What is a Crystal Radio | How to Construct a Crystal Radio

Crystal radio is a very simple radio receiver that was used in the earlier days for radio listening. It does not have much components in them and there were no amplification stages used to boost the signals that were received. This radio uses only the power of the transmitted radio signal to produce the audible sound. The earlier detector consisted of a thin wire or metal probe that  contacts a crystal detector that was placed in an enclosure or hold by a stand. The most common crystal used in these detectors were minerals such as Galena, pyrite etc. The detector was connected to a headphone through which the sound can be heard.

 

 

Image Source: Holger.Ellgaard, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Important Components & Operation of a Crystal Radio

1. Crystal detector: The crystal detector is the most important component in this radio. The crystal that was used as the detector here is called as Galena. It detects or demodulates the radio frequency alternating current to an audio signal. Nowadays, detectors such as semiconductor diodes are used for the same purpose.
2. Tuned coil and capacitor: The second important component is the tuned coil and capacitor. The coil used here is the enameled copper wire, which acts as an inductor. The capacitor used is a variable capacitor that can be varied manually to adjust the capacitance for tuning.
3. Hearing mechanism: The third component is the hearing mechanism that consists of the earphone. It is used to convert weak electrical signals into sound or audio signals. The crystal detector is connected to the earphone and the detected audio is heard through the earphone. The earphone operates on the energy of the transmitted radio wave to produce sound waves or audio which can be heard through the ear-piece.
4. Antenna and ground: The fourth component in a crystal radio is the antenna and the ground. The antenna helps to trap the energy of the radio waves and conveys it to the tuned coil and capacitor circuit. Good grounding is also needed for the circuit to achieve sufficient polarity and thus to function properly.

Image: Crystal radio receiver circuit

 

Connect the antenna terminal to a long wire antenna that is hung outside into the open in a safe area. Attach the ground terminal to a metal stud inserted into the ground. Tune the capacitor from one position to the other until you hear a signal from a radio station that breaks through the receiver. The resonant frequency of the tuned circuit is determined by the coil and capacitor. With the change in the variable capacitor value, you can change the resonant frequency of this tuned circuit. It acts like a band-pass circuit where the resonant circuit allows the signal from the required station to be tuned and the signal from a very small portion of the radio frequency band is allowed to pass through. The tuned radio frequency signal is detected using a small signal Germanium diode into very small audio electrical signals of high impedance. This high impedance output signal is fed to a high impedance earphone or it can be amplified using an audio amplifier.The detected audio from the diode can be heard through the earpiece/headphone or a speaker.

 

Disadvantages of Crystal Radio
1. Only the stations with stronger signals are heard through and the stations with weaker signals are not able to produce enough signal to be heard through the earpiece or speaker without the use of radio frequency amplification.

2. Because of the low selectivity, signals from multiple radio stations can be heard intermittently depending on the power of the signal from the station received.

Note: To improve the received signal through this radio, try different tuned circuits using different number of turns in the coil and different capacitor values and antenna coupling techniques. Type of antennas such as long wire antenna, dipole antenna, loop antenna etc. can be used to experiment with.

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