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Homework Help: Please explain Circuit resonance in a Crystal Radio?

  1. Mar 10, 2012 #1
    1. The problem statement, all variables and given/known data

    I'm in grade 11 working on a science fair project I found on this site:

    Grade 11 physics doesn't cover much so I'm confused about why the radio is set up the
    way it is...

    I know that the diode prevents current from going in the opposite direction, but
    what does the resistor do?

    The website says that the taps at various locations is for resonance, but how does that work?

    I found this paragraph on another site explaining resonance but I found it confusing.

    "For electricity, a coil and capacitor make a resonant circuit. The capacitor plates get an electric charge from other parts in the radio. That charge flows through the coil. As it does, it builds up a magnetic field in the coil. When all the charge is gone from the capacitor, the magnetic field makes the electricity keep on flowing a little. This charges the capacitor plates the opposite way. As the opposite charge builds up on the capacitor plates, it finally stops the charge flow in the coil. Then the charge in the capacitor plates makes electricity flow the opposite way through the coil. That builds up a magnetic field in the opposite direction. The charge swings back and forth between the coil and capacitor at one certain frequency. That is the resonant frequency of the coil and capacitor."

    If someone could explain all this or point me to some good ressources I would really appreciate it.

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Mar 10, 2012 #2
    Hi fchen720
    The circuit will, according to the different conditions of its components (resistor, capacitor, coil) have a different resonance frequency.

    This is because this circuit is an oscillator, as explained in your resource. You could think of it as another kind of oscillator:
    A spring with a certain strength (and mass), or a pendulum with a certain length will have different 'proper frequencies'

    You can see it easily for the pendulum: just think of pushing your little sister 'regularly' so that she can swing higher and higher.
    you just give a little push at a fixed frequency. if you give the little push at the wrong time, your sister is not going to go very high, but for some given length of the ropes, there is an exact frequency, so that if you push at exactly, the right time, well, she is going to swing very high.

    This is the same thing for the circuit, except the regular pushes are given by the radio waves getting into the circuit through the antenna.
    By taping at different places on the coil, you are modifying one of those values, and that makes you do the equivalent of, say, changing the length of the rope of a pendulum, so the frequency of you pushes would have to be different.
    So according to where you tap, your circuit will be more favourable to some definite, different frequency.

    Now the air is filled with many radio frequencies coming from many different radio stations, so what you do when taping at different places is in fact 'favouring one specific channel' or filtering out the others, that is, you are tuning on a specific channel.
    Thanks to your filter, only (mostly) the right channel will get to the end of the circuit where it will be rectified by the diode, which is all what is needed (for AM) to recreate the original signal which the ear-bud will finally turn back into sound.

  4. Mar 10, 2012 #3


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    Staff: Mentor

    It appears that the circuit in the attachment is using stray capacitance for the resonant circuit, as there is no capacitor shown. So having understood that both C and L are needed for a resonant circuit, you have to imagine a capacitor is present. Also, if you are constructing the circuit, the diode needs to be a signal diode, not a power diode. If you are not close to a radio station the signal from the aerial will be weak, and a germanium diode will perform better because it has less voltage loss. So buy a germanium signal diode if you can.

    There is a lot of clever engineering that went into the design of the crystal radio. It is entirely powered by the weak radio signal picked up by the aerial. Good luck!
  5. Mar 12, 2012 #4
    Thanks for all the replies.:smile:

    Yesterday I got the parts, put the radio together and success! I could actually hear a few
    radio stations. The sound was barely audible, and everyone had to be quiet if we wanted to hear anything. I'm not sure if that's the norm with crystal radios though.

    Details of setup:
    -15m of 24 guage magnet wire tethered by insulating string 1.5 m above the ground
    -4m of regular wire connected from set to an outdoor water pipe
    -24 guage magnet wire wrapped around an 8.5cm diameter thin plastic container,
    with sanded taps at every 5 turns
    -47kohm resistor in parallel circuit with earphone, as suggested by sciencebuddies.com
    -germanium diode
    Specifications of diode
    Peak Inv. Voltage(PIV): 60 Volts
    Max. Average Rectified Current: 50mA @ 25 deg C
    Junction Temperature (TJ): 100 deg C Max.
    FWD Voltage Drop(VF): 1.0V @ 5.0 mA
    Reverse Current: IR 15uA @ VR 10 volts

    Strange things that happened during experimentation:

    When the wire coming from the tap was connected to the junction (as planned in the diagram) , we heard a lot of interference. We removed the tap wire, and for some reason we got a clear signal without interference, so we reasoned that the signal must have passed through the coil, into the junction that was supposed to go to the ground (whoops), through the earphone diode setup, and finally was grounded through our fingers which held the alligator clip at the other end.
    (we must have installed the diode backwards :uhh: but it appears that the direction of current through the earphones doesn't make a difference)
    This setup is drawn in my attached diagram.

    So then we went with how things were, and moved the ground wire to the other side so that we didn't have to use our fingers to ground it. This setup is also drawn in my attached diagram. (The intended setup by sciencebuddies.com is drawn too.)

    After the final setup we removed the coil altogether to see if the coil does anything.
    And we still had the signal :uhh:.

    My questions:
    What is the coil's purpose?
    Why did my taps produce static?
    Why did sciencebuddies recommend wrapping around a 10cm container and making a tap
    every five turns? Did they use an equation to calculate that?

    Thanks to everyone who patiently read through my post.

    Attached Files:

  6. Mar 13, 2012 #5


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    Staff: Mentor

    You are using the word "tap" in two different contexts, and getting me confused. Can you refer to one as "the water pipe"? :tongue:

    The purpose of the coil is twofold: it makes the audio much louder, and also allows you to tune between different stations. If you couldn't turn between stations, then listeners in a big city would hear all their local stations chattering away together!

    If you can hear a station without the coil, then you are probably located very close to that station's transmitter.

    Yes, using body capacitance as 'ground' is a common technique, but a water pipe or some other object is usually better. (People in apartments where the water pipes are plastic may have no recourse to an earthed metal water pipe.)

    The diode can be connected any way 'round here. A lot of "static-sounding" interference is probably due to a poor connection, but there could be other electricity going through the water pipe to ground, and that will affect your crystal radio.

    Theirs is an unusual design, because they omit the usual tuning capacitor. I'm surprised that it works, if in fact it does. They must have found that it is good enough by testing it.
    Last edited by a moderator: Apr 30, 2017
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