Solenoid Bit Reader
Macro-scale hard-disk-reader analog decoding falling magnetic bits at 100% accuracy.
- Status
- archived
- Year
- 2026
- Category
- hardware
- Built at
- Physics 4BL
- Team
- 2 people
Stack
Highlights
- Built a macro-scale hard-disk-reader analog — a 387-turn copper coil on a high-permeability iron core, read by an ESP32 — decoding falling binary magnetic bits via Faraday's Law of Induction
- Amplified sensor output 20x with an LM358N op-amp to fit the ESP32 ADC's 0-3.3V window, and calibrated bit-time windows against gravitational acceleration to establish the system's 40 mm resolution limit
- Achieved 100% decoding accuracy across all binary test sequences using a start-bit clock synchronization scheme
A hard drive reads data by sensing the magnetic field of bits passing a coil. This is that mechanism rebuilt at a scale you can watch: magnets fall past a hand-wound solenoid under gravity, and the induced voltage spike is decoded back into the binary sequence they encode.
The problem
Faraday induction gives you a voltage proportional to the rate of change of flux, so a falling magnet produces a brief spike rather than a level. Two constraints shaped the design:
- The ESP32's ADC only reads 0-3.3V, and the raw coil output was far below that. An LM358N operational amplifier stage provides 20x gain to bring spikes into readable range.
- Gravity means the bits do not arrive at a constant rate. Each successive magnet is moving faster than the last, so the time window for a bit shrinks continuously down the drop.
Design decisions
Presence-based encoding over polarity-based. With a unipolar ADC, encoding a 1 as north-up and a 0 as south-up would put half the signal below the readable floor. Encoding instead as magnet-present versus empty-slot keeps every symbol inside the window, at the cost of needing a clock to know when an empty slot went by.
A start bit for clock synchronization. Since empty slots produce no signal, the decoder cannot count them directly. A known start bit establishes t=0, and a calibration matrix built across three intervals maps the shrinking windows that follow.
Results
Four binary sequences decoded at 100% accuracy. The calibration work established a 40 mm resolution limit — closer than that and adjacent spikes merge at the velocities reached toward the end of the drop.
Figures
