crystal-radio/piezoelectric-circuits-guide.md

# Piezoelectric Circuits: From Crystal to Communication

> *A hands-on guide to building microphones, speakers, and telephones from crystals—demonstrating the beautiful reversibility of nature's physics.*

---

## Table of Contents

1. [The Magic of Piezoelectricity](#the-magic-of-piezoelectricity)
2. [Understanding the Physics](#understanding-the-physics)
3. [Build 1: The Zero-Power Crystal Telephone](#build-1-the-zero-power-crystal-telephone)
4. [Build 2: The Amplified Version](#build-2-the-amplified-version)
5. [Build 3: The Full-Duplex Intercom](#build-3-the-full-duplex-intercom)
6. [Build 4: The One-Crystal Walkie-Talkie](#build-4-the-one-crystal-walkie-talkie)
7. [Growing Your Own Piezoelectric Crystals](#growing-your-own-piezoelectric-crystals)
8. [Demonstration Script](#demonstration-script)
9. [Troubleshooting](#troubleshooting)
10. [Further Experiments](#further-experiments)

---

## The Magic of Piezoelectricity

The piezoelectric effect is one of nature's most elegant symmetries: **the same crystal that converts mechanical stress into electricity will convert electricity back into mechanical motion.** No switching required—it's the *direction of energy flow* that determines whether your crystal is a microphone or a speaker.

This guide will take you from understanding this phenomenon to building practical devices that demonstrate it—culminating in a one-crystal walkie-talkie that switches between transmit and receive modes.

### What Makes It Magical

- **No batteries required** for basic operation (the crystal generates its own voltage!)
- **Perfect reversibility**—same device, same wires, energy flows both ways
- **Found everywhere**—watches, ultrasound, guitar pickups, lighters, even your bones
- **Grows in your kitchen**—you can make piezoelectric crystals from household ingredients

---

## Understanding the Physics

```
┌─────────────────────────────────────────────────────────────┐
│              PIEZOELECTRIC BIDIRECTIONALITY                 │
├─────────────────────────────────────────────────────────────┤
│                                                             │
│   DIRECT EFFECT (Microphone)     INVERSE EFFECT (Speaker)  │
│   ─────────────────────────     ─────────────────────────  │
│                                                             │
│   Sound Wave → Crystal          Voltage → Crystal          │
│        ↓                             ↓                     │
│   Compression/Expansion         Expansion/Compression      │
│        ↓                             ↓                     │
│   Charge Displacement           Physical Displacement      │
│        ↓                             ↓                     │
│   Voltage Output                Sound Wave Output          │
│                                                             │
│   Same crystal. Same wires. Energy flows BOTH ways.        │
└─────────────────────────────────────────────────────────────┘
```

### Why It Works

The crystal's molecular structure is **asymmetric**—the arrangement of atoms lacks a center of symmetry. When you apply pressure:

1. **Positive and negative ions shift** relative to each other
2. This creates a **charge separation** across the crystal faces
3. Connect wires → you get **voltage output**

Apply voltage instead:

1. The electric field **pushes those same ions**
2. They shift, causing the crystal to **physically deform**
3. That deformation pushes air → **sound!**

### The Profound Implication

Nature doesn't distinguish between cause and effect at the fundamental level. The physical law that says "stress creates charge" *simultaneously* says "charge creates stress." It's not two phenomena—it's **one phenomenon viewed from two directions.**

This same symmetry appears throughout physics:
- Motors ↔ Generators
- Heat pumps ↔ Refrigerators
- Microphones ↔ Speakers
- Even thermodynamic reversibility

---

## Natural Crystal Options

| Material | Effect Strength | Notes |
|----------|-----------------|-------|
| **Quartz** | Moderate | The classic! Used in watches. Natural crystals work. |
| **Rochelle Salt** | Very Strong | Can grow at home! Historically first discovered piezoelectric. |
| **Tourmaline** | Moderate | Beautiful natural crystals, pyroelectric too |
| **Ceramic PZT** | Very Strong | Most practical, what buzzers use |
| **Bone** | Weak | Yes, your bones are piezoelectric! |
| **DNA** | Weak | Life itself uses this effect |
| **Sugar (Sucrose)** | Weak | Ordinary table sugar is piezoelectric |

---

## Build 1: The Zero-Power Crystal Telephone

The most elegant demonstration: **two crystals, two wires, zero batteries—and you can talk through it.**

### How It Works

```
┌─────────────────────────────────────────────────────────────────────┐
│                    THE PIEZO TELEPHONE                              │
├─────────────────────────────────────────────────────────────────────┤
│                                                                     │
│   PERSON A                                         PERSON B         │
│                                                                     │
│   "Hello!" ──→ Sound waves                                          │
│                    │                                                │
│                    ▼                                                │
│              ┌──────────┐                      ┌──────────┐         │
│              │  PIEZO   │══════════════════════│  PIEZO   │         │
│              │ CRYSTAL  │     copper wire      │ CRYSTAL  │         │
│              │    A     │══════════════════════│    B     │         │
│              └──────────┘                      └──────────┘         │
│                    │                                  │             │
│           Sound compresses                    Voltage deforms       │
│           crystal → voltage                   crystal → sound       │
│                                                      │              │
│                                                      ▼              │
│                                               "Hello!" ──→ Ear     │
│                                                                     │
│   NO BATTERIES. NO AMPLIFIER. PURE PHYSICS.                        │
└─────────────────────────────────────────────────────────────────────┘
```

### Parts List

| Qty | Component | Where to Get | Cost |
|-----|-----------|--------------|------|
| 2 | Piezo discs (27mm or 35mm) | Amazon, electronics shop, or harvest from greeting cards/buzzers | ~$3 for 10 |
| 2 | Small plastic funnels or cups | Dollar store | ~$1 |
| 10m | Thin insulated wire (any gauge) | Hardware store | ~$3 |
| 2 | Alligator clips (optional) | Electronics shop | ~$2 |
| - | Hot glue or tape | You probably have this | - |

**Total: ~$5-10** (or free if you harvest piezos from old devices!)

### Piezo Disc Anatomy

```
             ┌─────────────────┐
             │   ┌─────────┐   │
             │   │ CERAMIC │   │  ← White/tan ceramic layer
             │   │  (top)  │   │     (this is the active element)
             │   └─────────┘   │
             │                 │
             │  BRASS PLATE    │  ← Gold/brass metal disc
             │    (bottom)     │     (this is the ground/base)
             └─────────────────┘

        Two solder points:
        • Center (ceramic) = Signal (+)
        • Edge (brass) = Ground (-)
```

### Wiring

```
    PIEZO A                              PIEZO B
    ───────                              ───────

    Center ●────── RED WIRE ──────────────● Center
    (signal)                              (signal)

    Edge ●──────── BLACK WIRE ────────────● Edge
    (ground)                              (ground)


    That's it. Two wires connecting two piezos.
    The circuit is COMPLETE.
```

### Building the Acoustic Couplers

The piezo alone is quiet. A funnel or cup acts as an acoustic amplifier:

```
    SIDE VIEW OF HANDSET
    ────────────────────

         ╱          ╲
        ╱            ╲        ← Funnel/cup (collects & focuses sound)
       ╱              ╲
      ╱                ╲
     ╱                  ╲
    ║                    ║
    ║   ┌────────────┐   ║
    ║   │   PIEZO    │   ║    ← Piezo glued across opening
    ║   │    DISC    │   ║
    ║   └────────────┘   ║
    ║                    ║
    ╚════════════════════╝
            │    │
            │    │            ← Wires exit through hole or notch
          to other piezo
```

**Construction Options:**

```
    Option A: Plastic Funnel          Option B: Paper Cup
    ────────────────────              ─────────────────

         ╲      ╱                         ┌─────┐
          ╲    ╱                          │     │
           ╲  ╱                           │     │
            ╲╱                            │     │
         [PIEZO]                          │     │
            │                           ──┴─────┴──
                                        [  PIEZO  ]
    Funnel focuses sound                    │
    onto crystal center              Cup resonates and
                                     amplifies vibrations
```

### Testing

```
┌──────────────────────────────────────────────────────────┐
│                                                          │
│   Person A                              Person B         │
│                                                          │
│      🗣️ ──→ [HANDSET A]═══════════[HANDSET B] ──→ 👂   │
│                                                          │
│   Speak firmly and clearly    Listen closely to cup      │
│   into your cup/funnel        held against ear           │
│                                                          │
│   Then SWAP roles - it works both ways!                  │
│                                                          │
└──────────────────────────────────────────────────────────┘
```

### Expected Results

- **Clear speech** at close range (1-3 meters)
- **Quieter but audible** at longer ranges (up to 30+ meters with good wire)
- **Best results** with higher-pitched voices and consonant-heavy words
- Sound is thin and "crystal radio"-like—because it literally is!

---

## Build 2: The Amplified Version

Adding a simple amplifier makes the crystal telephone much louder and more practical.

### Additional Parts

| Qty | Component | Notes |
|-----|-----------|-------|
| 1 | LM386 audio amplifier chip | Classic, cheap, easy |
| 1 | 8-pin DIP socket | Optional but recommended |
| 2 | 10µF electrolytic capacitors | Input/output coupling |
| 1 | 220µF electrolytic capacitor | Output to speaker |
| 1 | 10Ω resistor | Stability |
| 1 | 0.05µF ceramic capacitor | Stability |
| 1 | 10kΩ potentiometer | Volume control |
| 1 | Small speaker (8Ω) | Or use second piezo |
| 1 | 9V battery + clip | Power |
| 1 | DPDT switch | Mode selector |

### Circuit Diagram

```
                         AMPLIFIED PIEZO MICROPHONE/SPEAKER
    ┌─────────────────────────────────────────────────────────────────────┐
    │                                                                     │
    │                              +9V                                    │
    │                               │                                     │
    │                               │                                     │
    │    ┌──────────┐              ┌┴┐                                    │
    │    │          │         ┌────┤6├────┐                               │
    │    │  PIEZO   │         │    └─┘    │                               │
    │    │  DISC    ├────┬────┤           │                               │
    │    │          │    │    │  LM386    │    ┌────────┐                 │
    │    └──────────┘   ┌┴┐   │           ├────┤220µF   ├────┐            │
    │         │        10µF   │           │    └────────┘    │            │
    │         │         │     │    ┌──┐   │                  │            │
    │         │         │     │   ┌┤5 ├┐  │            ┌─────┴─────┐      │
    │         │         │     │   │└──┘│  │            │  SPEAKER  │      │
    │         │         │     └───┤1  8├──┘            │   (8Ω)    │      │
    │         │         │         │    │               └─────┬─────┘      │
    │         │         │         │2  4│                     │            │
    │         │         │         └┬──┬┘                     │            │
    │         │         │          │  │                      │            │
    │         └─────────┴──────────┴──┴──────────────────────┘            │
    │                              │                                      │
    │                             GND                                     │
    │                                                                     │
    └─────────────────────────────────────────────────────────────────────┘
```

### LM386 Pinout Reference

```
    ┌────────────┐
    │ 1  2  3  4 │     Pin 1 = Gain       Pin 5 = Output
    │ ○  ○  ○  ○ │     Pin 2 = Input(-)   Pin 6 = +V (9V)
    │    LM386   │     Pin 3 = Input(+)   Pin 7 = Bypass
    │ ○  ○  ○  ○ │     Pin 4 = GND        Pin 8 = Gain
    │ 8  7  6  5 │
    └────────────┘
```

### Adding the Mode Switch (Mic ↔ Speaker)

```
    BIDIRECTIONAL CIRCUIT WITH DPDT SWITCH
    ──────────────────────────────────────

                           DPDT SWITCH
                          ┌─────────┐
                          │ A     B │
         PIEZO ●──────────┤●       ●├──────────● AMP INPUT
                          │    ○    │
                          │    │    │
         GROUND ●─────────┤●   │   ●├──────────● AMP OUTPUT
                          │         │
                          └─────────┘
                               │
                          COMMON/CENTER
                               │
                         to speaker/line out


    Position A (UP):    Piezo → Amp → Speaker    (MICROPHONE MODE)
    Position B (DOWN):  Line in → Amp → Piezo    (SPEAKER MODE)
```

---

## Build 3: The Full-Duplex Intercom

Talk AND listen simultaneously—like a real telephone.

### Concept

```
    STATION A                                    STATION B
    ─────────                                    ─────────

    ┌─────────────────┐                    ┌─────────────────┐
    │                 │                    │                 │
    │  [PIEZO MIC]────┼────────────────────┼───►[SPEAKER]    │
    │       ▲         │                    │         │       │
    │       │         │                    │         ▼       │
    │    (speak)      │                    │      (hear)     │
    │                 │                    │                 │
    │  [SPEAKER]◄─────┼────────────────────┼────[PIEZO MIC]  │
    │       │         │                    │         ▲       │
    │       ▼         │                    │         │       │
    │    (hear)       │                    │      (speak)    │
    │                 │                    │                 │
    └─────────────────┘                    └─────────────────┘

    Four wires total (use CAT5 cable - you need 2 of its 4 pairs)
```

### Parts Per Station

| Qty | Component | Purpose |
|-----|-----------|---------|
| 1 | Piezo disc (35mm) | Microphone |
| 1 | Small speaker (8Ω, 2") | Output |
| 1 | LM386 amplifier | Boost signal |
| 1 | 9V battery | Power |
| - | Caps, resistors | Per schematic above |
| 1 | Project box | Housing |

---

## Build 4: The One-Crystal Walkie-Talkie

**The ultimate demonstration**: a single crystal that switches between microphone and speaker mode, just like a walkie-talkie. Press to talk, release to listen—all through ONE piezo element.

### Design Philosophy

Traditional walkie-talkies are "half-duplex"—you either transmit OR receive, never both simultaneously. This is actually perfect for demonstrating piezo reversibility because:

1. **One crystal does both jobs** (no separate mic and speaker)
2. **Same wires, same connections** (only the signal direction changes)
3. **The switch moment is the "aha!"** (same crystal, opposite function)

### System Overview

```
┌─────────────────────────────────────────────────────────────────────┐
│                  ONE-CRYSTAL WALKIE-TALKIE                          │
├─────────────────────────────────────────────────────────────────────┤
│                                                                     │
│   UNIT A                                         UNIT B             │
│   ──────                                         ──────             │
│                                                                     │
│   ┌─────────────┐                           ┌─────────────┐         │
│   │             │                           │             │         │
│   │   [PIEZO]   │═══════════════════════════│   [PIEZO]   │         │
│   │      │      │      signal wire          │      │      │         │
│   │      │      │═══════════════════════════│      │      │         │
│   │   [PTT]     │      ground wire          │   [PTT]     │         │
│   │   BUTTON    │                           │   BUTTON    │         │
│   │             │                           │             │         │
│   └─────────────┘                           └─────────────┘         │
│                                                                     │
│   PTT PRESSED:  Crystal = MICROPHONE (transmit)                     │
│   PTT RELEASED: Crystal = SPEAKER (receive)                         │
│                                                                     │
│   When A presses PTT and talks → B hears through their crystal      │
│   When B presses PTT and talks → A hears through their crystal      │
│                                                                     │
└─────────────────────────────────────────────────────────────────────┘
```

### The Key Insight

The "switching" isn't electrical routing—it's **which end is generating signal**:

- When YOU press PTT: Your voice → your crystal → voltage → wire → their crystal → sound
- When THEY press PTT: Their voice → their crystal → voltage → wire → your crystal → sound

The crystals are ALWAYS connected. The "mode" depends on who's speaking!

### Circuit Design

```
                    ONE-CRYSTAL WALKIE-TALKIE UNIT
    ┌─────────────────────────────────────────────────────────────┐
    │                                                             │
    │                         +9V                                 │
    │                          │                                  │
    │                          │                                  │
    │    ┌────────┐           ┌┴┐                                 │
    │    │        │      ┌────┤ ├────┐                            │
    │    │ PIEZO  │      │    └─┘    │                            │
    │    │  DISC  ├──┬───┤  LM386   ├────┐                        │
    │    │        │  │   │          │    │                        │
    │    └────────┘  │   └────┬─────┘    │                        │
    │        │       │        │          │                        │
    │        │      ┌┴┐       │     ┌────┴────┐                   │
    │        │     10µF       │     │ 220µF   │                   │
    │        │       │        │     └────┬────┘                   │
    │        │       │        │          │                        │
    │   ┌────┴───────┴────────┴──────────┤                        │
    │   │                                │                        │
    │   │         ACTIVE PATH            │                        │
    │   │    (selected by PTT switch)    │                        │
    │   │                                │                        │
    │   └───────────┬────────────────────┘                        │
    │               │                                             │
    │          ┌────┴────┐                                        │
    │          │  DPDT   │                                        │
    │          │   PTT   │  ← Push-To-Talk button                 │
    │          │ SWITCH  │                                        │
    │          └────┬────┘                                        │
    │               │                                             │
    │    ┌──────────┴──────────┐                                  │
    │    │                     │                                  │
    │   TO WIRE              ACTIVE                               │
    │   (other unit)         INDICATOR                            │
    │                        LED (red=TX)                         │
    │                                                             │
    └─────────────────────────────────────────────────────────────┘
```

### Detailed Schematic

```
                              +9V
                               │
                               │
        ┌──────────────────────┼──────────────────────┐
        │                      │                      │
        │                     ┌┴┐                     │
        │                     │ │ 10Ω                 │
        │                     └┬┘                     │
        │                      │                      │
        │    ┌─────────────────┴─────────┐            │
        │    │                           │            │
        │    │  ┌───┐     LM386     ┌───┐│            │
        │    │  │ 1 ├───────────────┤ 8 ││            │
        │    │  ├───┤               ├───┤│            │
        │    └──┤ 2 │               │ 7 ├┘            │
        │       ├───┤               ├───┤             │
    IN+ ├───┬───┤ 3 │               │ 6 ├─────────────┘
        │   │   ├───┤               ├───┤
        │  ┌┴┐  │ 4 ├───────────────┤ 5 ├──────┐
        │ 10µF  └───┘               └───┘      │
        │   │     │                            │
        │   │     │                        ┌───┴───┐
        │   │     │                        │ 220µF │
        │   │     │                        └───┬───┘
        │   │     │                            │
        └───┴─────┴────────────────────────────┤
                          │                    │
                         GND              TO PIEZO (speaker mode)
                                          OR TO WIRE (transmit mode)
                                               │
                                          ┌────┴────┐
                                          │  DPDT   │
                                          │   PTT   │
                                          └────┬────┘
                                               │
                                    ┌──────────┴──────────┐
                                    │                     │
                               TO WIRE OUT           BACK TO PIEZO
                              (when pressed)        (when released)
```

### PTT Switch Wiring Detail

```
    DPDT SWITCH (Push-To-Talk)
    ──────────────────────────

    The DPDT switch has 6 terminals arranged in 2 rows:

         ACTIVE ACTIVE
           ↓     ↓
        ┌─────────────┐
        │  ○     ○    │ ← Normally Connected (PTT released = RECEIVE)
        │             │
        │  ●     ●    │ ← Common (center terminals)
        │             │
        │  ○     ○    │ ← Connected when Pressed (PTT pressed = TRANSMIT)
        └─────────────┘


    WIRING:

                    PIEZO
                      │
                      ▼
        ┌─────────────●─────────────┐
        │          (common)         │
        │                           │
        ○ ← To Amp Input            ○ ← To Amp Output
        │   (RECEIVE mode)          │   (not used here)
        │                           │
        ○ ← To Wire Out             ○ ← From Amp Output
            (TRANSMIT mode)             (TRANSMIT mode)


    PTT RELEASED (Receive Mode):
    ────────────────────────────

    Wire In ──→ PIEZO ──→ Amp ──→ (internal speaker or earpiece)

    The incoming signal from the OTHER unit drives YOUR piezo as a speaker.
    Your amp boosts it so you can hear.


    PTT PRESSED (Transmit Mode):
    ───────────────────────────

    Your Voice ──→ PIEZO ──→ Amp ──→ Wire Out ──→ OTHER UNIT

    Your piezo acts as microphone.
    Your amp boosts the signal for transmission.
    The other unit's piezo reproduces it as sound.
```

### Complete Parts List (For Two Units)

| Qty | Component | Notes |
|-----|-----------|-------|
| 2 | Piezo discs (35mm preferred) | Larger = louder |
| 2 | LM386 audio amplifier IC | Or LM386N-1 for lower gain |
| 2 | 8-pin DIP sockets | Protects IC, allows replacement |
| 4 | 10µF electrolytic capacitors | 16V or higher |
| 2 | 220µF electrolytic capacitors | Output coupling |
| 2 | 10Ω resistors | Amp stability |
| 2 | 0.047µF ceramic capacitors | High frequency filtering |
| 2 | 10kΩ potentiometers | Volume control |
| 2 | DPDT momentary switches | Push-to-talk buttons |
| 2 | LEDs (red) | TX indicator |
| 2 | 330Ω resistors | LED current limiting |
| 2 | 9V batteries + clips | Power |
| 2 | Project enclosures | Altoids tins work great! |
| 20m | 2-conductor wire | Or use headphone cable |
| - | Hookup wire, solder, etc. | Standard supplies |

### Enclosure Ideas

```
    ALTOIDS TIN WALKIE-TALKIE
    ─────────────────────────

    ┌─────────────────────────────────────┐
    │ ┌─────┐                       ┌───┐ │
    │ │PIEZO│ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○│LED│ │  ← Holes for sound
    │ │     │ (sound holes)         └───┘ │
    │ └─────┘                             │
    │                              [VOL]  │  ← Volume knob
    │                                     │
    │    ════════════════════════════     │  ← Wire exit notch
    │    │                          │     │
    │    │      [PTT BUTTON]        │     │  ← Big, satisfying button
    │    │                          │     │
    └────┴──────────────────────────┴─────┘


    INSIDE VIEW:

    ┌─────────────────────────────────────┐
    │  ┌────────┐    ┌──────────────┐     │
    │  │ 9V     │    │   CIRCUIT    │     │
    │  │BATTERY │    │    BOARD     │     │
    │  └────────┘    │  (LM386 etc) │     │
    │                └──────────────┘     │
    │   ┌──────────────────────────┐      │
    │   │      PIEZO DISC          │      │
    │   │   (glued to lid from     │      │
    │   │    inside, sound holes   │      │
    │   │    drilled above it)     │      │
    │   └──────────────────────────┘      │
    └─────────────────────────────────────┘
```

### Operating Instructions

```
┌─────────────────────────────────────────────────────────────────────┐
│                    HOW TO USE YOUR WALKIE-TALKIE                    │
├─────────────────────────────────────────────────────────────────────┤
│                                                                     │
│   1. Connect the two units with the wire                            │
│                                                                     │
│   2. Turn on both units (if you added a power switch)               │
│                                                                     │
│   3. TO TRANSMIT:                                                   │
│      • Press and HOLD the PTT button                                │
│      • Speak clearly toward the piezo disc                          │
│      • Your LED lights up (red = transmitting)                      │
│                                                                     │
│   4. TO RECEIVE:                                                    │
│      • Release the PTT button                                       │
│      • Listen! Sound comes from your piezo                          │
│      • LED is off (receiving mode)                                  │
│                                                                     │
│   5. ETIQUETTE:                                                     │
│      • Say "over" when done speaking                                │
│      • Wait for the other person to respond                         │
│      • Don't press PTT while they're transmitting!                  │
│                                                                     │
│   ⚠️  SAME CRYSTAL IS YOUR MIC AND SPEAKER!                        │
│      That's the whole point. Magic. ✨                              │
│                                                                     │
└─────────────────────────────────────────────────────────────────────┘
```

### Advanced: VOX (Voice-Activated Switching)

Want hands-free operation? Add a VOX circuit that automatically switches to transmit when you speak:

```
    VOX CIRCUIT (Voice-Operated Switch)
    ───────────────────────────────────

    Piezo Output ──→ Amplifier ──→ Rectifier ──→ Comparator ──→ Relay/MOSFET
                                      │              │
                                      │              ▼
                                      │         Threshold
                                      │         Adjustment
                                      ▼              │
                                  Smoothing     ┌────┴────┐
                                  Capacitor     │  10kΩ   │
                                      │         │   POT   │
                                      ▼         └─────────┘
                                  To Comparator
                                  Reference

    When voice signal exceeds threshold → switches to TX mode
    When silent → returns to RX mode

    Add ~500ms delay on release to prevent cutting off words.
```

---

## Growing Your Own Piezoelectric Crystals

The most magical version uses crystals you grew yourself. **Rochelle salt** (potassium sodium tartrate) is strongly piezoelectric and can be made from kitchen ingredients!

### Rochelle Salt Crystals

#### Background

Rochelle salt was the first material discovered to have piezoelectric properties (1880, by the Curie brothers). It has one of the **strongest piezoelectric effects** of any known material—much stronger than quartz!

#### Ingredients

| Ingredient | Amount | Where to Get |
|------------|--------|--------------|
| Cream of tartar (potassium bitartrate) | 200g | Grocery store, baking aisle |
| Baking soda (sodium bicarbonate) | ~80g | Grocery store |
| Distilled water | 500ml | Grocery store or pharmacy |

#### Equipment

- Large glass jar or beaker (1L+)
- Saucepan for heating water
- Coffee filters or paper towels
- String or fishing line
- Pencil or stick (to suspend crystal)
- Thermometer (optional but helpful)
- Patience (crystals take days to grow!)

#### Process

```
    ROCHELLE SALT CRYSTAL GROWING
    ─────────────────────────────

    STEP 1: Make Sodium Tartrate Solution
    ──────────────────────────────────────

    Cream of Tartar + Baking Soda + Water → Rochelle Salt + CO₂

    KHC₄H₄O₆ + NaHCO₃ → KNaC₄H₄O₆·4H₂O + CO₂ + H₂O


    STEP 2: Dissolve & React
    ────────────────────────

        ┌─────────────────┐
        │   Boiling       │
        │    Water        │  ← 500ml distilled water
        │   ~100°C        │
        │                 │
        │  + 200g cream   │  ← Add gradually, stir to dissolve
        │    of tartar    │
        │                 │
        │  + baking soda  │  ← Add SLOWLY (fizzes!)
        │   until fizzing │     Stop when fizzing stops
        │     stops       │     (usually ~80g)
        │                 │
        └─────────────────┘


    STEP 3: Filter & Cool
    ─────────────────────

        Hot Solution ──→ Coffee Filter ──→ Clean Jar
                             │
                             ▼
                        (catches impurities)

        Let solution cool slowly to room temperature.
        Cover with cloth (allows evaporation, blocks dust).


    STEP 4: Seed Crystal Formation (3-7 days)
    ─────────────────────────────────────────

        ┌─────────────────┐
        │                 │
        │   Saturated     │
        │   Solution      │  As water evaporates:
        │                 │  • Solution becomes supersaturated
        │                 │  • Small crystals form on bottom/sides
        │    ░░░░░░░░     │  • These are your "seed crystals"
        │    ░░░░░░░░     │
        └─────────────────┘

        Select the clearest, most regular seed crystal.


    STEP 5: Growing the Main Crystal (1-4 weeks)
    ────────────────────────────────────────────

               ┌──────┐
               │pencil│
        ═══════╪══════╪═══════
               │string│
        ┌──────│──────│──────┐
        │      │      │      │
        │      │      ▼      │
        │      │   ┌─────┐   │
        │      │   │SEED │   │  ← Suspended seed crystal
        │      │   │XTAL │   │     grows larger over time
        │      └───┴─────┘   │
        │                    │
        │   Fresh Solution   │  ← Make new saturated solution
        │                    │     (same recipe, filter well)
        │                    │
        └────────────────────┘

        • Keep in stable temperature (fluctuations = cloudy crystals)
        • Cover to prevent dust
        • Check daily, remove any competing crystals
        • Crystal grows ~1-3mm per day


    STEP 6: Harvest & Protect
    ─────────────────────────

        Remove crystal when desired size reached (2-5cm ideal).

        ⚠️ IMPORTANT: Rochelle salt is WATER SOLUBLE!

        • Don't get it wet
        • Coat with clear nail polish or lacquer to protect
        • Store in dry place
        • Handle with dry hands
```

#### Tips for Best Results

| Factor | Recommendation |
|--------|----------------|
| **Water** | Use distilled—tap water minerals cause cloudiness |
| **Temperature** | Keep stable! Fluctuations ruin crystal clarity |
| **Patience** | Slower growth = clearer, stronger crystals |
| **Cleanliness** | Filter solution well, keep dust out |
| **Seed selection** | Pick the clearest, most regular small crystal |

### Testing Your Homegrown Crystal

```
    PIEZO TEST SETUP
    ────────────────

    ┌─────────────────────────────────────────────────────┐
    │                                                     │
    │   Option A: LED Test (visual)                       │
    │   ────────────────────────────                      │
    │                                                     │
    │       ┌─────────┐                                   │
    │       │ CRYSTAL │                                   │
    │       └────┬────┘                                   │
    │            │                                        │
    │        ┌───┴───┐                                    │
    │        │  LED  │    TAP crystal firmly              │
    │        └───┬───┘    LED should flash!               │
    │            │                                        │
    │           GND                                       │
    │                                                     │
    │   Note: May need larger crystal or harder tap       │
    │   for visible LED flash. Rochelle salt has          │
    │   strong effect so this usually works well.         │
    │                                                     │
    │                                                     │
    │   Option B: Multimeter Test (measured)              │
    │   ─────────────────────────────────────             │
    │                                                     │
    │       ┌─────────┐                                   │
    │       │ CRYSTAL │                                   │
    │       └────┬────┘                                   │
    │            │                                        │
    │        ┌───┴───┐                                    │
    │        │  DVM  │    Set to DC Voltage               │
    │        │(volts)│    Squeeze or tap crystal          │
    │        └───┬───┘    Watch for voltage spike!        │
    │            │                                        │
    │           GND       (Can see 1-10V from good        │
    │                      Rochelle salt crystal)         │
    │                                                     │
    │                                                     │
    │   Option C: Oscilloscope Test (waveform)            │
    │   ──────────────────────────────────────            │
    │                                                     │
    │       ┌─────────┐                                   │
    │       │ CRYSTAL │                                   │
    │       └────┬────┘                                   │
    │            │                                        │
    │        ┌───┴───┐                                    │
    │        │ SCOPE │    Speak near crystal              │
    │        │ probe │    See your voice as waveform!     │
    │        └───────┘                                    │
    │                                                     │
    └─────────────────────────────────────────────────────┘
```

### Attaching Electrodes

```
    ELECTRODE APPLICATION
    ─────────────────────

    Rochelle salt crystals are soft and water-soluble,
    so electrode attachment requires care:


    METHOD 1: Conductive Paint/Ink
    ──────────────────────────────

         ┌─────────────────┐
         │▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓│ ← Conductive silver paint
         │                 │    on top face
         │                 │
         │    CRYSTAL      │
         │                 │
         │                 │
         │▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓│ ← Conductive paint
         └─────────────────┘    on bottom face
               │     │
               │     └─────── Wire attached with
               │              conductive epoxy
               └───────────── or soldered to paint


    METHOD 2: Metal Foil Sandwich
    ─────────────────────────────

         ┌─────────────────┐
         │   COPPER FOIL   │ ← Thin copper/aluminum foil
         ├─────────────────┤    (from craft store)
         │                 │
         │    CRYSTAL      │ ← Crystal pressed between
         │                 │    foil layers
         ├─────────────────┤
         │   COPPER FOIL   │
         └────────┬────────┘
                  │
             To circuit


    METHOD 3: Pressure Contacts
    ───────────────────────────

        ┌──────────────────────┐
        │    SPRING CLIP       │ ← Gentle spring pressure
        │         │            │    holds contact
        │         ▼            │
        │    ┌─────────┐       │
        │    │ CRYSTAL │       │
        │    └─────────┘       │
        │         ▲            │
        │         │            │
        │    SPRING CLIP       │
        └──────────────────────┘

    Good for testing, easy to reposition.
    Use alligator clips with rubber covers.
```

### Other Growable Piezoelectric Crystals

| Crystal | Difficulty | Piezo Strength | Notes |
|---------|------------|----------------|-------|
| **Rochelle Salt** | Easy | Very Strong | Best for beginners |
| **Potassium Dihydrogen Phosphate (KDP)** | Medium | Moderate | Used in optics |
| **Ammonium Dihydrogen Phosphate (ADP)** | Medium | Moderate | Similar to KDP |
| **Triglycine Sulfate (TGS)** | Hard | Very Strong | Ferroelectric, complex |
| **Sugar (Sucrose)** | Easy | Very Weak | Yes, it's piezoelectric! Fun fact. |

---

## Demonstration Script

Use this when presenting to friends, students, or curious minds:

### Act 1: "Crystals Make Electricity"

> *"This crystal has a special property discovered in 1880. When I squeeze it, it generates electricity—no batteries needed."*

**Demo:** Connect piezo or Rochelle salt crystal to multimeter (DC voltage) or LED. Tap crystal → voltage spike / LED flicker.

```
    TAP! ──→ [CRYSTAL] ──→ LED FLICKERS ✨

    "The atoms inside are arranged asymmetrically.
     Pressure shifts them, separating + and - charges.
     That's voltage. From NOTHING but pressure!"
```

### Act 2: "Electricity Moves Crystals"

> *"Now watch—if I PUT electricity into the same crystal..."*

**Demo:** Connect function generator or 9V battery through momentary switch. Crystal clicks/ticks.

```
    VOLTAGE ──→ [CRYSTAL] ──→ CLICK! 🔊

    "The voltage pushes those same atoms.
     They shift, the crystal flexes.
     That flex pushes air—that's SOUND."
```

### Act 3: "The Same Wires, Both Directions"

> *"Here's the magical part. I haven't changed anything—same crystal, same wires. Energy just flows... either way."*

**Demo:** Crystal telephone. Person A talks → Person B hears. Swap.

```
    "There's no switch that says 'microphone' or 'speaker.'
     The crystal doesn't KNOW which one it's being.
     It just... converts. Either direction.
     Nature doesn't care about cause and effect."
```

### Act 4: "This is Everywhere"

> *"This isn't just a party trick. This is how..."*

- ⌚ Your watch keeps time (quartz oscillator)
- 🔊 Ultrasound sees babies (piezo transducers)
- 🎸 Electric guitars sing (piezo pickups)
- 💡 Lighters spark (piezo igniter)
- 🦴 Your bones heal (stress-generated charges guide cell growth!)

### Act 5: "You Can Grow Your Own"

> *"And the most amazing part? You can grow these crystals at home, from ingredients in your kitchen."*

Show the Rochelle salt crystal (if you've grown one).

```
    "Cream of tartar and baking soda.
     That's all. Let it evaporate, and nature does the rest.
     The crystal structure itself has this property built in.
     No manufacturing. No special treatment.
     Just... the way atoms like to arrange themselves."
```

---

## Troubleshooting

| Problem | Likely Cause | Solution |
|---------|--------------|----------|
| **No sound at all** | Bad solder joint or broken wire | Check continuity with multimeter |
| **Very quiet** | Small piezo, weak acoustic coupling | Use larger disc (35mm+), improve chamber |
| **Distorted/scratchy** | Piezo touching housing | Ensure only edge is mounted, center free |
| **Works one direction only** | Wire break or bad joint | Check all connections end-to-end |
| **Hum/buzz (amplified version)** | Ground loop or RF pickup | Use shielded cable, add 0.1µF bypass cap |
| **Crystal not responding** | Electrodes not contacting well | Improve electrode contact, try conductive paint |
| **Grown crystal is cloudy** | Temperature fluctuations during growth | Start over with stable temperature |
| **Grown crystal dissolves** | Got wet! | Rochelle salt is water-soluble—keep dry, seal with lacquer |

---

## Further Experiments

### Experiment 1: Distance Test
How far apart can two crystal telephone stations be before signal degrades? Test with 10m, 30m, 100m wire. What factors limit distance?

### Experiment 2: Frequency Response
Connect piezo to oscilloscope, play different frequencies through it. What frequencies does it respond to best? (Hint: there's a resonant frequency!)

### Experiment 3: Crystal Comparison
Compare piezoelectric response of:
- Different size ceramic piezos
- Homegrown Rochelle salt
- Natural quartz (if you can find/afford one)
- A guitar piezo pickup

### Experiment 4: The Bone Question
Your bones are piezoelectric. This is thought to help them respond to stress (Wolff's Law). Can you detect piezoelectric signals from a chicken bone? (Hint: you'll need sensitive equipment and proper preparation)

### Experiment 5: Piezo Energy Harvesting
How much energy can you harvest from walking? Put piezos in your shoes, rectify the output, charge a capacitor. Can you light an LED from walking?

### Experiment 6: Ultrasonic Communication
Piezos can vibrate at ultrasonic frequencies. Can you build a communication system that humans can't hear? (Dogs might notice though!)

---

## Resources

### Where to Buy Components

- **Piezo discs**: Amazon, Adafruit, SparkFun, DigiKey
- **LM386 amplifier**: Any electronics supplier
- **Rochelle salt ingredients**: Grocery store (cream of tartar, baking soda)
- **Natural quartz crystals**: Mineral shops, eBay, geology suppliers

### Further Reading

- *Piezoelectricity* by Walter Cady (the classic textbook)
- *Piezoelectric Transducers and Applications* by Antonio Arnau
- Crystal growing guides at growyourowncrystals.com

### Historical Notes

- 1880: Curie brothers discover piezoelectricity in quartz and Rochelle salt
- 1917: First practical application—submarine sonar in WWI
- 1921: First piezoelectric oscillator for radio
- 1969: Quartz watches revolutionize timekeeping
- Today: Billions of piezo devices in phones, cars, medical equipment

---

## Final Thoughts

The piezoelectric effect reveals something profound about nature: **at the fundamental level, cause and effect are interchangeable.** The same physical law that says "stress creates charge" simultaneously says "charge creates stress."

When you build these devices, you're not just making toys—you're holding a demonstration of deep physical symmetry in your hands. The crystal doesn't know if it's a microphone or a speaker. It doesn't care. It just converts, in whichever direction energy flows.

That's not engineering. That's nature.

---

*Document created: January 2026*
*For the curious minds who want to hold physics in their hands.*