HC-SR501 PIR Sensor (Passive Infrared Sensor) + IR Proximity Sensor

🔍 1. HC-SR501 PIR Sensor (Motion Detection)
📌 What it does
Detects human/animal movement
Works based on body heat (infrared radiation)
👉 It does NOT detect objects, only motion of warm bodies
⚙️ Key Specifications
Voltage: 4.5V – 20V
Output: Digital HIGH (3.3V) when motion detected
Range: ~3 to 7 meters
Angle: ~110°–120°
Delay time: 5s – 300s adjustable
🧠 Features
✔️ Adjustable sensitivity + delay
✔️ Two modes:
Repeatable (keeps detecting)
Non-repeatable
✔️ Very low power (<50µA standby)
✔️ Uses Fresnel lens for wide detection
🔌 Pinout
VCC → 5V
GND → GND
OUT → Digital pin
💡 Applications
🚨 Security systems
💡 Automatic lights
🏠 Smart home automation
👣 Human presence detection
⚠️ Important Notes
Needs 30–60 sec warm-up
Cannot detect stationary person
Sensitive to heat changes
🔍 2. IR Proximity Sensor (Obstacle Detection)
📌 What it does
Detects objects (any material)
Works using IR reflection
⚙️ Key Specifications
Voltage: 3.3V – 5V
Range: ~2 cm – 30 cm
Output: Digital LOW when obstacle detected
IC: LM393 comparator
🧠 Features
✔️ Adjustable detection distance
✔️ Fast response
✔️ Works for any object (not just humans)
✔️ Cheap & simple
🔌 Pinout
VCC → 3.3V / 5V
GND → GND
OUT → Digital pin
💡 Applications
🤖 Obstacle avoiding robot
📦 Object detection
📏 Line following robot
🚗 Smart vehicles
⚖️ PIR vs IR Sensor (VERY IMPORTANT)
Feature
PIR Sensor
IR Sensor
Detects
Human motion
Any object
Range
Long (3–7m)
Short (2–30cm)
Type
Motion sensor
Proximity sensor
Works on
Body heat
IR reflection
Accuracy
Good for motion
Good for distance (short)
🧠 When to use which
👉 Use PIR if:
You want human detection
Example: automatic light ON when someone enters
👉 Use IR if:
You want object detection
Example: robot avoiding obstacles
🔥 Pro Tip (Very Useful)
👉 Combine BOTH sensors:
PIR → detect human presence
IR → detect distance/obstacle
➡️ Used in:
Smart robots
Smart doors
Advanced IoT systems
⚠️ Common Beginner Mistakes
PIR not working → didn’t wait warm-up time
IR giving false output → not adjusted potentiometer
Using PIR for object detection ❌ (wrong sensor choice)