Crystal radio

AC circuits and filters · Intermediate · about 15 minutes · part of Electronics Engineering

Two AM stations reach the antenna together. The coil and tuning capacitor resonate at one frequency, building up that station and shunting the others away; the germanium diode detects its envelope, and the earpiece turns that straight into sound. No battery: the radio runs on the energy of the radio wave.

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How it works

Try it on the bench

1. Set Tuning to about 0.29 of its range: you now hear station B’s 400 Hz tone.
2. Change the antenna resistor to 10 kΩ: louder, but both stations bleed through (less selective).

Parts on the bench

• 2 × Function generator
• Resistor
• Axial choke
• Variable capacitor
• Germanium diode
• High-impedance earphone

In the course

Electronics Engineering, module “Filters and frequency response”.

More on ac circuits and filters

• RC low-pass filter
• RC high-pass filter
• RLC series circuit
• RLC parallel circuit
• Single-phase AC circuit
• Peak, peak-to-peak and RMS
• Reactance of L and C
• Capacitors in series and parallel