$25 or less
None, unless soldering is done.
Available in kit form, or as components, such as from Radio Shack
Less than 1 hour
To demonstrate basic radio communications.
Note: Many moderately priced kits are available and come with all the necessary parts. These can be found online, at stores such as Radio Shack, hobby stores, and many other places. This is the recommended method for those with limited access to electronic components.
Also easily available online, from the library, etc. are dozens of circuit designs. Most are very similar and will give a specific parts list for that design. If the student chooses to make the receiver without a kit, additional research will be needed to buy or build the components for that particular circuit.
If built from components, the connections can be made by twisting bared wire together, or made more secure with solder. Actual connectors, solderless or soldered, may also be used.
In general, a crystal radio consists of:
- Coil of wire (a cardboard tube and fine gauge wire can be used)
- Capacitor, sometimes variable (tunable)
- Extra wire to make connections
- Length of wire for the antenna (the longer, the better)
The basic radio circuit consists of a tuned circuit, in which a coil and a capacitor resonate at a particular frequency. A diode acts as a rectifier to change the AC current of the radio wave to the DC current used by the earphone.
Dozens of possible crystal radio circuits are easily available. The student will select one of these and use that circuit diagram, plus the assembled radio, for illustration. If the recommended method of using a kit is chosen, the kit will almost certainly come with its own circuit diagram. Note that with so many different kits and possible circuits, the student may wish to build more than one crystal radio and compare them.
Optional drawings could include those of radio wave propagation, frequency vs. wavelength, antenna construction, etc.
- What is a radio wave?
- How are frequency and wavelength related?
- What is a tuned circuit?
- What is the function of a diode in a simple crystal radio circuit?
- Why is “grounding” so important for a crystal radio?
- frequency: the number of cycles per second
- wavelength: the distance the wave travels from one peak to the next peak
- inductor: usually, a coil of wire
- capacitor: an electronic device that can store an electric charge, and can also pass an AC signal while blocking a DC signal
- detector: a diode rectifier
The electromagnetic spectrum is measured in frequency (cycles per second, or hertz) and in wavelength. The higher the frequency, the shorter the wavelength. “Bands” of frequencies have different characteristics.
- What is electromagnetic propagation?
- What is AM, and what is FM?
- How does a tuned circuit work?
- How does an antenna work?
- Gather the materials (depending on the design used)
- Build each of the components (if necessary)
- Assemble the components