The Crystal Radio
Created | Updated Jan 28, 2002
One of the small wonders of our time is the Tuned Circuit. A 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. Almost every electronic device uses a tuned circuit in one form or another and 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)).
I hope the advent of digital radio, television and communications does not mean that we lose the simplest form of radio receiver, the Crystal Set. The crystal set, with only four components, is capable of receiving amplitude modulated (A.M.) signals in the short, medium and long wavebands and two of those components comprise a tuned circuit.
I built my first crystal set when I was about twelve years old and the thrill of hearing sounds coming from the headphone was nearly as good as the many hours of illicit fun listening to late night radio under the bedclothes.
The original components were a coil of 60 turns of 24 s.w.g. (0.56mm) insulated copper wire closely wound along a 3” (75mm) diameter by 4” (100mm) long 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. If you’d like to try to make one, you will also need a long wire aerial (antenna) about 20 metres long and as high as possible and a suitable ground or earth connection. (DO NOT 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 thered end of the diode, the other end of which is also connected to the aerial.
The whole thing was 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.
I hope the advent of digital radio, television and communications does not mean that we lose the simplest form of radio receiver, the Crystal Set. The crystal set, with only four components, is capable of receiving amplitude modulated (A.M.) signals in the short, medium and long wavebands and two of those components comprise a tuned circuit.
I built my first crystal set when I was about twelve years old and the thrill of hearing sounds coming from the headphone was nearly as good as the many hours of illicit fun listening to late night radio under the bedclothes.
The original components were a coil of 60 turns of 24 s.w.g. (0.56mm) insulated copper wire closely wound along a 3” (75mm) diameter by 4” (100mm) long 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. If you’d like to try to make one, you will also need a long wire aerial (antenna) about 20 metres long and as high as possible and a suitable ground or earth connection. (DO NOT 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 thered end of the diode, the other end of which is also connected to the aerial.
The whole thing was 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.
