Lesson 8: Sensor-Controlled Outputs with Arduino

Use sensor readings to control LEDs, buzzers, and other outputs

32-Lesson Course • Semester 1

Now that you can read sensor values with analogRead(), you'll learn to use those values to make decisions and control outputs like LEDs. This combines everything you've learned: conditionals, loops, functions, and sensors.

Learning Objectives

By the end of this Lesson, you will:

Use sensor values in conditional statements
Control LEDs based on sensor thresholds
Write functions that respond to sensor data
Create a complete sensor-controlled project

Skills Developed:

Combining sensors with output devices
Setting thresholds for sensor decisions
Building responsive hardware systems
Debugging sensor-controlled circuits

Section 1: From Input to Output

In Lesson 7, you learned to read sensor values. Now you'll use those values to control outputs. The pattern is simple: read → decide → act.

The control loop pattern

Read the sensor with analogRead()
Compare the value to a threshold (conditionals)
Control the output accordingly

This pattern works for any sensor and any output. For example: read a light sensor, check if it's dark, turn on an LED.

Section 2: LED Controlled by Potentiometer

Let's start with a simple example: turn on an LED when the potentiometer reads above a certain value.

Hardware setup: potentiometer and LED

Potentiometer (from Lesson 7)

Left pin → 5V
Middle pin → A0
Right pin → GND

LED (with resistor)

Long leg → Pin 13 (through 220Ω resistor)
Short leg → GND

const int sensorPin = A0;
const int ledPin = 13;

void setup() {
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
}

void loop() {
  int sensorValue = analogRead(sensorPin);
  Serial.print("Sensor: ");
  Serial.println(sensorValue);

  if (sensorValue > 500) {
    digitalWrite(ledPin, HIGH);  // LED on
  } else {
    digitalWrite(ledPin, LOW);   // LED off
  }

  delay(200);
}

What happens?

When sensorValue > 500, the LED turns on
When sensorValue <= 500, the LED turns off
The threshold (500) is roughly in the middle of the 0-1023 range

Section 3: Choosing a Threshold

The threshold value (like 500) is important. It determines when your output changes. Here's how to think about it:

Finding a good threshold

Run the sensor reading program from Lesson 7
Watch the Serial Monitor or Plotter
Note the values when the condition is true/false
Pick a value in between

Example thresholds

Potentiometer: 512 (middle)
Light sensor: 300 (dim/dark)
Temperature: depends on your sensor

Section 4: Using Functions for Sensor Control

You can organize your code with functions (from Lesson 6). This makes it easier to read and maintain.

const int sensorPin = A0;
const int ledPin = 13;

void setup() {
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
}

void loop() {
  checkSensorAndControlLED();
  delay(200);
}

void checkSensorAndControlLED() {
  int sensorValue = analogRead(sensorPin);
  Serial.print("Sensor: ");
  Serial.println(sensorValue);

  if (sensorValue > 500) {
    turnOnLED();
  } else {
    turnOffLED();
  }
}

void turnOnLED() {
  digitalWrite(ledPin, HIGH);
  Serial.println("LED is ON");
}

void turnOffLED() {
  digitalWrite(ledPin, LOW);
  Serial.println("LED is OFF");
}

Using functions makes the code more organized. Each function has a clear job: read the sensor, turn the LED on, or turn the LED off.

Section 5: Multiple Thresholds

You can use multiple thresholds to create different response levels. For example, show "Low", "Medium", or "High" based on the sensor value.

const int sensorPin = A0;
const int ledPin = 13;

void setup() {
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
}

void loop() {
  int sensorValue = analogRead(sensorPin);
  Serial.print("Sensor: ");
  Serial.print(sensorValue);
  Serial.print(" - Level: ");
  printSensorLevel(sensorValue);
  delay(300);
}

void printSensorLevel(int value) {
  if (value < 341) {
    Serial.println("Low");
    digitalWrite(ledPin, LOW);
  } else if (value < 682) {
    Serial.println("Medium");
    digitalWrite(ledPin, HIGH);
  } else {
    Serial.println("High");
    digitalWrite(ledPin, HIGH);
  }
}

How it works

0-340: Low → LED off
341-681: Medium → LED on
682-1023: High → LED on
You can add more LEDs for visual feedback

Section 6: Looping with Sensors

You can combine sensor readings with loops (from Lesson 5). For example, take 5 readings and average them for a more stable value.

const int sensorPin = A0;
const int ledPin = 13;

void setup() {
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
}

void loop() {
  int averageValue = getAverageReading();
  Serial.print("Average: ");
  Serial.println(averageValue);
  controlLED(averageValue);
  delay(500);
}

int getAverageReading() {
  int sum = 0;
  for (int i = 0; i < 5; i++) {
    sum += analogRead(sensorPin);
    delay(50);
  }
  return sum / 5;
}

void controlLED(int value) {
  if (value > 500) {
    digitalWrite(ledPin, HIGH);
    Serial.println("LED ON");
  } else {
    digitalWrite(ledPin, LOW);
    Serial.println("LED OFF");
  }
}

Why average readings?

Sensor readings can be noisy. Taking multiple readings and averaging them gives a smoother, more stable value. This is a common technique in real-world projects.

Section 7: Common Mistakes

Mistake 1

Forgetting pinMode(ledPin, OUTPUT) in setup().

Mistake 2

Using the wrong pin number (e.g., A0 for LED output instead of a digital pin).

Mistake 3

Not using a resistor with the LED, which can damage the LED.

Mistake 4

Choosing a threshold that's too close to the edge of the range.

Hands-On Activity: Light-Activated LED

Create a system that turns on an LED when it's dark. You can use a potentiometer to simulate a light sensor, or if you have a photoresistor, use that instead.

Steps:

Wire a potentiometer to A0 (or use a light sensor)
Wire an LED with resistor to pin 13
Write a program that turns the LED on when the sensor reads below a threshold (dark)
Test by covering/uncovering the sensor or turning the potentiometer
Use functions to organize your code

Extension: Add a second LED and create three brightness levels: dim, medium, bright.

Sneak Peek: Variable Brightness

Arduino has special pins (marked with ~: 3, 5, 6, 9, 10, 11) that can dim an LED smoothly instead of just on/off. Using analogWrite(), a sensor can control brightness like a dimmer switch. We'll cover this in a future lesson!

Assessment and Homework

Lesson 8 quiz topics

Using sensor values in conditionals
Choosing thresholds for sensor decisions
Controlling LEDs based on sensor input
Writing functions for sensor control
Using loops with sensor readings

Homework ideas

Create a sensor-controlled LED system
Experiment with different threshold values
Write a function that returns a sensor "level" (low/medium/high)
Try averaging multiple sensor readings
Be ready to build a complete project next

You now have all the pieces: reading sensors, making decisions, and controlling outputs. This is the foundation for building real projects!

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