Infrared Sensors Notes

Infrared Radiation

• Invisible light; differs in wavelength & frequency from visible spectrum
• Associated with heat emitted by any object (sun, bulbs, human body)
• Requires direct line-of-sight for transmission (unlike Wi-Fi/Bluetooth)
• Position in spectrum: between radio waves and visible light
  • Approx. 700\,\text{nm} (red edge) to 1\,000\,\text{nm+}

IR Sensors – General

• Devices that emit and/or detect IR to sense surroundings
• Two main categories:
  • Active IR Sensors (emit + detect)
  • Passive IR Sensors (detect only)

Active IR Sensors

• Measure IR based on difference between emitted and reflected/received radiation
• Core elements:
  • Infrared source / transmitter: IR LED
  • Infrared detector / receiver: photodiode or phototransistor
• Typical uses:
  • Remote controls (e.g., TV channel change)
  • Break-beam motion detectors
• Signal path: transmitter sends IR ➝ object interacts ➝ receiver senses change ➝ electronic output

Passive IR Sensors (PIR)

• Detect natural IR emitted by objects; no emission of their own
• Components:
  • Pyroelectric sensor: senses changes in IR energy (motion/heat variations)
  • Fresnel lens: concentrates incoming IR onto sensor
• Applications:
  • Security & occupancy detectors (banks, airports, lighting control)
• Operation: object moves ➝ IR pattern changes ➝ sensor output triggers (e.g., lights LED)