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The Physics of Resonance

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Crystal Radio Demonstrations

Crystal radios are one of the best devices for demonstrating a resonating electrical circuit. They are low cost and easy to understand.

When radio was new, crystal sets were the first receivers available. They are so simple they don't even need a battery or other power source. They literally pull power out of the air. However, before you start believing in perpetual motion machines, we must state that the first and second laws of thermodynamics are rigorously obeyed even by crystal radios. The power they pull out of the air is put into it by the radio station's transmitter. In fact, the transmitter puts a lot more power in than thousands of crystal radios could remove.

Tesla was definitely not as eccentric (a polite word for crazy) as he may have seemed. Electrical power actually can be transmitted without wires. We do it on a regular basis with radio and television. However, there are some serious problems involved with transmitting enough power, to say, run a city.

     
Crystal radios contain an inductor and a capacitor in an electrical circuit tuned to resonate at the carrier frequency of a radio broadcast. The signal is then passed through a diode to transform it from AC to DC (see Figure 1) form so that it can drive a very sensitive ear phone.

The diode at one time was made of a naturally occurring crystalline material, hence, the name crystal radio. An antenna is also usually attached to the circuit and the circuit is grounded.

RadioShack produces a crystal radio kit which sells for about $10 and can easily be put together in less than 30 minutes. It works fine for most purposes.

 
Figure 1. Effect of Rectifying a Wave
     

More adventuresome individuals will want to make their own from scratch. It can even be done as a class project in a reasonable amount of time. The Xtal Set Society has plans for a very inexpensive unit made from a Quaker Oats box. Owen Pool's Crystal Radio Resources page has lots of construction information and about a bizillion links. Plans for our personal favorite can be found on Ian Purdies's Amateur Radio Tutorial Pages. Once you have a crystal radio in hand it's time to proceed to the demos.

Demo 1- Using a Crystal Radio to See Radio Waves:
It's possible to amplify a crystal radio's output by connecting it to a set of computer speakers rather than using the ear phone. The first step is to make a jumper wire with the correct fitting on each end so that it can be connected to both the radio and the speakers. Generally, the parts can be obtained at RadioShack. If the radio is simultaneously connected to an oscilloscope the signal from the radio station can be heard while looking at the distinctive AM radio wave form.

Playing a crystal radio over an amplified speaker system takes some of the wonder out of it. However, unless everyone in the room has their own radio its a problem to demonstrate that the unit is indeed picking up a station.

A small transistor radio can also be connected to an oscilloscope and used for viewing either AM or FM radio signals. Simply cut the ear plug off and strip off some of the insulation, then connect the ends to the oscilloscope. Be careful not to short the wires together. This is the best system to use if you have trouble tuning into a radio station with the crystal set.

Demo 2- Evaluating the Resonant Frequency of a Crystal Radio:
When it's excited by a voltage pulse, a crystal radio will oscillate at its resonant frequency (see circuits). This is similar to striking a bell with a hammer. The hammer provides a mechanical impulse which makes the bell ring.

To make a crystal radio's circuit oscillate, select a long connecting wire. Attach one end of the lead wire to the ground on a power supply set for a voltage of about 12 volts. Wrap about two turns of wire around the coil. Attach the other end of the wire to either a 12 volt light bulb fixture or a 5 ohm resistor. This is used to limit the current flow in the wire and keep the fuses in the power supply from blowing. Connect an oscilloscope to the radio instead of the earphone. Attach a second connecting wire to the light fixture.

Using the free end of the second connecting wire, briefly touch the positive lead on the power supply. You should observe a decaying sin wave on the oscilloscope. The wave represents the oscillation of the crystal radio's circuit. It decays since the circuit has resistance.

Note: the crystal radio does become a low powered transmitter when it is pulsed. Although the signal will be too weak to transmit any appreciable distance, it could cause interference to nearby radio receivers. If the crystal set is tuned to a station, it should be possible to hear the interference by tuning a separate transistor radio to the same station. Listen for the static sound when the crystal set is pulsed.

Required Equipment

Demo Number Item Comments
1&2 1 Crystal Radio See above
1 1 Set of Amplified Computer Speakers These generally come with any computer. No special type is needed.
1&2 1 Oscilloscope  
2 1 12 volt Power Supply 12 volts is suggested, but other DC voltages are useable as long as they are reasonably low powered
2 1 12 volt Light Fixture (any kind) Another device such as a 10 ohm resistor can be used for limiting current.
1 1 Specialized Connector Wire to Connect Between the Radio and Speakers This wire will probably have to be built.
2 3 Misc. Connecting wires  

 

For more information about wireless communication and the electromagnetic spectrum visit The Hidden World of the Electromagnetic Spectrum.

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