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Abstract

Objective

The objective of this science fair project is to build a simple crystal radio from scratch, then experiment to make improvements.

Introduction

Before the 1950s and the dawn of television, people would gather in their homes with family and friends around the radio, just as we do with our TVs today, to listen to sports, music, news, and entertainment programs. Then as now, the radio acted as a gateway to the world. But what exactly is a radio and how does it work?

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 1 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 is between 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.

Figure 1. The top part of this figure shows a 1-cycle wave, the second a 3-cycle wave.

Radio stations send out waves and the radios in our homes receive that wave. But how do our radios turn these waves into sounds and how do stations transmit different sounds? Waves have both a frequency and an amplitude, which is the height of the wave. If some one 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 vary or modulate the amplitude of the wave and that is how we hear the different levels of sound. Stations that transmit signals via amplitude modulation are called AM radio stations.

Now that we know how radio waves work, let's learn how a simple crystal radio receives and converts a wave into sounds we recognize. A crystal radio is made up of an antenna, an insulated wire coil, a diode, an earphone, and a connection to an electrical ground. This type of radio doesn't require a battery or electrical power to function. It just runs on the power it receives from the radio waves. The function of the antenna is to capture the wave and send it to the coil. Once in the coil the wave will bounce back and forth and cause electrons, negatively charged particles, in the coil to move. At certain lengths of coil, the wave strengthens, but at other lengths the wave weakens or completely disappears. A strengthened wave is said to have resonance. Each frequency has resonance with a certain length of coil. Signals that lack resonance with a particular length of coil will eventually disappear. As shown in Figure 2 in the Procedure below, you can make a crystal radio that receives a variety of signals by wrapping wire around an oatmeal canister and then creating "taps" at different locations. The taps correspond to different lengths of wire.

The crystal radio in this project is an example of an electrical circuit. A circuit is a system through which electrical current (moving electrons) flows. The top end of the coil is connected to the antenna and the other (bottom) end, to the electrical ground. The ground acts as the baseline for measuring other currents and voltages in a circuit and serves as the return path for current in an electrical circuit, like that of the crystal radio. Without ground, current wouldn't flow and the circuit would not work properly. Any electrical spikes that could damage the circuit are sent harmlessly away to and absorbed by the ground. It is important to ground your crystal radio circuit in order to prevent damage to the circuit and to yourself. Finally, the diode removes the bottom half of the radio wave, converting it into signals that an earphone can pick up. The diode is what gives the name "crystal" to the radio. In the days before the diode was invented, an actual crystal functioned as the diode.

In this project you will build a crystal radio from materials you find around the house and a few items from an electronics store. Try to pick up as many radio stations as you can by connecting to the various taps. This is called "tuning" the radio. You will need help from an adult to string up the antenna.

Terms, Concepts, and Questions to Start Background Research

To do this science fair project, you should do research that enables you to understand the following terms and concepts:

• Radio wave
• Electromagnetic radiation
• Frequency
• Hertz (Hz)
• Diode
• Electrical ground
• Electrons
• Resonance

Bibliography

Here are some sources to get you started:

• Brain, M. (n.d.). How Radio Works. Retrieved April 10, 2009, from http://electronics.howstuffworks.com/radio.htm
• Wikipedia Contributors. (2009, March 26). Crystal radio: Wikipedia: The Free Encyclopedia. Retrieved April 10, 2009, from http://en.wikipedia.org/wiki/Crystal_set

If you want to follow an interest in radio further, try The Amateur Radio Relay League website:

• ARRL. (2009). ARRL: The National Association for Amateur Radio. Retrieved April 10, 2009, from http://www.arrl.org

Materials and Equipment

Many of the supplies listed below are available from the following suppliers:

• Antiques Electronic Supplies: www.tubesandmore.com
• Computer Controlled Automation: www.angelfire.com/electronic2/index1
• Dan's Small Parts and Kits: www.danscloseoutsandspecialdeals.com
• RadioShack: www.radioshack.com

To do this science fair project, you 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); available from RadioShack, part # 278-1224
• Germanium diode (1n34, 1n34a, 1n60 etc.) (1); available from:
  • Antiques Electronic Supplies, part # P-Q972
  • Dan's Small Parts and Kits (no part numbers available, must search for the part name on their site)
  • Computer Controlled Automation (no part numbers available, must search for the part name on their site)
• 47-kohm resistor, 1/4- or 1/2-watt (1); available from:
  • Antiques Electronic Supplies, part # R-A47K
  • RadioShack, part # 271-1342
  • Dan's Small Parts and Kits (no part numbers available, must search for the part name on their site)
• Alligator clip (1); available from:
  • RadioShack, part # 270-346
• High-impedance ceramic earphone (1) Note: An earphone, headphones or "ear buds" from RadioShack will not work. It must be a high-impedance ceramic earphone, not the kind used on transistor radios); available from:
  • Antiques Electronic Supplies, part # P-A480
  • Computer Controlled Automation (no part numbers available, must search for the part name on their site)
• Optional: PVC pipe coupling, ¾ inch, (2). Used to make antenna insulators if needed. See Figure 3 for details.
• Optional: Fahnestock clips (4); you could also use a machine screw through the bottom of the board, secured by a nut. Then use a knurled nut or another nut to tighten the wires down. Available from:
  • Antiques Electronic Supplies, part # S-H11-4043-6
• Multimeter or microammeter and voltmeter (optional)
  • Microammeter must be capable of reading as low as 5 microamps (uA)
  • Voltmeter must be capable of reading as low as 100 millivolts (mV)

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Experimental Procedure

Important Safety Notes Have an adult help you with this science fair project. Do not listen to a crystal radio during a thunderstorm. Make sure that you follow the instructions and ground your antenna properly.

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 2, 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.

   Figure 2. Diagram showing how to wrap the tapped coil.

2. Wrap five turns of wire around the oatmeal box and make a "tap," see Figure 2. 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 2.
5. Next you 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 3, below.

   Figure 3. Crystal radio antenna diagram.

6. The ground can be made by connecting to a water pipe, or to a metal rod that is pounded at least 2 ft. into the ground.
7. Figure 4, below shows the remaining connections that you need to make.

   Figure 4. Diagram showing completed crystal radio.

8. Hook it all up and connect the alligator clip to one of the center taps. You should hear something! To tune the radio, try connecting to another tap.

Troubleshooting

For troubleshooting tips, please read our FAQ for Crystal Radio.

Variations

Building a radio is your first step. To make a good science fair experiment, you should investigate some of the following questions.

• A number of interesting experiments become possible if you add meters to measure the current and voltage in your radio. "To measure the current in our radio, we will need to have the current flowing through the meter. To do this, we connect the microammeter between the earphone and the ground connection, so that any electricity that is going to flow through the earphones to make noise is going to have to flow through the meter also. The meter can be connected in two ways, one is forward and one is backward. If the meter is connected backward, the needle will start reading below zero. If this happens, just reverse the connections, so the needle reads above zero. To measure the voltage, we connect the meter to both of the earphone wires." (Field, date unknown) Depending on the radio stations near you, you need a meter that will read as low as 5 microamps and around 100 millivolts. You can calculate the power (in watts) that your radio receives by multiplying the volts times the amps.

• Simon Field suggests these variations: "Try different lengths of antenna, and watch the current go up as the longer antennas catch more of the power from the radio station. Try more than one antenna. Try connecting the ground wire to different things that are connected to the ground, such as pipes, metal fences, etc. As you try each test, make sure you tune the radio again, because your changes may affect the tuning." (Field, date unknown) You should check out this source for more details and additional ideas.

• Lewis Whaley suggests another variation for this project, based on the "foxhole radios" which American soldiers in World War II built using razor blades and a safety pin (or graphite pencil point) in place of the germanium diode. How could this work? Build one and compare its performance to the circuit described above. For more information, see: http://bizarrelabs.com/foxhole.htm.

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

This science fair project was inspired by projects on the following websites:

• Boyd, D. (2005). Build a Crystal Radio. Retrieved January 30, 2006, from http://www.crystalradio.net/beginners/index.shtml

Note for this source: See "Power from radio waves—hooking up a meter to measure the voltage and current" about half way down the page.

• Field, S. Q. (n.d.). Science Toys You Can Make with Your Kids: Building a Radio Out of Household Implements. Retrieved November 21, 2004, from http://www.scitoys.com/scitoys/scitoys/radio/homemade_radio.html

Last edit date: 2012-01-24 10:27:00