From 33c3d2483826e22b13040ce2bdd75b9abfa75660 Mon Sep 17 00:00:00 2001
From: Jeff Emmett <jeffemmett@gmail.com>
Date: Tue, 6 Jan 2026 00:03:42 +0100
Subject: [PATCH] Initial commit: Piezoelectric circuits guide and backlog
 tasks
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Add comprehensive guide covering:
- Zero-power crystal telephone (two piezos, no batteries)
- Amplified version with LM386
- Full-duplex intercom
- One-crystal walkie-talkie with PTT switching
- Rochelle salt crystal growing instructions
- Physics explanations and demonstration scripts

Backlog tasks created:
- task-1: Build Unpowered Crystal Telephone (high priority)
- task-2: Build One-Crystal Walkie-Talkie (medium priority)
- task-3: Grow Rochelle Salt Piezoelectric Crystals (medium priority)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
---
 backlog/config.yml                            |   15 +
 ...k-1 - Build-Unpowered-Crystal-Telephone.md |   35 +
 ...ask-2 - Build-One-Crystal-Walkie-Talkie.md |   38 +
 ...ow-Rochelle-Salt-Piezoelectric-Crystals.md |   34 +
 piezoelectric-circuits-guide.md               | 1100 +++++++++++++++++
 5 files changed, 1222 insertions(+)
 create mode 100644 backlog/config.yml
 create mode 100644 backlog/tasks/task-1 - Build-Unpowered-Crystal-Telephone.md
 create mode 100644 backlog/tasks/task-2 - Build-One-Crystal-Walkie-Talkie.md
 create mode 100644 backlog/tasks/task-3 - Grow-Rochelle-Salt-Piezoelectric-Crystals.md
 create mode 100644 piezoelectric-circuits-guide.md

diff --git a/backlog/config.yml b/backlog/config.yml
new file mode 100644
index 0000000..fdee00e
--- /dev/null
+++ b/backlog/config.yml
@@ -0,0 +1,15 @@
+project_name: "Crystal Radio Project"
+default_status: "To Do"
+statuses: ["To Do", "In Progress", "Done"]
+labels: []
+milestones: []
+date_format: yyyy-mm-dd
+max_column_width: 20
+default_editor: "nvim"
+auto_open_browser: true
+default_port: 6420
+remote_operations: true
+auto_commit: false
+bypass_git_hooks: false
+check_active_branches: true
+active_branch_days: 30
diff --git a/backlog/tasks/task-1 - Build-Unpowered-Crystal-Telephone.md b/backlog/tasks/task-1 - Build-Unpowered-Crystal-Telephone.md
new file mode 100644
index 0000000..bbafd86
--- /dev/null
+++ b/backlog/tasks/task-1 - Build-Unpowered-Crystal-Telephone.md	
@@ -0,0 +1,35 @@
+---
+id: task-1
+title: Build Unpowered Crystal Telephone
+status: To Do
+assignee: []
+created_date: '2026-01-05 23:02'
+labels:
+  - hardware
+  - piezoelectric
+dependencies: []
+priority: high
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Build the zero-power piezoelectric crystal telephone that demonstrates the reversibility of the piezoelectric effect. Two crystals connected by wire - no batteries needed\!
+
+This project demonstrates:
+- Direct piezoelectric effect (sound → electricity)
+- Inverse piezoelectric effect (electricity → sound)
+- Energy flows both directions through the same crystal
+
+Refer to piezoelectric-circuits-guide.md for full instructions.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [ ] #1 Acquire 2x piezo discs (27mm or 35mm)
+- [ ] #2 Build acoustic couplers (funnels or cups)
+- [ ] #3 Solder wires between the two piezos
+- [ ] #4 Test communication at 1-3 meters
+- [ ] #5 Test maximum distance achievable
+- [ ] #6 Document results with photos/video
+<!-- AC:END -->
diff --git a/backlog/tasks/task-2 - Build-One-Crystal-Walkie-Talkie.md b/backlog/tasks/task-2 - Build-One-Crystal-Walkie-Talkie.md
new file mode 100644
index 0000000..2ecb26a
--- /dev/null
+++ b/backlog/tasks/task-2 - Build-One-Crystal-Walkie-Talkie.md	
@@ -0,0 +1,38 @@
+---
+id: task-2
+title: Build One-Crystal Walkie-Talkie
+status: To Do
+assignee: []
+created_date: '2026-01-05 23:03'
+labels:
+  - hardware
+  - piezoelectric
+  - electronics
+dependencies: []
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Build the half-duplex walkie-talkie that uses a SINGLE piezo crystal as both microphone AND speaker. Press PTT to talk, release to listen - demonstrating the ultimate reversibility of piezoelectricity.
+
+Key components:
+- LM386 amplifier
+- DPDT momentary switch (PTT)
+- Single piezo disc per unit
+- 9V battery power
+
+Refer to piezoelectric-circuits-guide.md Build 4 section.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [ ] #1 Source LM386 chips and support components
+- [ ] #2 Acquire DPDT momentary switches for PTT
+- [ ] #3 Build amplifier circuit on breadboard/perfboard
+- [ ] #4 Wire PTT switching logic
+- [ ] #5 Build enclosures (Altoids tins work great)
+- [ ] #6 Test two-unit communication
+- [ ] #7 Add TX indicator LED
+<!-- AC:END -->
diff --git a/backlog/tasks/task-3 - Grow-Rochelle-Salt-Piezoelectric-Crystals.md b/backlog/tasks/task-3 - Grow-Rochelle-Salt-Piezoelectric-Crystals.md
new file mode 100644
index 0000000..6bc7e2a
--- /dev/null
+++ b/backlog/tasks/task-3 - Grow-Rochelle-Salt-Piezoelectric-Crystals.md	
@@ -0,0 +1,34 @@
+---
+id: task-3
+title: Grow Rochelle Salt Piezoelectric Crystals
+status: To Do
+assignee: []
+created_date: '2026-01-05 23:03'
+labels:
+  - chemistry
+  - piezoelectric
+  - crystals
+dependencies: []
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Grow piezoelectric crystals from kitchen ingredients (cream of tartar + baking soda). Rochelle salt has one of the strongest piezoelectric effects known\!
+
+This adds a "made from scratch" dimension to the crystal radio project - demonstrating that the piezoelectric property comes from the crystal structure itself, not manufacturing.
+
+Refer to piezoelectric-circuits-guide.md crystal growing section.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [ ] #1 Acquire cream of tartar (200g) and baking soda
+- [ ] #2 Make saturated Rochelle salt solution
+- [ ] #3 Grow seed crystals (3-7 days)
+- [ ] #4 Grow main crystal from seed (1-4 weeks)
+- [ ] #5 Apply electrodes (conductive paint or foil)
+- [ ] #6 Test piezoelectric response with LED/multimeter
+- [ ] #7 Compare to commercial ceramic piezo
+<!-- AC:END -->
diff --git a/piezoelectric-circuits-guide.md b/piezoelectric-circuits-guide.md
new file mode 100644
index 0000000..5eef723
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+++ b/piezoelectric-circuits-guide.md
@@ -0,0 +1,1100 @@
+# 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.*