Chapters:

1. Electronics overview
2. Assembly overview
3. TODO: Audio filter (state variable filter) and electromagnet driver details
   
4. TODO: Audio output and speaker driver details
5. TODO: Unsolved problems

Electronics overview

The electronic circuit is divided in four parts: power input, audio input, analyzer, and audio output. The heart of the project is the analyzer, which consists in a tunable state variable filter. It takes audio as input, and outputs into an electromagnet to make the ferrofluid dance. The gain, bandwidth and center frequency of the state variable filter can be tuned to the music with three potentiometers.

Parallel to the analyzer is the audio output. It takes audio as input, amplifies it with adjustable gain, and directs it to a speaker. 

Audio input for the analyzer and the audio output comes from a TRRS jack connector. The signal is buffered and directed straight into the audio output. It is however amplified before going into the analyzer: this is because the analyzer works on a +/-12V range but the typical audio signals from computer or smartphones are within +/-2V.

The audio output can be muted with a switch, in which case music from the audio input can still be heard with an alternate device, because the audio input has two TRRS jack connectors that are wired in parallel.

The power input takes +/-12V and dispatches it to the electronics. To drive the electromagnet and the speaker, peak current can go as high as 1.5Arms.

Assembly overview

The VU-Meter is composed of many mechanical parts, assembling in a functional case. The main piece is a bottle of ferrofluid, obtained directly from Concept Zero: Klock model, 50 mL which sits directly in front of a 50 mm electromagnet (12 VDC, holding force 50 kg, Heschen, obtained from Amazon).

They are held together in a 3D printed part (approx. 200 mm x 110 mm x 70 mm) which, as we will see later, also holds four tuning potentiometers and a speaker.

The front face is covered by two decorative layers. The first on is 3D printed, it maintains the bottle inside the pocket and leaves space for the potentiometer and speaker nuts and screws. The second layer is laser cut in wood, with a decorative purpose. Both layer have an opening for the ferrofluid bottle to appear through.

The circuit board is located in the back, and fits around the ferrofluid bottle and electromagnet.

The ensemble is hidden in a laser cut and bent metal enclosure, completed with a wooden laser cut back panel.