Own radio receiver

We’ll be able to listen to AM radio broadcasts with something we made our self.  A nice feature of this project is that you can make it as easy or as advanced as we want.

A radio is an electrical device that receives an invisible signal, or radio wave, from a radio station and converts the signal into sound that we hear and understand. A radio wave is a type of electromagnetic radiation that can be used to convey audio information. Radio waves have energy associated with them. Radio stations, using a transmitter and an antenna, transmit waves like the ones in Figure below, which shows both a 1-cycle wave and a 3-cycle wave, each occurring in the span of 1 second. The number of cycles per second is called frequency. The unit for frequency is the hertz (Hz). A 1-cycle-wave per second is a 1 Hz wave and a 3-cycle-wave per second is a 3 Hz wave. Every AM radio station transmits its signal at a given frequency, and the frequency band for AM radio stations in the United States is from 530,000 Hz to 1,710,000 Hz. So a radio station transmitting at 1,590,000 Hz (expressed in kilohertz as 1,590 kHz) is sending out a signal that is 1,590,000 cycles per second.

Waves have both a frequency and amplitude, which is the height of the wave. If someone yells at you from across a room, the amplitude of the sound wave is high. Conversely, if someone whispers to you from across the room, the amplitude of that sound wave is very low. When radio stations transmit sound (or music) waves, they can vary or modulate the amplitude of the wave and that is one way we hear the different levels and frequencies of sound. Stations that transmit signals via amplitude modulation are called AM radio stations.

A crystal radio is a very simple radio that was popular in the early history of radios. It can pick up and play sound from AM radio stations. Rather than rely on outside electrical sources, like a batteries or plugs, crystal radios get their power directly from the radio waves. The diagram in Figure shows the parts of a crystal radio: antenna, coiled wire tuner, diode, earphone, and a connection to an electrical ground.

The antenna picks up AM radio waves which create an alternating current (AC) in the antenna wire. An alternating current is one with a voltage that oscillates between positive and negative.

A diode is an electrical component that allows current to flow in only one direction (positive OR negative). Consequently, when a diode is in a circuit with an alternating current (positive AND negative), it blocks either the positive or the negative half of the wave. The other half of the wave passes through unchanged. This process is called rectification, and it results in alternating current being changed to direct current or DC. The rectified wave only has the positive portion of the original AC wave. When crystal radios were first made the diode was composed of a thin wire that scratched against the surface of a crystal of semi conductive material thus imparting the name "crystal" radio.

The earphones convert the DC to sound. The electrical current is converted into vibration, and that vibration generates sound waves. The sound waves are not very strong though, which is why earphones that fit close to the ear drum, as opposed to a speaker, are required to hear the sound.

The tuner allows you to select the AM frequency for the crystal radio to zone in on. The tuner has many coils of wire. Each AM frequency has resonance with a different length of coil. By changing how much coil is used you can alter the radio’s preferred resonance and thus "tune" in to a specific radio station. Taps, outcroppings of wire at regular coil intervals, are used as places to connect the antenna and/or diode at different wire lengths during the tuning process.

The electrical ground allows current to flow through the circuit (the crystal radio is in fact an electrical circuit). All circuits need a ground to work properly.

To do this fair project, we will need the following materials and equipment:

• Cylindrical oatmeal box, 4-in. diameter (1)
• Masking tape
• Mounting board, wood, about 6 in. × 9 in. (1)
• Screw, any size to tether wire to mounting board (1)
• Screwdriver

Spool of 20 or 22 gauge solid plastic insulated wire, 75-foot (1);

Germanium diode(1n34, 1n34a, 1n60 etc.);

47-kohm resistor, 1/4- or 1/2-watt (1);

Alligator clip (2);

High-impedance ceramic earphone (1);

PVC pipe coupling, ¾ inch, (2);

Fahnestock clips (4);

Steps for the making project:

1. Take the oatmeal container (empty, of course) and on the open end, come down about a 1/2 in. and carefully poke two holes. Thread the wire through one hole and back out through the other, as shown in Figure, below. Pull about 1 ft. of wire out, for making the connection to the rest of the circuit. Tape the wire on the inside of the oatmeal box, to keep it from slipping out.

2. Wrap five turns of wire around the oatmeal box and make a "tap,". Remove a short span of insulation, and twist the wires together.
3. Continue wrapping, and every five turns, make a tap, until you get to 40 turns.
4. At 40 turns, poke two holes next to the last turn of wire. Cut the wire off so that you have 1–2 ft. extra to connect to the rest of the circuit. Poke the wire into the first hole and back out the second hole. Tape the wire in place inside the box. You now have your coil wound, as in Figure
5. Next we will need an antenna and a ground. The antenna can be any wire (insulated or bare), as high and as long as possible. Make sure not to place it near electrical wires for your safety and the performance of the radio. Also don't let the antenna "ground out" to trees or the earth (ground). You can make insulators from plastic water pipe or couplings. See Figure, below.

The ground can be made by connecting to a metal water pipe (plastic pipes won't work), or to a metal rod that is pounded at least 2 ft. into the ground.