Spanda Piezoelectric Generator Build Guide (Energy Harvesting Prototype) A practical, low-power piezoelectric energy harvester with rectification, storage, and regulated output. Scope and intent This document explains how to build a small piezoelectric generator that converts mechanical oscillation ("spanda": any periodic vibration, flex, tap, or pulse) into usable DC power for low- energy electronics. It is an energy-harvesting project, not a mains-power generator. 1. Concept overview Piezoelectric materials produce an electric charge when mechanically stressed. If you repeatedly flex or compress a piezo element, you get an AC voltage. A rectifier converts AC to DC, a storage element (capacitor or rechargeable cell) accumulates energy, and a regulator provides a stable voltage for a load (e.g., a microcontroller sensor, LED blink, or low-power radio burst). System block diagram Mechanical spanda Piezo element(s) Rectifier (low-loss) Storage Regulator Load → → → → → 2. Bill of materials (BOM) Item Example choices Notes Qty Piezo element Piezo disc (27–35 mm), piezo film strip, piezo stack Disc is cheap/easy; film is flexible; stack gives higher current 1–4 Bridge rectifier 4× Schottky diodes (e.g., SS14) or a low- loss energy- harvesting IC Schottky reduces drop vs. standard diodes 1 Storage Supercapacitor (0.1– 10 F) OR LiFePO4/Li- ion cell with charger Start with supercap for simplicity/safety 1 Regulator Low-quiescent- current buck/boost Pick based on your load voltage; ultra- 1 or LDO low Iq matters Protection (recommended) Zener/TVS clamp, series resistor, bleed resistor Prevents overvoltage spikes, reduces stress on electronics 1–3 Load (test) LED + resistor, low- power MCU board, e- ink tag Use a known load for measurement 1 Mechanical mounting Cantilever beam, spring, foam pad, clamp, enclosure Mechanical design determines output more than electronics 1 Tools Soldering iron, multimeter, hot glue/epoxy, small hand tools Oscilloscope optional but helpful — 3. Choose your mechanical spanda source Three practical ways to create repeatable oscillation: Cantilever harvester (recommended): a piezo element bonded to a flexible beam with a small tip mass. Vibration flexes the beam repeatedly. Compression harvester: piezo disc or stack compressed by footsteps, a spring, or a press. Robust, simple, but needs force. Impact/tap harvester: a striker taps the piezo. Very high voltage spikes, low average power unless repeated quickly. Rule of thumb: match the mechanical resonant frequency of your beam/spring system to the vibration you can reliably provide. A weak but frequent vibration usually harvests more energy than rare strong hits. 4. Electrical design (simple, reliable version) This section uses a minimal circuit you can build on perfboard. 4.1 Piezo element connection Most piezo discs have two electrodes: the brass plate and the ceramic top electrode. They behave like a high-voltage, low-current AC source with a built-in capacitance. Use flexible wire and strain relief; piezo tabs are easy to tear off. 4.2 Rectifier Use a full-wave bridge so both halves of the piezo AC contribute to charging. For low-power harvesting, Schottky diodes are preferred because their forward drop is lower. Connect the piezo leads to the two AC inputs of the bridge; the bridge outputs are + and - DC. 4.3 Storage For a first prototype, use a supercapacitor (e.g., 1 F to 10 F). It tolerates charge/discharge cycles and simplifies safety. If you use a rechargeable cell, add a proper charging IC designed for energy harvesting. 4.4 Regulation and output If your load needs a stable voltage, add an ultra-low quiescent-current regulator. Many harvesting builds store energy at a higher, variable voltage (e.g., 2–10 V on a supercap) and then use a buck/boost converter to deliver a regulated rail only when the capacitor is above a threshold. 4.5 Protection Piezo elements can generate surprisingly high open-circuit voltages (tens to hundreds of volts) during sharp impacts. Add a clamp (Zener or TVS diode) across the rectified DC side to limit voltage to your storage component’s rating. A small series resistor (100–1k ohm) can reduce peak currents and ringing. If using a supercap, consider a bleed resistor (e.g., 100k–1M) so it discharges slowly when idle. 5. Step-by-step build (prototype) 1. Mount the piezo on your mechanical structure (beam/spring/press). Ensure the element flexes or compresses consistently. 2. Solder flexible leads to the piezo electrodes and add strain relief (hot glue or epoxy). 3. Build the bridge rectifier on perfboard (4 Schottky diodes in a standard bridge). Label the + and - outputs. 4. Connect piezo leads to the AC inputs of the bridge. Polarity doesn’t matter on the AC side. 5. Connect the bridge + and - to the storage capacitor (observe polarity on electrolytics/supercaps). 6. Add the clamp diode across the storage (respect polarity). Choose a clamp voltage below the capacitor’s max rating. 7. Add the regulator module (or harvesting IC) to the storage node. Connect its output to your test load. 8. Optionally add a switch or a load-enable MOSFET so the load only turns on above a capacitor threshold (prevents brownout). 9. Enclose and secure everything so mechanical motion doesn’t rip wires or short the circuit. 6. Testing and tuning Measure in this order: (1) piezo open-circuit AC voltage, (2) rectified DC voltage, (3) capacitor charge rate, (4) regulated output under load. A basic multimeter works; an oscilloscope helps you see spikes and resonance. Open-circuit check: vibrate/tap the piezo and measure AC across its leads (careful: voltage spikes). Rectifier check: measure DC across the bridge output while vibrating; confirm polarity. Charge check: watch the storage capacitor voltage rise over time. If it rises then falls, your load is too heavy or there’s leakage. Load check: connect a known load (e.g., LED + resistor) and observe whether the capacitor can reach the regulator’s minimum input. Tuning: adjust beam length, tip mass, or mounting stiffness to increase amplitude at your available vibration frequency. 7. Example configurations A. Quick demo (LED blink) Piezo disc Schottky bridge 0.47–1.0 mF electrolytic (or 0.1–1 F supercap) LED + → → → series resistor. You’ll see energy accumulation as brighter/longer flashes with repeated taps. B. Practical harvester (sensor burst) Cantilever piezo + tip mass low-loss bridge or harvesting IC 1–10 F supercap → → → buck/boost with low Iq MCU wakes when cap > threshold, samples sensor, transmits, → sleeps. 8. Safety notes Piezo harvesters are generally safe, but sharp impacts can generate high voltage. Avoid touching exposed conductors during strong impacts. Never connect a piezo harvester directly to mains circuits. Use appropriately rated capacitors, and include voltage clamps. Appendix: Bridge rectifier wiring (text diagram) If using four diodes D1–D4: - Piezo lead A connects to: D1 anode and D2 cathode - Piezo lead B connects to: D3 anode and D4 cathode - DC + output is where D2 anode meets D4 anode - DC - output is where D1 cathode meets D3 cathode This is the standard full-wave bridge arrangement.