1

Record LDR readings in different lighting conditions.

Task 2

Determine the best threshold value for your classroom.

Task 3

Test three threshold values (300, 500, 700) and compare results.

Task 4

Explain why calibration is important in sensor-based projects.

Task 5

Create a chart showing LED behavior for different LDR readings.