================================================================================ SCHEMATIC: CLASSIC CRYSTAL RADIO TM-CR-001 Rev A Ferrite rod antenna, 1N34A detector, AM broadcast 530–1700 kHz + 160M Fully passive — no battery required ================================================================================ OVERVIEW -------- The classic crystal radio converts AM broadcast RF energy directly into audio using only passive components: a resonant tank circuit selects the station, a germanium diode rectifies (detects) the AM envelope, and a high-impedance earphone converts the audio current to sound. No power source is required. All operating energy comes from the received radio frequency signal. Practical minimum detectable signal: ≈5 mV/m at antenna, corresponding to a medium-power broadcast station at 5–20 km range with a good outdoor antenna. DESIGN SPECIFICATIONS ---------------------- Frequency range: 530–1700 kHz (AM broadcast); selectable for 160M (1.8–2.0 MHz) Antenna: Ferrite rod loopstick (7" = 178 mm, mix-61 or mix-77 ferrite) or long-wire (10–30 m) via coupling winding Detector: 1N34A germanium diode (V_f ≈ 0.2V, C_j ≈ 1 pF) Earphone: Crystal (piezoelectric): 50 kΩ–2 MΩ OR high-Z magnetic: 2000–4000 Ω OR 8 Ω headphones via matching transformer (see sch_headphone_transformer.txt) Tuning: 365 pF single-gang air variable capacitor Selectivity: −3 dB bandwidth ≈ 10–20 kHz at 1 MHz (Q ≈ 50–100) COMPLETE SCHEMATIC — AM BROADCAST CRYSTAL RADIO ------------------------------------------------- ┌─────────────────────────────┐ ANT COUPLING │ MAIN TUNED CIRCUIT │ (ferrite rod or │ │ link winding) │ L1 (ferrite rod, 220 µH) │ │ ┌────┐ │ ANTENNA ── (optional) ── ├──────┤ ├──────────────────┤ │ └────┘ │ Or: antenna directly │ C1 (365 pF variable) │ ┌── DETECTOR to tap on L1 │ ┌───┤├───┐ │ │ ├──────┤ ├─────────────┤ │ GROUND ───────────────── ├──────┴─────────┴─────────────┤ │ └───────────────────────────── │ │ │ │ Simplified one-line diagram: │ │ │ │ ANT ──┬──────────────────────────────────────────────────┘ │ │ DETECTOR│ [L1] ══ ferrite rod coil, 220µH │ │ │ [C1] 365pF variable │ │ │ GND ────────────────────────────────────────────────── ┘ DETAILED SCHEMATIC ------------------ ANTENNA ─────────────────────────────────────── A │ ┌─────── L1 ────────┐ │ (Ferrite rod) │ 220 µH loopstick│ │ │ or long-wire │ │ │ link coupling │ │ │ │ ┌──── ANT ────────┤ ├──────── A ─────┐ │ │ │ │ │ ┌───── WIPER ──────┐ │ │ │ │ C1: 365 pF │ │ │ │ │ variable cap │ │ │ │ └──────────────────┘ │ │ │ │ │ │ └── GND ──────────┴──────────────── GND ─────────┴──── GND Point A (top of tank circuit) connects to detector: Point A │ [D1] 1N34A (anode toward A, cathode toward earphone) │ ├──────── [R_load: 470 kΩ or earphone] ──── GND │ AF output (to earphone or headphone transformer) DETECTOR DETAIL: Tank top ─── [D1: 1N34A] ─────────────────────── AF (+) (anode → cathode →) │ [C2: 0.1 µF] [EARPHONE or R_load] │ │ GND ──────────────────────────────── ┴────────────────────┴──── GND C2 (0.1 µF) is the RF bypass/detector filter capacitor. It short-circuits the RF component of the detected signal to GND, leaving only the audio-frequency AM envelope at the output. WITHOUT C2: RF and audio both appear at output (not desirable). WITH C2: Only AF envelope remains → audio to earphone. COMPONENT SELECTION AND VALUES -------------------------------- L1 — FERRITE ROD LOOPSTICK COIL: Core: 7-inch (178 mm) ferrite rod, 9.5 mm diameter Mix: Type 61 (permeability µ = 125, good to 3 MHz) or Type 77 (µ = 2000; better sensitivity, slightly less Q) or Type 73 for AM broadcast (µ = 2500) Wire: #28 AWG enameled (magnet wire), single-layer Turns: 110 turns for 220 µH on 9.5 mm rod Winding length: 50 mm; position on rod adjustable for Q optimization Coil form: 3D printed (see openscad/coil_former.scad) Tap: At 20–25 turns from ground end: low-impedance antenna coupling tap At 50 turns from ground end: medium-impedance tap MEASURING INDUCTANCE: Use LC meter or function generator + oscilloscope. Target: 220 µH ± 10%. At 1 MHz: X_L = 2π × 10⁶ × 220×10⁻⁶ = 1382 Ω C1 — VARIABLE CAPACITOR: Type: 365 pF air variable (classic broadcast type) or dual-gang 365+365 pF (for TRF version; use single gang here) Min C: ≈15 pF (with stray); Max C: 365 pF Tuning range at 220 µH: f_max = 1/(2π√(220µH × 15pF)) = 2.77 MHz (covers 160M if desired) f_min = 1/(2π√(220µH × 365pF)) = 563 kHz (covers 530 kHz AM low end) Knob: Direct drive (no vernier) for simple sets; 10:1 vernier reduction for fine tuning (recommended) D1 — DETECTOR DIODE: Part: 1N34A germanium point-contact diode Alternatives: OA79, OA47, OA91, AAZ15 (all germanium Schottky) For small signals: 1N270 also works WHY GERMANIUM: Forward voltage V_f ≈ 0.15–0.25V vs. silicon V_f ≈ 0.6V. Crystal radios have very limited RF voltage (often <0.5V peak). Silicon diodes require >0.6V to conduct; germanium starts conducting at much lower levels → critical for sensitivity. DO NOT use 1N914, 1N4148 (silicon) — too high threshold voltage. Schottky (BAT43, 1N5711) can work; V_f ≈ 0.25V at low currents. C2 — RF BYPASS / DETECTOR FILTER: Value: 0.001 µF (1000 pF = 1 nF) for standard audio 0.01 µF for lower audio bandwidth (less hiss) 0.1 µF for very strong signals (cleaner audio) Type: Ceramic disc (any type — not in resonant circuit, so C0G not required) EARPHONE / LOAD: Crystal earphone: Z = 50 kΩ – 2 MΩ (piezoelectric); highest sensitivity, no transformer needed; use in parallel with 470 kΩ – 1 MΩ. High-Z magnetic: Z = 2000–4000 Ω; requires 500:1 impedance matching at detector to prevent loading the tank circuit. Low-Z (8 Ω): Requires transformer (see sch_headphone_transformer.txt). DETECTOR LOAD IMPEDANCE RULE: R_load should be >> Z_tank to avoid detuning and loading. Z_tank = Q² × (L/C) at resonance; for Q=80, L=220µH, C=100pF: Z_tank = 80² × (220µH/100pF) = 6400 × 2200 = 14 MΩ (theoretical) In practice (with antenna and losses): Z_tank ≈ 200 kΩ – 2 MΩ. Crystal earphone (> 500 kΩ) presents negligible loading. ✓ 8 Ω headphone (without transformer) would load and kill Q. ✗ COIL WINDING PROCEDURE (FERRITE ROD) -------------------------------------- 1. Print coil form from openscad/coil_former.scad. Form slides on 9.5 mm ferrite rod; allows position adjustment. 2. Wind 110 turns of #28 AWG magnet wire, single layer, close-wound. Wind from one end toward center. Do NOT overlap turns; keep even spacing. 3. After 20 turns: bring out a tap wire (tie a loop and continue winding). Label this "ANT TAP" (low-Z antenna coupling). 4. After 50 turns: bring out a tap wire. Label this "MED TAP". 5. Continue to 110 turns total. 6. Secure winding ends with a drop of nail varnish at each end. 7. Measure inductance. Adjust by spreading or compressing turns. INDUCTANCE vs TURNS (approximate, 9.5 mm rod, Mix 61, 50 mm winding): 50 turns: ≈ 50 µH 80 turns: ≈ 130 µH 110 turns: ≈ 220 µH 150 turns: ≈ 400 µH For 160M (1.8–2.0 MHz) with 100 pF: L needed: 78 µH → use 75 turns Or: use separate 80-turn coil on a short air-core form (50 mm dia, 50 mm long) 160M (1.8–2.0 MHz) CRYSTAL RADIO VARIANT ------------------------------------------ Same circuit; replace ferrite rod with air-core coil: Coil: 50 mm diameter × 50 mm long, 30 turns #24 AWG L ≈ 78 µH C: 100 pF fixed + 100 pF variable (or air-var trimmed) Antenna: Long-wire (10–30 m + counterpoise) for best sensitivity Diode: 1N34A (same) Earphone: Crystal or high-Z type At 160M: Signal levels are generally higher than AM broadcast (amateur operators often run 100–1500 W). Detection is more reliable; audio stronger. PERFORMANCE PREDICTIONS ------------------------ Q factor (unloaded): 80–150 (ferrite rod, good wire, correct core) Selectivity (−3 dB BW): 7–13 kHz at 1 MHz (adequate for AM stations) Minimum detectable E: ≈5–20 mV/m with 15m long-wire antenna Audio output (strong): ≈0.1–2 mW into crystal earphone Range (typical AM xmtr): 10–100 km depending on station power and antenna PARTS LIST ----------- Ref Qty Value/Part Description --- --- ---------- ----------- L1 1 220 µH Ferrite rod loopstick, 7" rod (see text) C1 1 365 pF variable Air variable, single gang D1 1 1N34A Germanium point-contact detector diode C2 1 0.01 µF Ceramic disc, RF bypass R1 1 470 kΩ Earphone shunt (optional; improves audio) J1 1 Binding posts (2) Antenna and ground terminals J2 1 3.5 mm stereo jack Earphone output J3 1 3.5 mm jack External antenna (optional bypass) ================================================================================