Intermediate Level • Lesson 5

📡 Reading Sensors

⏱️ 50 minutes 📚 Intermediate 🎯 Sensor Input

📡

Reading Sensors

Learn how to read input from buttons and sensors - this lets your programs interact with the real world!

What are Sensors?

Sensors are devices that detect things in the real world - like temperature, light, sound, or movement. They send information to your Raspberry Pi so your program can react to what's happening around it!

💡 Real-World Example: Your phone has many sensors - it knows when you tilt it (gyroscope), how bright it is (light sensor), and when you touch the screen (touch sensor). Your Raspberry Pi can use similar sensors!

GPIO Pins: Input vs Output

GPIO pins can be used in two ways:

📤

Output

Send signals OUT (like turning LEDs on/off)

GPIO.setup(pin, GPIO.OUT)

📥

Input

Read signals IN (like reading button presses)

GPIO.setup(pin, GPIO.IN)

Reading Button Input

Buttons are the simplest input device. When you press a button, it connects two wires, sending a signal to your Raspberry Pi.

Basic Button Setup

import RPi.GPIO as GPIO
import time

# Set up button on GPIO 2
GPIO.setmode(GPIO.BCM)
GPIO.setup(2, GPIO.IN, pull_up_down=GPIO.PUD_UP)

# Read button state
button_state = GPIO.input(2)

if button_state == GPIO.LOW:
    print("Button is pressed!")
else:
    print("Button is not pressed")

Understanding the Code:

GPIO.IN: Sets the pin as an input (for reading)
GPIO.PUD_UP: Enables pull-up resistor (keeps pin HIGH when button not pressed)
GPIO.input(2): Reads the current state of pin 2
GPIO.LOW: Means button is pressed (connected to ground)
GPIO.HIGH: Means button is not pressed

Button Wiring

How to Wire a Button:

One side of button: Connect to GPIO pin (e.g., GPIO 2)
Other side of button: Connect to GND (ground)
Pull-up resistor: Use GPIO.PUD_UP in code (built-in on Raspberry Pi)

Note: When button is not pressed, pin reads HIGH. When pressed, it connects to GND and reads LOW.

Reading Button in a Loop

To continuously check if a button is pressed, use a loop:

Continuous Button Reading

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(2, GPIO.IN, pull_up_down=GPIO.PUD_UP)
GPIO.setup(18, GPIO.OUT)  # LED

try:
    while True:
        button_state = GPIO.input(2)
        
        if button_state == GPIO.LOW:
            GPIO.output(18, GPIO.HIGH)  # Turn LED on
            print("Button pressed - LED ON")
        else:
            GPIO.output(18, GPIO.LOW)   # Turn LED off
            print("Button not pressed - LED OFF")
        
        time.sleep(0.1)  # Check every 0.1 seconds

except KeyboardInterrupt:
    GPIO.cleanup()

Reading Analog Sensors

Some sensors give analog values (like temperature or light level). Raspberry Pi GPIO pins are digital, so we need special techniques:

🌡️

Temperature Sensors

DS18B20 (digital) or DHT11/DHT22 (digital)

Read temperature values

💡

Light Sensors

LDR (Light Dependent Resistor)

Detect light levels

📏

Distance Sensors

Ultrasonic (HC-SR04)

Measure distance

Project: Interactive Button-Controlled LED System

Let's build a system where pressing a button controls an LED!

Materials Needed:

Raspberry Pi
Push button
LED (any color)
Resistor (220Ω for LED, 10kΩ for button - optional)
Breadboard and jumper wires

Wiring Instructions:

Button: One side to GPIO 2, other side to GND
LED: Long leg to GPIO 18 via resistor, short leg to GND
Make sure all connections are secure

The Complete Code:

import RPi.GPIO as GPIO
import time

# Set up GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(2, GPIO.IN, pull_up_down=GPIO.PUD_UP)  # Button
GPIO.setup(18, GPIO.OUT)  # LED

# Keep track of LED state
led_on = False

try:
    print("Press the button to toggle the LED. Press Ctrl+C to exit.")
    
    while True:
        button_state = GPIO.input(2)
        
        # Check if button is pressed (goes LOW)
        if button_state == GPIO.LOW:
            # Toggle LED state
            led_on = not led_on
            
            if led_on:
                GPIO.output(18, GPIO.HIGH)
                print("LED turned ON")
            else:
                GPIO.output(18, GPIO.LOW)
                print("LED turned OFF")
            
            # Wait a bit to avoid multiple toggles from one press
            time.sleep(0.3)
        
        time.sleep(0.05)  # Small delay to prevent too much CPU usage

except KeyboardInterrupt:
    GPIO.output(18, GPIO.LOW)
    GPIO.cleanup()
    print("\nProgram stopped!")

Understanding the Code:

led_on variable: Tracks whether LED is currently on or off
not led_on: Toggles the state (True becomes False, False becomes True)
time.sleep(0.3): Prevents the button from toggling multiple times from one press (debouncing)
The loop continuously checks the button and toggles the LED when pressed

Debouncing

When you press a button, it might "bounce" - quickly turning on and off multiple times. This is called "bounce" and can cause your program to think the button was pressed many times. We fix this with a small delay after detecting a press - this is called "debouncing".

💡 Tip: Always add a small delay (like 0.2-0.3 seconds) after detecting a button press to prevent multiple triggers from one press!

Common Mistakes to Avoid

⚠️ Watch Out For:

Wrong Pin Mode: Make sure to use GPIO.IN for inputs, not GPIO.OUT
Missing Pull-up: Use GPIO.PUD_UP to prevent floating pins
No Debouncing: Add a delay after button press to prevent multiple triggers
Wrong Logic: Remember LOW means pressed when using PUD_UP
Forgetting Cleanup: Always call GPIO.cleanup() when done

Summary

You've learned:

✅ Sensors let your program detect the real world
✅ GPIO pins can be inputs (read) or outputs (write)
✅ Use GPIO.IN to set a pin as input
✅ Use GPIO.input(pin) to read the pin state
✅ Buttons read LOW when pressed (with PUD_UP)
✅ Always add debouncing delay to prevent multiple triggers
✅ You can combine inputs and outputs to create interactive systems

🎉 Great Progress! You can now read input from the real world! In the next lesson, you'll learn to display information on screens.

🎮 Try It: Practice Reading Inputs!

Practice writing code to read sensors. Try these challenges:

📝 Challenge 1: Simple Button Check

Write code that reads a button on GPIO 2 and prints "Pressed" or "Not Pressed":

📝 Challenge 2: Button Counter

Write code that counts how many times a button is pressed:

💡 Tip: Remember to use GPIO.IN for reading, GPIO.PUD_UP for pull-up resistor, and add debouncing delays!

🎯 Activity: Interactive Button-Controlled LED System

What You'll Build:

Create a system where pressing a button toggles an LED on and off!

Step-by-Step Instructions:

Wire the Button: Connect button between GPIO 2 and GND
Wire the LED: Connect LED to GPIO 18 with resistor
Write the Code: Use the example code from the Learn tab
Test: Press the button and watch the LED toggle
Experiment: Try changing the debounce delay time
Enhance: Add a second button to control a second LED

Testing Checklist:

✅ LED turns on when button is pressed first time
✅ LED turns off when button is pressed second time
✅ Button doesn't trigger multiple times from one press
✅ Program stops cleanly with Ctrl+C

🏆 Bonus Challenge: Can you create a system with two buttons - one to turn LED on, another to turn it off?

💪 Practice Challenges

Challenge 1: Button State Monitor

Write code that continuously monitors a button and prints its state every second:

# Should print "Button: Pressed" or "Button: Not Pressed" every second

Challenge 2: Press Counter

Create a program that counts button presses and displays the count. Reset the count when it reaches 10:

# Count presses, print count each time
# When count reaches 10, reset to 0 and print "Reset!"

Challenge 3: Hold to Light

Create a system where the LED only stays on while the button is held down:

# LED should be ON when button is pressed
# LED should be OFF when button is released

Challenge 4: Double Button Control

Use two buttons to control one LED:

Button 1 (GPIO 2): Turn LED on
Button 2 (GPIO 3): Turn LED off

Challenge 5: Code Detective

What's wrong with this code? Find and fix the mistakes!

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(2, GPIO.OUT)  # Should this be OUT?

button = GPIO.input(2)

if button == True:
    print("Pressed")

Click to see the answer

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setup(2, GPIO.IN, pull_up_down=GPIO.PUD_UP)  # Fixed: use IN, add PUD_UP

button = GPIO.input(2)

if button == GPIO.LOW:  # Fixed: use GPIO.LOW, not True
    print("Pressed")

GPIO.cleanup()  # Added: cleanup