Write Spooky Messages with an Ultrasonic Phased Array Device

Halloween is a great time of year to build projects that showcase something new and exciting, which is exactly what Dan Beaven who runs the Adventures in DIY Engineering blog set out to do. His project aims to create a spooky drawing system that utilizes invisible sound waves to draw shapes, text, and images onto a metal plate covered in baking soda with a human-like quality for an extra scary effect.

Theory of operation

As with any other moving entity, sound waves exert a force upon surfaces when they collide, which our ears interpret as sound through our eardrums and sensory hairs. But what if instead of merely sending a single wave towards a given object, a whole matrix of speakers were used to pin-point the energy at a location via wave physics? One team known as UpnaLab have created just that by arranging 256 transducers into a 16 by 16 grid and driving them to produce 40kHz waves, which they call the SonicSurface. This allows for users to levitate objects in 3D space, draw on a surface, and even showcase art.

Building the SonicSurface

Thanks to UpnaLab's helpful Instructable, Beaven was able to gather all of the necessary components for the SonicSurface, including 128 MOSFET drivers, 32 shift registers, 256 ultrasonic emitters, and a CoreEP4CE6 FPGA module for controlling everything. Once the board had been assembled, Beaven constructed a small and ornate box for the PCB to reside in while facing downwards. Also on the underside of the box is an ESP32 board that can receive commands over WiFi and forward them to the FPGA for remote access. And finally, there is a small time-of-flight (ToF) sensor that takes measurements between itself and the table that allows for the project to be taken anywhere and still accurately draw.

Drawing a simple image

For now, the ESP32-based webserver still is not working fully, so Beaven must still send animations over from his PC. However, he plans on implementing his own algorithm on the board if he can figure out the data format for the animations. As seen in his demonstration video below, the ultrasonic phased array levitation device can sketch out the word "Hi" with an eerie slowness that makes it ideal as a Halloween decoration. It will be exciting to see what other IoT integrations can come from the onboard ESP32, such as a virtual canvas where people are able to draw something and have it displayed in nearly real-time.