In this rapidly changing hi-tech world of digital radio, television and communications, it is to be hoped we do not lose the simplest form of radio receiver, the Crystal Set. This simple coil of wire (an inductor) connected to a pair of metal plates separated by an insulator (a capacitor) is capable of unscrambling the thousands of radio signals buzzing around this planet.
What is a Crystal Set?
A crystal set has only four components.
A coil of 60 turns of 24swg (0.56mm) insulated copper wire closely wound along a 3" diameter (75mm) by 4"-long (100mm) cardboard tube.
A 500 picofarad tuning capacitor
A germanium diode (OA90 is the modern equivalent)
And, most difficult to find today, a pair of high impedance headphones, 2000–4000 ohms.
Two of these components comprise a tuned circuit, which is one of the small wonders of our time. Almost every electronic device uses a tuned circuit in one form or another though quartz crystals and frequency synthesizers have replaced it in many applications. It is still used to filter, produce and modify electromagnetic signals from audio frequencies to hundreds of megahertz. For those who like and can remember formulae, the resonant frequency, in Hertz or Cycles per second of a tuned circuit is determined by The reciprocal of (2 * pi * (square root of (Inductance of the coil in Henrys * Capacitance in Farads)).
Making a Crystal Set
If you would like to try to make one, you will also need a long wire aerial (antenna) about 20 metres long and positioned as high as possible, and a suitable ground or earth connection. Never connect to any part of a main electricity supply. A metal water supply pipe or a metal spike driven into the ground works well.
One end of the coil, the moving plates of the capacitor and one wire from the phones are connected to ground. The other end of the coil and the fixed plates of the capacitor are connected to the aerial (antenna). The remaining phone lead is connected to the red end of the diode. The other end of which is also connected to the aerial. The whole thing is mounted on a wooden base, connections being made by fixing the wires under washered screw heads.
More complex designs may give better results but the simplicity of this receiver is one of its greatest charms and shows the properties of the tuned circuit at its best.