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Basic crystal set

Crystal Set Basics

- an overview of the crystal set giving a basic circuit and description of how it operates.

The crystal set has been in use for many years, finding their first uses at the very beginnings of wireless technology. Crystal sets were also widely used as some of the first broadcast receivers. But with the introduction of more effective radio receivers, the use of the crystal set has declined. Nevertheless, they can still be constructed quite easily, and pull in a variety of the stronger stations that are transmitting.

Crystal set basics
The crystal set is the most basic form of radio receiver consisting simply of a tuned circuit and a demodulator or detector. The circuit of a simple set can be seen to possess very few components. Naturally the simplicity reflects the level of performance which can be achieved, but some more complicated receivers are able to achieve higher levels of performance.

Circuit of a simple crystal set

In the circuit of the crystal set, L1 and VC1 act together to form a resonant circuit. This performs the function of accepting signals on and around the wanted one and rejecting the others. As the inductor and variable capacitor form a parallel tuned circuit, the impedance reaches a maximum at resonance. This means that signals on frequencies either side of the resonant frequency pass to earth. At resonance the signals do not pass through the tuned circuit. Instead they pass into the diode. This rectifies the signal so that the amplitude variations from the signal can be obtained. The small smoothing capacitor C1 acts to remove any remaining radio frequency components and smooths the signal.

AM demodulation

The rectified signals from the diode can be heard by connecting a pair of headphones to the output of the set. However it is soon discovered that the signal strengths are weak even when a good aerial is used. Also the tuning is very broad and a single strong station may be heard over a large part of the tuning range.

Antenna for the crystal set
Crystal set receivers are not at all sensitive. If any signals are to be heard then they require a good aerial or antenna. This should be as long and as high as reasonably possible.

Summary
Although crystal sets are not the height of technology these days, they can prove to be an interesting constructional project. It is easy to get the crystal set working, although a good antenna is required if signals are to be hear at good strength.