Introduction
The Fresnel lens is often overlooked, but it’s crucial to PIR sensor performance. Different lenses create different detection patterns. Choosing the right lens can make or break your project.
What Does a Fresnel Lens Do?
The lens focuses infrared onto the pyroelectric element and divides the field of view into discrete detection zones. The pattern of these zones determines where motion is detected.
Common Lens Patterns
1. Wide Angle (Standard)
Coverage: 90-120° horizontal, 5-10m range.
Zones: Multiple zones spread across the area.
Best for: General room coverage, indoor security, lighting control.
2. Long Range (Curtain)
Coverage: Narrow (5-20°) but long distance (up to 20m).
Zones: Zones arranged in a line.
Best for: Hallways, corridors, perimeter security.
3. Pet Immune
Coverage: Similar to wide angle but with reduced sensitivity near ground.
Zones: Pattern designed to ignore animals below a certain height.
Best for: Homes with pets.
4. High Density / Multiple Zones
Coverage: Wide angle but with more zones for increased sensitivity to small movements.
Best for: Occupancy sensing where sitting/working movements need detection.
5. Custom Patterns
Some manufacturers offer lenses for specific applications like ceiling mounting (360°), corner mounting, etc.
How to Choose
- Define the coverage area: Measure the space. Is it a long hallway or a square room?
- Identify potential false trigger sources: Pets, heaters, windows? Choose pet-immune or adjust placement.
- Consider mounting height: Higher mounting may require a different lens to avoid blind spots.
- Check sensor compatibility: Lenses are often specific to sensor series. Ensure it fits your sensor module.
Where to Buy Lenses
- Some PIR modules (like HC-SR501) have interchangeable lenses available on AliExpress, eBay.
- Manufacturers like Panasonic offer various lens options for their sensors.
- 3D printing your own lens is possible but challenging due to optical requirements.
DIY: Modifying Lens Pattern
You can partially mask a lens with opaque tape to narrow its field, or use multiple sensors with different lenses to cover complex areas.
Conclusion
Don’t just use the stock lens without thought. Consider your coverage needs and choose a lens that matches. It’s a simple way to improve sensor performance.