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Question about radio

  1. Feb 23, 2013 #1
    What are the most basic components I need to generate and recieve a radio signal and how would I arrange them?

    For instance imagine I want to make an LED in my bedroom flicker when my front door opens. Firstly how would I make this happen at the most basic level, so that some signal is recieved and secondly how would I amplify this signal so that, for instance, the light stays on for 5 seconds? I'm guessing for the latter I just need to learn how a 555 chip works so I'm more interested in the former question.

    Thanks in advance.
     
  2. jcsd
  3. Feb 23, 2013 #2
    The easiest way is to send the signal along wires. Fit a switch on the door jamb.
    Do this with low voltage ac current (not batteries) and the signal IS INDEED a travelling wave field (radio!) guided by the wire.

    To light the led you need a source of power. 2 volts at 25 mA and a resistor to limit the current below 30 mA..

    To send the signal wirelessly, you need an oscillator.
    Any one will do. A transistor with positive feedback! A 555 chip wired fo feedback.
    But TRY not to annoy the neighbours - choose a frequency (and one alone!) not in the radio bands.

    How about using ultrasound (provided you have no dog,cat ar canary)
    Piezo-electric transducers are cheap
    Get two. Rig up one as transmitter (oscillator) - the other as receiver

    The old Space Command" Zenith Tvs used ultrasound. In your case a littl "hammer" strikes an aluminium rod (in the Space command unit) (think of a xylophone) and the receiver picks up the untrasound and switches on your LED instead of the TV.

    Go to a model shop and ask about model airoplane and car radio-control components.
    They might have a "broken" model they'd give you the radio parts from.
    Above all, have FUN.
     
  4. Feb 23, 2013 #3
    A transistor is capable of sending a radio signal? How does this work? How would I receive it?
     
  5. Feb 23, 2013 #4







    You need a power supply - enough to turn on the transistor 2 or 3 volts
    Feed the transistors oupput to its input, via a capacitor to block dc
    It will oscillate.
    This will launch a radio wave - using the tansistor circuit wires as antenna
    Any wire or loop will pick up this signal
    Another transistor will amplify it (use it to turn on current from a powr supply)
    This current can light your LED
     
  6. Feb 24, 2013 #5
    To the base or the collector? Would you be able to show me a circuit diagram where this oscillation occurs?
     
  7. Feb 24, 2013 #6
    The "Grounded emmitter" is the easiest. From the power supply to ground joinn 2 1K resistors in series
    If the supply is 3v the mid point will be at 1.5 volts.
    This would turn the transistor on, if of the right type for the polarity of your power supply.

    Though 1.5v from base to emitter (emitter grounded) would turn the transistor on, it is thermally unstable.
    To stabilise it we apply "negative feedback"
    All this means is "The higher the current the LOWER the base volts".
    This is so SIMPLE to do. Put a 470 ohn resistor from ground to emitter as the only emitter connection.
    Now it will sit there with current flowing.

    Now to get an output voltage, put a 1K resistor from collector to power supply so the collector current flows thru it
    Now join a 1 uF capacitor from collector to base
    Expect oscillations around t=1/RC = 1/(500 times 10^-6) = 2 KHz
    Hear these with headpones.
    To up the frequency to 200 KHz try .01 uF
    If it needs more gain (unlikely with modern transistors) place a .1uF capacitor in parallel with the 470 ohm resistor

    Have FUN
     
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