PIR Sensor Not Detecting Behind Plastic Cover

Introduction

Sometimes you want to hide a PIR sensor behind a plastic cover for aesthetics or protection. But many plastics block infrared, rendering the sensor useless. This article explains what works and what doesn’t.

Why Most Plastics Block IR

Common plastics like acrylic (Plexiglas), polycarbonate (Lexan), and ABS are opaque to infrared in the 8-14µm range. They may look clear to visible light, but they absorb thermal IR.

What Materials Work?

Materials that transmit IR:

• Polyethylene (PE): Thin film (e.g., plastic bags) transmits IR well. Thicker sheets may work but lose transmission.
• Polypropylene (PP): Similar to polyethylene.
• Silicon: IR-transparent, but expensive and fragile.
• Germanium: Excellent IR transmission, very expensive.
• Special IR-transmitting plastics: Some manufacturers make IR-transparent plastics for sensor windows.

Testing Your Cover Material

1. Place the sensor facing a warm object (like your hand).
2. Measure the output signal (use oscilloscope or LED).
3. Place the candidate material between sensor and hand.
4. If signal drops significantly, the material is not suitable.

Symptoms

• Sensor works without cover, fails with cover.
• Detection range severely reduced.

Solutions

1. Use IR-Transparent Material

If you need a cover, use thin polyethylene film (0.1-0.5mm). A plastic bag stretched over the opening can work.

2. Leave an Opening

Design the enclosure with an open window for the sensor. Cover with a mesh if needed for protection (mesh must be IR-transparent or have large openings).

3. Use a Sensor with Built-in Cover

Some outdoor sensors come with IR-transparent housings designed for the purpose.

4. Recess the Sensor

Mount the sensor so it’s recessed behind an opening, not behind a cover.

Case Study: Hidden Sensor in Smoke Detector

A user tried to hide a PIR sensor inside a plastic smoke detector enclosure. It didn’t work. Replacing the front panel with thin polyethylene film restored detection.

Conclusion

Don’t assume that clear plastic is IR-transparent. Test your cover material or use an open design.