Crystal Radio project

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.

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)

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Experimental Procedure

Note Before Beginning: This science fair project requires you to hook up one or more devices in an electrical circuit. Basic help can be found in the Electronics Primer. However, if you don't have experience in putting together electrical circuits you may find it helpful to have someone who can answer questions and help you troubleshoot if your project isn't working. A science teacher or parent may be a good resource. If you need to find another mentor, try asking a local electrician, electrical engineer, or person whose hobbies involve building things like model airplanes, trains, or cars. You may also need to work your way up to this project by starting with an electronics project that has a lower level of difficulty.

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.