Crystal Radio Ppt

Presented by: Shruti Nair(EEE-3rd yr.)  Akanksha Garg(EEE-3rd yr.)

Structure of Presentation 

Introduction  Main parts of crystal radio  Working Principle  Summary 

Introduction  

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. Crystal radios were the first widely used type of radio receiver, and the main type used during the wireless telegraphy era. Crystal radios are the simplest type of radio receiver, and can be handmade with a few inexpensive parts, like an antenna wire, tuning coil of copper wire, crystal detector and earphones . They are passive receivers, while other radios use a separate source of electric power such as a battery or the mains power to amplify the weak radio signal

What is a crystal Radio? A crystal radio receiver, also called a crystal set or cat's whisker receiver, is a very simple radio receiver, popular in the early days of radio. It needs no battery or power source and runs on the power received from radio waves by a long wire antenna. It gets its name from its most important component, known as a crystal detector, originally made with a piece of  crystalline mineral such as galena. This component is now called a diode.

[source: Wikipedia]

The radio components

Main Components:  Antenna  Tuned circuit  Crystal detector  Earphone 

 Antenna The antenna converts the energy in the electromagnetic radio waves striking it to an alternating electric current in the antenna, which is connected to the tuning coil.  Since in a crystal radio all the power comes from the antenna, it is important that the antenna collect as much power from the radio wave as possible. 

Tuned circuit The tuned circuit, consisting of the coil and capacitor, acts as a resonator, analogous to a tuning fork for sound waves .  Electric charge flows rapidly back and forth between the plates of the capacitor through the coil, oscillating at the frequency of the radio signal . 

Crystal Detector  A semiconductor crystal detector which extracts the audio signal (modulation) from the radio frequency carrier wave. It does this by only allowing current to pass through it in one direction, blocking half of the oscillations of the radio wave.  Thisrectifies the alternating current radio wave to a pulsing direct current, whose strength varies with the audio signal. This current can be converted to sound by the earphone. 

Cat’s Whisker Detector

Diode

Schematic Diagram of the Radio

[ Source: Scott Lowe, http://www.angelfire.com/electronic2/index1/DmCrystalSet.html ]

Working Model of Crystal radio

 AM radio signal demodulation A. The Amplitude Modulated carrier is tuned in by the coil and variable capacitor.

B. The signal is rectified (detected) by the diode.

C. The carrier is filtered out and removed by the other capacitor leaving the audio signal.

Uses These have become obsolete for commercial use. However they continue to be built by hobbyists, youth groups as a way of learning about the technology of radio. Today they are sold as educational devices .   A crystal radio tuned to a strong local transmitter can be used just as a power source for a second amplified receiver for distant stations that cannot be heard with a plain crystal radio. 

Modern Crystal Radio with piezoelectric earphone

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