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simple radio building

Discussion in 'General Electronics Discussion' started by scientifico, May 21, 2014.

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  1. scientifico

    scientifico

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    Mar 13, 2012
    Hi, can you tell me what is R2 for in this project and what happens if I remove it ?
    In your opinion, can a shortwave receiver build like this receive well something ?

    Thanks.

    Schermata-3.png
     
  2. BobK

    BobK

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    1,688
    Jan 5, 2010
    It is biasing TR2 into the linear region. If you remove it (why would you?) you would get distorted sound as only the positive half of the waveform would come through.

    Bob
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    As Bob says, R2 is needed to bias TR2 into its linear region so it can amplify. If you remove it, you will probably get no sound at all, or possibly very weak and distorted sound, depending on how leaky the transistor is.

    Yes this circuit will receive short wave transmissions. Connect the bottom rail (the connector marked T) to an earth ground, and connect a long antenna wire to A1, A2 or A3. You will need to reduce the inductance of L1 and the capacitance of C1.

    The diode and transistors must be germanium types.
     
  4. scientifico

    scientifico

    62
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    Mar 13, 2012
    So do transistors let current flow only in one direction like diodes?

    What do i need to modify in order to make it work with a speaker instead of headphones?
     
  5. BobK

    BobK

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    Jan 5, 2010
    Easiest, replace the headphones with a 1K resistor then connect the junction of the collector and the resistor through a capacitor (1uF) to your stereo system through an AUX input.

    Bob
     
  6. duke37

    duke37

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    Jan 9, 2011
    Transistors will pass curent in eiher direction depending on the circuit but you need to know what you are doing if you run them in unusual modes. Why would you want to do this?

    The circuit is just an amplified crystal set and will have very poor sensitivity and selectivity.

    Depending on transistor 1 (Q1), you may be better off not using the diode at all, just connect the coil direct to the transistor, the transistor can do the detection if it is fast enough. This circuit may rely on a slightly leaky Q1.

    The diode should be a small germanium diode or small schottky diode to minimise voltage drop.
     
  7. scientifico

    scientifico

    62
    0
    Mar 13, 2012
    I wonder how can a diode separate the high frequency from the low frequency since it cut them both in half ?
    Wouldn't we have more power by connecting the headphones directly after the tuning stage ?
     
  8. BobK

    BobK

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    Jan 5, 2010
    If you allow both sides of a sine wave through, the average voltage is 0. If you allow only the positive part, the average voltage is > 0 and varies with the amplitude of the sine wave.

    Bob
     
    duke37 likes this.
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Google how does a crystal set work?
    No. The signal level there is a lot lower.
     
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