By Arpa Ghazarian
What is a Crystal Radio?
Crystal Radios are used to receive amplitude modulated (AM) signals without power or batteries, but with only the signals transmitted through the antenna of the crystal radio.
History of the Crystal Radio
Crystal Radios were discovered and worked out in the early 1900s and were even used to send Morse Code. Read more about the history of the crystal radio.
How A Crystal Radio Works
Key Concepts: Electromagnetic waves, radio waves, amplitude, frequency, amplitude modulation (AM), resonant, diode, and variable capacitor
Crystal Radio Diagram and Schematic:
1. Radio waves: electromagnetic waves that can travel through the air. Below is a picture of a one-cycle wave with its amplitude (strength) and frequency.
a. Frequency is measured in hertz (Hz)
b. Notice above that there three wave cycles, or 3 Hz, for the frequency in the second figure versus the first with 1 Hz.
c. The AM radio band ranges from 530,000 Hz to 1,710,000 Hz (or 530 kHz to 1710 kHz). When your on the radio station 710 AM that means the radio is broadcasting at 710 kHz (wave cycles 710, 000 times a second).
2. Amplitude Modulation- varied amplitude and fixed cycle rate
a. You can have a radio station that has the same frequency but the strength, or amplitude is varied as in the picture below:
b. Unmodulated wave:
1. Antenna & Ground Wire 2. Coil ("resonant") 3. Germanium diode
c. Now radio waves are traveling in the air. However, they need to be converted to audio waves so that we could actually hear them. Crystal Radios assist with this!
3. Crystal Radio Workings
4. Tuning Capacitor 5. Cold water pipe (ground) 6. Headphone
- A small amount of energy from the radio wave is captured by the antenna wire and is taken to the coil.
- The coil has to be designed just right to capture only the frequency we are trying to receive. By winding just the right amount of wire on just the right diameter coil form, the coil will be what we call "resonant" and "ring". In other words it will be able to store the energy of the radio wave we want to hear. All other radio waves not "resonant" will pass through the coil and out the other side to the ground.
- A small amount of the radio wave energy stored in the coil moves to the detector or the device called a diode. The energy is an alternating current signal (AC) at this point. The detector (diode) rejects half of the alternating current signal and the signal looks like the figure below. Now the signal is a pulsating direct current (DC) signal.
- This allows the earphone to use the energy. If both sides of the wave were used, they would cancel each other out as they are opposite of each other. As this energy go into the earphone, the amplitude or strength of the signal varies because the wave is "modulated". This energy is converted by mechanical means in the internal workings of the earphone. The sound waves exit the earphone which you perceive as the original sounds from the radio station.
- The tuning capacitor allows for the change in the radio station. It adds or removes capacitance to the coil and changed the resonant frequency the coil will tune to.