Fun with Radio and Electronics!

Print out this page and have a go at building a crystal radio set yourself!*

INTRODUCTION
If you're stuck inside on a Saturday afternoon with a junk box full of electronic parts and nothing much to do, you can spend an hour or so and build this very simple but effective Crystal Radio receiver. Often, the satisfaction of building these simple radios is lost amidst the latest computer program or the latest digital marvel chip!

THEORY
Briefly, a Crystal Radio receiver derives it energy from. the radiated power from the AM radio stations in your area, This energy travels in waves, and is induced into your Aerial/Earth wire system (more about safe Aerial and Earth wires later on) and then fed into the coil/capacitor circuit. This coil/capacitor combination is known as a "tank" circuit, because it. stores radiated energy. The coil stores this energy in a pulsating electro-magnetic field, while the. variable capacitor stores energy in 'an electro-static field between its fixed and moving sets of plates.

The Ferrite Rod acts as a bar magnet for radio waves. concentrating them inside the turns of the coil. When.you turn the knob of the variable capacitor you change the value of thisdevice. Changing the value of the capacitor will vary the frequency at which"the Crystal Radio receives. When you tune into a local AM radio station, the tank circuit experiences what is called "resonance". This is like a musician using a tuning fork to tune up the strings of his instrument. Each string will 'resonate' with the chosen tuning fork when they are both pitched at the same frequency.

!n the same way, when a radio station and the Crystal Radio are tuned into the same frequency, resonancetakes place. This state of resonance allows for maximum signal transfer between transmitter and receiver. The selected frequency is then fed into the Germanium Diode detector. This corn portent separates (rectifies) the radio station's "carrier wave" from the program signal (voice and music). The rectified signal is thenfed in the Crystal Earpiece where it is converted into sound waves that we can hear.

CONSTRUCTION
Our radio set can be made up on a square of wood or plastic around 10cm by 12cm by 1cm. If you or a parent can solder, then feel free to drive nickel plated nails into the wooden baseboard. Then you can "tin" these with a bit of solder and use each nail head as a tie point to interconnect the component leads. BE VERY CAREFUL with soldering tools - wear eye safety glasses at all times. Alternatively, you can use a screw/washer combination with the DSE plastic baseboard. These should be the #4x9mm screws and the washers around 5mm in diameter (see parts list). If you decide to solder, take special care with soldering the Diode and the solder tags from the Variable Capacitor. The Diode should only be soldered by using a heat sink ( a simple device that will draw any excess heat away from the Diode itself) and this can be a crocodile clip attached to either end of the Diode's leadouts (close to the glass bulb) during soldering. The caseing of the Variable Capacitor is made of plastic, and any excessive heat applied during soldering may well melt a part of this fragile case.

You can use other coils in this simple Crystal Radio design. You can either wind an air-cored coil (if you can't get hold of a Ferrite Rod easily) as shown below (comprised of the same number of turns -70) wound on a paper roll tube complete with taps (at 15 turns and 25 turns) or you can retrieve a factory made coil from an old, disused transistor radio. This coil actually has two windings, and you can tune the Crystal Radio by wiring a 120pF capacitor (instead of the variable capacitor) across the main winding (usually 'white and black wires) and sliding the square Ferrite Rod up and down inside the coil former to tune in local stations (see diagram below).

AERIALS AND EARTHS
The best Aerial is one that is at least two metres high off the ground (higher if possible) and at least ten metres long. You MUST stay well clear of overhead power and telephone cables for safety's sake. The safest Earth wire is one that's connected to a cold water pipe that is embedded in moist soil. A garden tap fitting just outside a window is ideal. Do NOT use the earth pin of a 240 volt power outlet, nor a hot water pipe, nor a gas pipe, as these are all unsafe to use as Earth connections, and may well result in electrical shocks or explosions occuring.

DO NOT operate this Crystal Radio receiver during an electrical thunderstorm.