Project story
Two flexible PCB coils, an ESP32, and no permanent magnet — can it produce recognizable audio? A contest entry led to a prototype that answered the question, one way or another.
The question
Can you make an audio speaker from two flexible PCB’s and without the use of a permanent magnet? This question came to me when I had been working on an RFID project that required a PCB coil. I wondered if I could make a flexible PCB (FPC) coil and pass an analog audio signal through it it would create a magnetic field. If I took a second identical PCB coil and ran it with opposite polarity, then the FPC’s would create a Push-Pull system that would create audible vibrations and sound. That was the theory anyway, so I simply needed a platform to test it out on!
Context
I stumbled across a contest on Hackaday.IO for a unique business card design, and I thought this would be the perfect application for my idea. The thin profile of the FPC speaker would fit the slim profile of a traditional PCB business card, and the cost would make the concept feasible to hand out these cards at an event.
You can find my entry to the contest here: https://hackaday.io/project/196562-business-card-bluetooth-speaker
Design considerations
I didn’t want to fight with the Bluetooth protocol, I2S audio formatting, and creating an A2DP sink for allowing a phone to connect to the card, and luckily I didn’t have to because Espressif provides an example code that does all of that on the older ESP32-WROOM-32 module.
Found here: https://github.com/espressif/esp-idf/blob/master/examples/bluetooth/bluedroid/classic_bt/a2dp_sink
The ESP32 definitely bumps up the total cost of the BOM, but as a proof of concept and a huge time saver, it fit the bill nicely. I designed the board and FPC coils, ordered a small quantity from JLCPCB, and within a week I had them in hand and soldered them up.
Result
Review the post on Hackaday for more details, but there was a degree of success with this concept because the speaker did make sound! It was very quiet and inefficient, but there was sound. Performance was drastically improved with the introduction of a permanent magnet, and slightly improved by driving one of the coils with a steady DC current to create an electromagnet. Finding a way to introduce a slim, but strong constant magnetic field will be the next hurdle of the project to improve audio quality and level of output.
Reflection
The original question didn’t require a permanent magnet — but the FPC-only proof of concept did technically answer it. The real takeaway is that a tiny proof-of-concept on a Hackaday timeline can produce results that are worth following up on, and that adding a magnet is sometimes the difference between a demo and a product.