Signals & Wireless (RF)
Build a crystal radio: tank, diode and earpiece
A crystal radio is the simplest radio receiver there is: an antenna, a tuned circuit that picks one station, a diode detector that pulls out the sound, and an earpiece. It has no battery. The station's own radio wave, caught by the antenna, powers the whole thing.
The parts
- Antenna: a long wire, the longer and higher the better, plus a ground connection
- Tank: a coil and a capacitor in parallel, from the antenna to ground
- Detector: a diode from the tank to the earpiece, and a capacitor across the earpiece
- Earpiece: high impedance, 47 kΩ here; a crystal earpiece is ideal
The tank picks the station
A coil and capacitor in parallel are the opposite of a series pair: at resonance their impedance peaks, so they're nearly an open circuit. Wired from the antenna to ground, the tank lets its own frequency stand across it at full strength, and shorts every other frequency away. With the 2.53 mH coil and 1 nF:
A station on 100 kHz arrives whole. One on 150 kHz finds the tank at under 2 kΩ, which against the antenna's 10 kΩ lets through only about a fifth.
Live experiment
Tune in one station
Two 3 V stations, at 100 and 150 kHz, come in through the antenna's 10 kΩ to a tank, a 2.53 mH coil with the capacitor you pick, then a diode and a 47 kΩ earpiece with a capacitor across it. 1 nF in the tank and 10 nF across the earpiece pass all four checks: one station in, the other out.
“Project: Crystal set” is the project that ends Carrier Cove, Signals & Wireless (RF) World 1. Free with an account. Lessons unlock in order, so you’ll do the 6 before it first.
The detector pulls out the sound
The diode, anode on the tank and cathode toward the earpiece, passes the tops of each carrier cycle. A 10 nF capacitor across the 47 kΩ earpiece holds the level between peaks: 47 kΩ × 10 nF is 470 µs, nearly fifty cycles at 100 kHz, yet short enough to follow a voice. The earpiece hears that level rising and falling with the sound.
For real stations
Broadcast AM sits between about 540 kHz and 1.6 MHz, so real sets use a smaller coil, about 240 µH, wound on a tube, and a variable capacitor of up to 365 pF as the tuning knob. Use a germanium or Schottky diode: it drops only a few tenths of a volt, where a silicon one's 0.6 V would swallow a weak station entirely. The first broadcast listeners, a century ago, heard their stations exactly this way.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Project: Crystal set” is the project that ends Carrier Cove, Signals & Wireless (RF) World 1. Free with an account. Lessons unlock in order, so you’ll do the 6 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: About 100 kHz. 1 ÷ (2π√(2.53 × 10⁻³ × 10⁻⁹)) = 1 ÷ (2π × 1.59 × 10⁻⁶) ≈ 100 kHz.
- Why not “About 10 kHz”? 10 kHz would need a hundred times more L × C. Worked through, the pair gives about 100 kHz.
- Why not “About 1 kHz”? 1 kHz is an audio frequency. It would need ten thousand times more L × C.
- Why not “About 1 MHz”? 1 MHz would need a hundred times less L × C than this pair has.
Show the answer
Answer: An open circuit: the station's full voltage appears across it. A parallel LC is the opposite of a series one: at resonance its impedance peaks. Off resonance it collapses and shorts the other stations to ground.
- Why not “A short circuit”? That's a series LC at resonance. A parallel tank does the opposite: its impedance peaks.
- Why not “50 Ω”? At resonance an ideal tank's impedance is very high, limited only by its losses, nothing like 50 Ω.
- Why not “A diode”? A tank is a coil and a capacitor. The diode is the detector that comes after it.
Show the answer
Answer: Across the earpiece, from its top to ground. It has to hold the earpiece's voltage up between peaks, so it sits across the earpiece. In series it would block the DC level altogether.
- Why not “In series with the diode”? In series it would block the steady level the earpiece needs.
- Why not “Across the generator”? Across the source, before the diode, it would only load the signal, and nothing would hold the earpiece's level between peaks.
- Why not “Nowhere”? With no capacitor the earpiece gets half-wave pulses at the carrier frequency, far too fast to hear.
“Project: Crystal set” is the project that ends Carrier Cove, Signals & Wireless (RF) World 1. Free with an account. Lessons unlock in order, so you’ll do the 6 before it first.
Questions people ask
Does a crystal radio need a battery?
No. The only power it has is the radio wave the antenna catches, which is why it needs a long antenna, a good ground and a sensitive, high-impedance earpiece. It can only play strong, nearby AM stations, and quietly.
Why is it called a crystal radio?
The first detectors were a lump of crystal, usually galena, touched by a fine wire called a cat's whisker. The point contact acted as a diode. Modern sets use a germanium or Schottky diode, which does the same job with no fiddling.
What coil and capacitor tune the AM band?
A coil of about 240 µH with a variable capacitor of up to about 365 pF covers roughly 540 kHz to 1.6 MHz. The frequency goes as 1 ÷ √C, so turning the capacitor from 365 pF down to about 40 pF sweeps the band.
Related topics
- How an envelope detector works, and how to pick R and CA diode lets the peaks through, a capacitor joins them up, and the outline is the sound.
- How an LC band-pass filter picks one stationA coil and a capacitor in the signal's path cancel at one frequency, and fight every other.
- LC resonant frequency explainedA coil and a capacitor swapping energy at one frequency: f₀ = 1 ÷ (2π√(LC)), tanks and tuning.
- How to calculate antenna lengthWavelength is c ÷ f: a quarter of it makes a good antenna, and a loading coil tunes one that's too short.