Questions by kids: How do Halloween animatronics know exactly when to scare you

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Chris Dagher

Today, Chris Dagher, an adjunct faculty in the Department of Mechanical and Biodemical Engineering, answers a child's question about everyone's favorite Halloween decorations: How do Halloween animatronics know exactly when to scare you?

At Boise State, Dagher teaches classes in mechanical engineering while also pursuing his doctoral degree in computing. Dagher is also a founder and the CEO of the startup QTEX AI, a business that impacts how engineers source, optimize and manage electronic components.

Question: How do Halloween animatronics know exactly when to scare you?

Animatronic Halloween candy bowl, created by McKenna Howard

Many Halloween animatronics that spring to life when you least expect it rely on a clever little device called a passive infrared (PIR) sensor to know when someone's nearby, Dagher said.

Instead of using cameras or sound, these sensors look for changes in heat. Everything warm, whether it's a person, a pet (or even a cup of tea!) gives off invisible heat waves known as infrared light. Our eyes can't see infrared light, but we can feel it. Think about standing in the sunshine: you can feel warmth on your skin even though the air might still be cool. That's infrared light from the sun reaching you across space.

A passive infrared sensor works a bit like a "heat detector." Inside, it has two tiny heat sensors placed side by side. These sensors are constantly measuring how much infrared light they receive from whatever's in front of them. If the scene stays still, like an empty porch, both sensors see the same amount of infrared light, and nothing happens. But if something warm moves across the field of view, like a person walking by, one sensor suddenly sees more heat than the other. The sensor notices this quick change and sends out an electrical signal that says, essentially, "Something warm just moved!"

A headless horseman Halloween animatronic. Chris Dagher says "The horse's eye is a dead giveaway that it's using a PIR sensor; the white plastic lens is typical for a PIR sensor." Photo provided by Dagher.

That electrical signal can trigger anything connected to it, whether it's a light, a motor, a sound chip, or a fog machine. In a Halloween animatronic, it's what makes the witch cackle, the zombie lurch forward, or the skeleton suddenly pop up from its coffin. The passive infrared sensor doesn't know what moved, just that something warm moved, which makes it perfect for jump scares. Humans glow brightly in infrared compared to the cooler night air. That makes it easy for the sensor to spot us, even in total darkness.

The processing electronics on a circuit board for easily reading a PIR sensor. Photo provided by Chris Dagher.

Some animatronics combine passive infrared sensors with other triggers, like pressure mats, ultrasonic sensors, or sound detectors, to make them even more responsive. For example, a haunted house might use a passive infrared sensor to start a sound effect when someone walks by, then a motorized trigger a moment later to move the prop in sync. The result feels perfectly timed and almost alive.

Passive infrared sensors aren't just for spooky decorations, though. They're used everywhere: in security systems, automatic lights, wildlife cameras, robotic obstacle detection, and even paper towel dispensers in bathrooms. They're cheap, reliable, and power-efficient, which makes them ideal for battery-powered devices like animatronics that sit outside for hours waiting to surprise trick-or-treaters.

So next time you walk past a haunted scarecrow and it suddenly shrieks to life, remember - it wasn't magic or ghosts at work. It was a tiny, heat-watching sensor quietly keeping an eye out for you, waiting for just the right moment to make you jump.

Want to Learn More?

• Passive Infrared Sensor by Science Direct
• Adafruit Learning Page, How PIRs Work

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