Do they work in the dark?

Yes, because they use invisible waves or heat, not visible light, so they work perfectly at night.

Town & Transport

How Do Automatic Doors See You?

April 10, 2026Ages 3–9

Key Insight

Automatic doors use Microwave Motion Sensors or Passive Infrared (PIR) sensors to detect changes in energy. When your body moves or its heat is detected, the door triggers its motor.

📖 Explanation

🧒 For 3-5 Years Old

Automatic doors have a 'robotic eye' at the top. It watches for anything moving near it. When it sees you coming, it tells the door to open wide so you can walk through!

🎒 For 6-9 Years Old

Microwave vs. Infrared

There are two main types of sensors. Microwave sensors send out invisible radio waves that bounce off you and return to the sensor. If you are moving, the waves change. Infrared sensors (PIR) don't send anything; they just 'look' for the heat coming from your body. When a 'hot' object moves into their field of view, they trigger the motor.

Safety Beams

Have you seen a small light near the floor of the door? That's a safety beam. If you stand in the middle, you block the light, telling the door 'Don't close yet!' This keeps you from getting squished.

❓ Frequently Asked Questions

Why don't the doors open for cats?: Sometimes they do! But many sensors are set to ignore small objects so the door doesn't open for every leaf that blows by.
Do they work in the dark?: Yes, because they use invisible waves or heat, not visible light, so they work perfectly at night.

🧠 Quick Knowledge Check

Q1 / 20%

Why don't the doors open for cats?

Step 1 / 2

🧪 Detecting Invisible Light

~10 min

Use your phone camera to see the invisible light that sensors use.

🛒 Supplies

📋 Steps

1
📺 Find a Remote Control
Grab a TV remote. It uses infrared light, just like some door sensors.
2
🤳 Use the Camera Hack
Open your smartphone camera and point it at the front of the remote. Press a button. You will see a purple light on the screen that your eyes can't see!

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