Stop Whining!

LED light bulbs are far superior to incandescent bulbs in terms of energy efficiency, but they have some annoying quirks. Manufacturers of lower-quality LED bulbs, in particular, tend to skimp on electronic filtering components. This most commonly leads to a maddening flicker that makes incandescent bulbs look pretty appealing, no matter how much energy they waste as heat. When used with a dimmer, these bulbs also have a tendency to make high-pitched whining noises — as if the flicker wasn't bad enough already!

Aaron Shady has a shelf with built-in LED lighting. He likes the shelf, but is completely fed up with the constant whining noise it makes when the lights are dimmed. So, rather than getting rid of the shelf, he built a custom controller board to replace the questionable electronics that came with it. Shady also open sourced the entire project, so you can borrow from it if you've got some LED lighting that is driving you batty.

Shady started by reverse engineering the original controller. Oscilloscope measurements showed that it drove the 24 V tunable white/orange LED strip with a 2 kHz H-bridge waveform. At lower brightness levels, the controller inserted periods where the output sat at 0 V, creating additional harmonics right in the range where human hearing is most sensitive. A microphone placed 5 cm away confirmed the culprit: the 4 kHz harmonic became roughly 16 dB louder when the lights were dimmed.

The replacement board fixes the problem by moving the switching frequency completely out of the audible range. An STM32F303 microcontroller generates a three-level H-bridge waveform running at 25 kHz, while an MP6528 gate driver controls four MOSFETs that switch the LED strip between +24 V, -24 V, and 0 V. Bluetooth Low Energy connectivity through a Microchip RNBD350 module lets the controller communicate with a smartphone, and the firmware remembers brightness and color settings across power outages by storing them in the microcontroller's flash memory.

Acoustic measurements of the completed controller show that the original 2 kHz harmonic comb has disappeared entirely because the controller now switches at 25 kHz, placing the fundamental frequency and all of its harmonics above the audible range. Better yet, the quieter design doesn't consume any more power than the original controller. For more details, or to snag the schematics and firmware, head on over to the project's GitHub repository.