Introduction
Every now and then you build something that’s more than just a “cool gadget”; it actually surprises people. That’s exactly what happened when we turned a simple 3D-printed pumpkin into a motion-activated Halloween prop. Using a transparent printed shell, a compact board Kypruino (or manually wire a regular Arduino UNO), a PIR motion sensor, and a light-and-sound sequence, the result was a pumpkin that flickers like a candle and then bursts into the first four seconds of the “Halloween” theme song whenever someone walks by.
This blog post walks you through how we did it — covering model edit, build, code, and the final effect.
Why We Built It
Halloween props are fun — but many of them just sit there. We wanted something interactive: something that senses motion and reacts with light and sound. The transparent material for the 3D-print added a layered effect. Add a USB-C cable for power and it becomes a reusable decoration when illuminated by the LEDs on the Kypruino board.
Goal: a pumpkin shell + flat base for mounting + motion trigger → flicker lights + theme song.
Components & Tools
- Kypruino board: – NeoPixels & Buzzer
- PIR motion sensor
- 3x male-female DuPont wires
- USB-C cable for power (through a back hole in the shell)
- 3D-printed “Jackie Jack-o-Lantern” model from Thingiverse (by BirdBott) - modified version. You can download the mod in our GitHub!
- PLA or PETG transparent filament for the shell
- Arduino IDE for programming
Kypruino
The Kypruino is essentially what brings the project to life — the brain that coordinates everything.
It’s a small, programmable printed circuit board used to build electronic projects that can sense and interact with the world. Think of it as a simple but powerful microcontroller that can read input (like motion or light) and respond by producing output (like lighting LEDs or playing sounds).
It has a series of connection points (pins) to attach sensors, buttons, LEDs, and other components. What makes the Kypruino special are its thoughtful enhancements over a standard Arduino UNO R3: it includes built-in buzzer, buttons, and NeoPixel LEDs, DAC and more, making it much easier to experiment without needing extra wiring.
You write instructions on your computer and upload them via USB. Once uploaded, the board runs your code automatically — continuously checking for motion, controlling LEDs, and triggering effects.
PIR Motion Sensor
This Passive Infrared (PIR) Motion Detector senses infrared radiation naturally emitted by all warm objects. Humans and animals (yep, the pumpkin can scare your adorable cat too!!) constantly emit infrared radiation, and the PIR sensor detects changes in that energy pattern. When someone passes in front, it detects the shift in heat and sends a trigger signal.
- Sensing angle: ~100°
- Range: up to 7 m
- Voltage: 4.5–12 V
- Two adjustable potentiometers: sensitivity & time delay
Designing & 3D Printing the Pumpkin
We started with this model by BirdBott on Thingiverse.
- Edit the model so the inside has a flat base — easier to mount the Kypruino (e.x. double face tape).
- Add a hole at the back of the shell to route the USB-C power cable.
You can find our modified version in our GitHub repository.
The transparent filament lets internal lights shine through beautifully — when the LEDs flicker, the shell glows softly.
Once printed and cleaned, we mounted the board and sensor, routed the cable, and closed the shell.
Wiring & Electronics
Inside the shell sits the Kypruino board, with its power port facing the back. Both the Buzzer and NeoPixel switches are turned on.
The PIR sensor is mounted behind the pumpkin’s eyes and mouth so it can detect passers-by. Using 3 male-female DuPont wires:
- PIR VCC → Kypruino 5V
- PIR GND → Kypruino GND
- PIR OUT → Kypruino digital pin 7
Power is supplied via the USB-C cable entering from the rear hole, keeping everything clean and hidden. We used double-faced tape for easy mounting, but the flat base allows for screw or standoff options if you want a more permanent setup.
Important tip: Face the motion sensor though the Pumpkin's mouth to easily detect motion!
Programming the Effect
The heart of the build is the code: when the PIR detects motion, it triggers a flickering LED animation and plays four seconds of the Halloween theme through the buzzer.
Sequence overview:
- Motion detected → LEDs turn orange/red
- Brightness randomly varies between 75–255 for flicker effect
- Buzzer plays the first 4 seconds of the Halloween theme
- Lights turn off → cooldown period → re-arm
You can find the complete code in our GitHub repository.
Final Result & Experience
Once powered via USB-C and placed on a table, the pumpkin patiently waits for someone to walk by. On motion detection, the LEDs flicker orange (like a real candle), and the theme song buzzes briefly. The transparent shell gives the glow a soft, ghostly halo.
People tend to walk away and come back again just to trigger it — the best sign of an engaging interactive prop!
Possible Future Enhancements
- Experiment with different NeoPixel colour transitions or gradients
- Create custom light patterns or sequences
- Try new sounds or songs
- Add a small speaker instead of the buzzer for richer sound
- Include a rechargeable battery pack to make it cordless
Conclusion
This build combines 3D printing, electronics, and programming to turn a simple design into something that “comes alive.” If you’re looking for a fun interactive prop — for Halloween or otherwise — a motion-triggered light-and-sound combo is a high-impact, low-cost way to go.
We hope this inspires you to create your own reactive decorations!
Attribution
Original 3D model: Jackie Jack-o-Lantern by BirdBott on Thingiverse — thank you for releasing the design!