Our corporate mission is a joy to live up to: Contribute to culture and industry through creativity. We hit the trifecta with our recent boombox project with Joe Grand; it highlights all three key elements of our motto – culture, industry, and creativity. It involves hacker royalty, breakdancers, state-of-the-art circuit board manufacturing technology, rappers, a hark back to TDK history, and one of the world’s thinnest piezoelectric speakers. We’re proud to say those speakers — our speakers — were the inspiration for the whole idea.
Hacker royalty
Joe Grand (aka Kingpin) is an engineer, a well-known cybersecurity expert, and one of the world’s most accomplished hardware hackers. Grand was a member of L0pht Heavy Industries, one of the first cybersecurity think tanks; he and other members of the collective famously testified before Congress in 1998. In early 2022, Grand made news by hacking a cryptocurrency wallet at the request of its owner who forgot his own PIN. In recent years Grand has also been taking on engineering projects simply for fun. That includes building a pizza compass, which points not to magnetic north but instead to the nearest pizza parlor.
Defining the project
We originally reached out to Grand in hopes that he would build a project using our components. Grand agreed but wanted to not only do something that would earn the interest of engineers but would also appeal to a wider audience, and to do something that in some way tied to TDK’s history.
“I have an affection for the ’80s,” said Grand, who was born in 1975. “I remember seeing everything that was coming out of New York City — hip-hop, breakdancing, boomboxes. I grew up in Boston and had a friend with a boombox. We’d listen to The Fat Boys, Grandmaster Flash. Walking around the neighborhood with my buddy and his boombox? People thought we were crazy.”
So when Grand began combing through TDK’s product portfolio and found the PiezoListen, TDK’s piezoelectric speakers (used widely in the automotive industry) it all came together for him.
He’d create a boombox. And not one of those unassuming little modern “boomboxes,” either. One reminiscent of those big 1980s monsters — but with multiple high-tech twists. It would be a work of art unto itself. And it would push the limits of the world’s most sophisticated manufacturing techniques.
Reimagining the ’80s boombox
TDK had a direct connection to boomboxes; back in the heyday of cassette players — in the 1970s and ’80s, we were one of the biggest sellers of cassette tapes. That would be the germ for one of Grand’s more clever design ideas — but we’ll get to that in a moment.
Grand envisioned a boombox as tall and wide as some of those classic ’80s boomboxes. But playing off the PiezoListen’s extreme thinness (only 0.76mm thick) it would also be the skinniest boombox ever.
Piezoelectric speakers generate sound due to the piezoelectric effect – essentially using voltage to control the deformation of a piezoelectric material. TDK’s PiezoListen, based on patented multilayer and material technologies, can turn nearly any surface (glass, plastic, ceramic, etc.) into a speaker by causing it to resonate.
Grand said that got him to thinking: could his boombox’s PCB itself be the speaker?
“I hooked it up to some circuit board material, and it worked,” Grand said. “And it was pretty loud.”
State of the art manufacturing
If he was going to build the world’s thinnest boombox, and the PCB was going to contain all the electronics including the speakers, would it be possible to have no shell at all? What if the boombox had no box?
In that case, the circuit board was going to be completely exposed. Grand wanted the boombox to be visually stimulating on the front side and to have a stark, clean look on the back. He decided to hide all the traces on inside layers of the PCB so they wouldn’t be visible to the outside world. This would require the use of via-in-pad, where the PCB’s inter-layer connections are fabricated directly into the component’s landing pads. It’s not uncommon and is often used for microBGA or chip-scale packages, but there was a question if it could be expanded to the dimensions Grand was proposing: roughly 22″ x 14″.
Grand had worked with PCB manufacturer Summit Interconnect on previous projects. He called them up to ask if what he was envisioning was even possible. The company said it would be a laborious and costly process, but – yes – it could be done.
Many of those old ’80s boomboxes incorporated LEDs. Often enough the lights were functional (stacks of diodes used to indicate volume levels, also known as a “VU meter,” for example), though plenty of models were studded with LEDs that were largely decorative. Grand didn’t want his boombox to simply have lights — he wanted the PCB itself to light up.
He asked the manufacturer if, instead of drilling through-holes for the LEDs, it could mill cavities only partly through the PCB. That way, when the LEDs were inserted from the back side, the emitted light would diffuse through the front of the PCB material.
Yes, the manufacturer could do controlled-depth milling. At that point, incorporating the capacitive touch buttons for operating the boombox (play, pause, volume, etc.) would be the easy part.
While the PCB itself would end up being 0.125″/3.17mm thick, Grand added a frame to protect the components and battery, the thickest element of the system — bringing the total thickness to 0.45”/11.43 mm.
Artists, rappers, breakdancers, and filmmakers
The original idea was to create an interesting engineering project. It certainly was that. But then Grand turned it into an elaborate multimedia art project, too, and teamed up with filmmakers Fred and Freddy Macdonald of Macdonald Entertainment Partners to capture the journey.
The designs of many ’80s-era boomboxes are iconic. Grand wanted his creation to combine retro-themed elements with a cyberpunk future, so he invited artist Mar Williams to create a design for the front surface.
Grand wanted to show off the boombox’s audio capabilities using an anthem well-known in the hacking and gaming communities. He asked int eighty (David Martinjak), a member of the rap group Dual Core, for permission to feature their song “All the Things” in the video that he and the Macdonalds were shooting.
He also invited New Birth Crew, a Portland, Oregon-based breakdancing crew, to appear in the finale and perform a routine to int eighty’s raps.
Pièce de résistance
Grand never had any intention of building in an actual cassette player, but if this was going to be an ’80s style boombox, then it needed to have a cassette tape. And if it was going to have a cassette tape, then it had to do something.
An actual TDK cassette tape in the spot literally carved out of the circuit board and used to activate the boombox.
His solution was metaphorically apt. You couldn’t play a 1980s boombox without a cassette, and you can’t play Grand’s without one either (though as an emergency fail-safe, he also built a standard on/off switch into the boombox). The magnets would not only hold the tape in place; Grand designed the electronics so that the system won’t turn on until the magnets are paired — until the cassette is “loaded.” (By the way, creating the cassette slot was another milling challenge.)
Joe Grand’s TDK boombox
And here’s the final product. The World’s Thinnest Boombox. An engineering marvel. Loud. Lit. In 4K video.
The World’s Thinnest Boombox. Engineering by Joe Grand. Artwork by Mar Williams.
I think the coolest part of this project was being able to combine art, music, and engineering,” Joe Grand said. “It was a lot to bite off, but that’s what got me excited, and I hope that’s what will get other people excited about it, too.
Let’s get technical
It started with the speakers
One manifestation of the piezoelectric effect is when an electric current is applied to a material causing it to deform. It is possible to cycle the current to get the material not just to deform but to vibrate at desired frequencies, effectively using the piezoelectric effect to create sound. Piezoelectric speakers have a number of applications. Still, they are becoming increasingly popular in automotive audio systems where their minimal size and light weight are as important as their acoustic performance.
TDK’s PiezoListen speakers feature an operating frequency range of 400 to 20,000 Hz and achieve a high sound pressure of 80 dB, even at low voltages. The speakers have a maximum output power of 34 W. With a thickness of just 0.49 mm, PiezoListen devices are among the thinnest piezo speakers available.
Grand’s boombox design affixes two PiezoListen speakers directly to the circuit board, in effect turning the PCB itself into a speaker.
Raspberry Pi board
For system control, Grand selected a Raspberry Pi Zero 2 W, which the Raspberry Pi Foundation describes as “the original $5 computer.” The Pi Zero controls everything: the capacitive touch unit, the piezo driver and stereo audio DAC (digital to analog converter), and controls the NeoPixel LEDs through a buffer used as a level translator.
Piezo driver and DAC
Grand received a recommendation that TDK’s PiezoListen pairs well with the LM48580 piezo driver from Texas Instruments.
The LM48580 is a fully differential, high-voltage driver for piezo actuators (and also for ceramic speakers) for portable multimedia devices. TI says that the LM48580 Class H architecture offers power savings compared to traditional Class AB amplifiers. It is a single supply driver with an integrated boost converter which allows the device to deliver 30 VP-P from a single 3.6 V supply.
TI was also source for the DAC. Grand used the PCM5100A, a 100dB audio stereo DAC with a 32-bit, 384kHz PCM Interface. TI’s PCM510xA devices use the latest generation of TI’s advanced segment- DAC architecture, which the company says achieves “excellent dynamic performance and improved tolerance to clock jitter.”
Capacitive touch driver, buffer, and LEDs
Grand’s design includes capacitive touch buttons for all the functions of the boombox (play/pause, stop, volume up, volume down, track forward, track back). The driver he selected was Microchip’s CAP1166. The 1166 is a turnkey capacitive touch controller for low-cost touch interfaces. It contains 6 LED drivers that offer full-on / off, variable rate blinking, and dimness controls.
A non-inverting buffer from Onsemi, the 74VHC1GT125, was selected to serve as a leveler for a two dozen AdaFruit NeoPixel LEDs (SK6812 MINI-E) that the design called for embedding into the PCB to make the PCB material glow from within.
The thickest part
By one measure, the most important element of the design was the battery. The marquee feature of the world’s thinnest boombox is its depth. As the largest component in the boombox, the size of the battery would determine exactly how thin the boombox would be.
Grand selected a Li-Polymer, 5V @ 2A, 5000mAh, battery that measured 123 x 66mm. Those dimensions ultimately led to a boombox that was 0.45 inches (11.43 mm), which included a protective frame that Grand also created out of PCB material to protect the components and battery
Software choices
Grand considers himself a hardware engineer who can code when he needs to, so going the open-source route was a natural decision.
Pi Zero comes with the Raspberry Pi OS of course. Grand turned to Pimoroni because it already had software modules written for controlling the audio DAC, a capacitive touch driver, and basic VU meter, though he modified some of the code to fit his exact needs.
For a music player, Grand chose Mopidy. Mopidy is an extensible music server written in Python. Mopidy plays music from a local disk, and with extensions can support streaming sources such as Spotify, SoundCloud, and TuneIn. Grand wrote a custom extension to allow the capacitive touch buttons to directly control the Mopidy software.
The boombox as a DIY project
Grand has released the schematics and documentation of his boombox on his website. The design is open source under a Creative Commons International License
Big costs That could easily be evaded
Grand’s boombox has no case or cabinet. It lights up as from within by virtue of its LEDs being embedded directly in the PCB material. The face of the PCB is itself a piece of art. These were some of the aesthetic choices mentioned earlier that had significant cost ramifications.
Engineers or hobbyists looking to build their own versions of the worlds thinnest boombox can easily avoid these costs.
Artwork: Grand elected to work with a friend of his, artist Mar Williams, to design the graphics for the face of the boombox. Having seven different Pantone colors required seven solder mask layers that had to be aligned precisely.
Controlled depth milling: in order to get a boombox that lit up from within, Grand directed the PCB manufacturer to mill cavities that go only part of the way through the PCB, rather than drill through-holes. When the LEDs are inserted into the cavities and activated, their light permeates the PCB material, giving it the appearance of being lit from within.
Via in pad: a PCB manufacturing technique typically used with highly dense component packages like migcroBGA or chip-scale in order for their signals to fan out properly to the PCB. Essentially the PCB’s vias (inter-layer connections) are fabricated directly into the component’s landing pads instead of using traditional vias that stand alone on the PCB. Via In Pad was required for the piezo driver, so Grand decided to use it for all components on the board, thus hiding every signal trace and connection.
Capacitive touch buttons: Traditional buttons would work just as well
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