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Project Summary

Difficulty	4  –  9
Time required	Short (several days)
Prerequisites	None
Material Availability	Specialty items
Cost	Low ($20 - $50)
Safety	Don't listen to a crystal radio during a thunderstorm. Make sure that you follow the instructions and ground your antenna properly. Adult assistance and supervision is required.

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Objective

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

Introduction

Cell phones, satellite TV, walkie-talkies, car radios, GPS, and baby monitors all rely on radios to function. This fun science fair project will enable you to understand the basics of how a radio receiver works and give you the satisfaction of hearing radio broadcasts on something you built from scratch.

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, frequency
• Amplitude modulation (AM)
• Antenna
• Tuner
• Resonance
• Detector or demodulator
• Diode
• Electrical power

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)
• Spool of #24 or #22 solid plastic insulated wire, 100-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:
  • Antiques Electronic Supplies, part # S-H301-103
  • 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: 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-H11-4034
• Multimeter or microammeter and voltmeter
  • Microammeter must be capable of reading as low as 5 microamps (mA)
  • Voltmeter must be capable of reading as low as 100 millivolts (mV)

Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies receives no consideration, financial or otherwise, from suppliers for these listings. (The sole exception is any Amazon.com or Barnes&Noble.com link.) If you have any comments (positive or negative) related to purchases you've made for science fair projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

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 1, 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 1. Diagram showing how to wrap the tapped coil.

2. Wrap five turns of wire around the oatmeal box and make a "tap," see Figure 1. 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 1.
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 2, below.

   Figure 2. 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 3, below shows the remaining connections that you need to make.

   Figure 3. 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.

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 throught 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: 2009-04-10 10:27:00

Career Focus

If you like this project, you might want to think about career opportunities in Electricity & Electronics.

Electricians are the people who bring electricity to our homes, schools, businesses, public spaces, and streets—lighting up our world, keeping the indoor temperature comfortable, and powering TVs, computers, and all sorts of machines that make life better. Electricians install and maintain the wiring and equipment that carries electricity, and they also fix electrical machines. Learn more about this career: Electrician.