AM Transmitter Isn't Transmitting Audio

Discussion in 'General Electronics Chat' started by ajm113, Oct 2, 2011.

  1. ajm113

    Thread Starter Member

    Feb 19, 2011
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    Hello I'm trying to play with radio signals and I thought this tutorial would be best for a intro: http://www.instructables.com/id/Easy-AM-Transmitter/

    I've made some modifications to the circuit since I was lacking parts, so I had to do some changes to the design in hope that it should still work.

    The good news:
    It's transmitting a signal though to my radio!

    The bad news:
    Instead of transmitting the sound off my PC it's just outputting a Oscillation type of waveform sound or "Ringing" though the radio.

    Could someone give me advice on what to change in hopes to fix this? I've attached my circuit down below including the circuit layout I used:

    Thank you, Ajm.
     
    Last edited: Oct 2, 2011
  2. Audioguru

    New Member

    Dec 20, 2007
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    INSTRUCTABLES are designed by little kids who know NOTHING about electronics.
    The original circuit cannot work because the transistor has no emitter resistor and the modulation simply turns the oscillator on and off.

    Your modification shorts pin 4 of the 555 to ground which turns it off.
     
  3. ajm113

    Thread Starter Member

    Feb 19, 2011
    176
    5
    Yeah come to think of it, most of the schematics I've seen really don't look like they would work, plus I just remembered, this schematic is missing a capacitor on the antenna for protection against current coming in contact with the antenna.

    I don't suppose you know of a better schematic to work with on a AM transmitter that doesn't require lots of parts? I'm kinda real short on cash at the moment and yes I could scavenge for parts, but surprisingly enough, old junk electronics on the street are hard to come by.
    -__-

    Though I may be able to get my hands on a PSU which would work, because it has inductors, a transformer, and capacitors.
     
    Last edited: Oct 3, 2011
  4. Audioguru

    New Member

    Dec 20, 2007
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    A 555 cannot produce an amplitude modulated carrier. It is simply on or off like Morse Code. Maybe you could modulate its supply voltage for it to produce AM.
    The transistor does not do anything and can be removed. A capacitor feeding the antenna would prevent a short circuit from destroying the 555.
     
  5. ajm113

    Thread Starter Member

    Feb 19, 2011
    176
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    If I may ask, why modulate the supply voltage? Wouldn't it mess up the 555 timer? I'm a bit confused, if the 555 unable to perform that task, so will I need to get something such as a crystal oscillator?

    And I went ahead and removed the NPN, and added a capacitor to the antenna. ;) I get the same results of course, but I'm getting there.

    EDIT: Is it possible to generate a AM/FM modulated signal with a PIC microcontroller? If so I could work with that.
     
    Last edited: Oct 3, 2011
  6. Audioguru

    New Member

    Dec 20, 2007
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    AM stands for Amplitude Modulation where the audio changes the amplitude of the carrier frequency from zero to double. The only way a 555 oscillator can have its output amplitude varied is by changing its supply voltage from 4.5V to 16V. It cannot linearly go down to zero but it can feed a transistor circuit that can be modulated from zero to double.

    The circuit you found is not an AM transmitter.
    An AM transmitter that works is here:
    http://www.bowdenshobbycircuits.info/page6.htm#amtrans.gif
    It can use a crystal or not. The IC is the oscillator that drives the lower RF output transistor. The upper transistor varies the output amplitude by changing the supply voltage of the RF output transistor.
     
  7. ajm113

    Thread Starter Member

    Feb 19, 2011
    176
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    Ohhh, I see what you mean now, makes a lot more sense.

    Thanks for the schematic! I'll give it a try after I try to scavenge parts for it. :) If I may ask, if by happen I stumble upon a crystal for this project. What Mhz would I be looking for? And does anyone know if maybe what scrap electronic these most likely are going to be in? I was thinking something like a crystal maybe my best bet.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The AM broadcast band goes from 540kHz (0.54MHz) to 1600kHz (1.6MHz). You will need a crystal with a frequency that is not occupied by a radio station that can be received.

    I can't think of anything that has a crystal that uses a broadcast band frequency because then it would cause illegal interference.
    Your AM transmitter also might cause illegal interference unless its output power is low and its antenna is small.
     
  9. ajm113

    Thread Starter Member

    Feb 19, 2011
    176
    5
    hhhmmm, well I dont see any reason why not to make my own. :S It would give me more flexibility on the mhz too.

    Idk if this is a reliable source, but it could save me time and money.

    http://how-to.wikia.com/wiki/How_to_build_an_oscillator_circuit

    Or... I could just create my own inverters like the schematic shows, but I'm a bit confused by the first inverter, I'm not quite sure 100% what the circuit looks like, unless it's just the voltage used for the inverting, which I'm guesting.
     
    Last edited: Oct 3, 2011
  10. Audioguru

    New Member

    Dec 20, 2007
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    I don't know which oscillator circuit you want that uses inverters because there are a few of them. Post its schematic.
     
  11. ajm113

    Thread Starter Member

    Feb 19, 2011
    176
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    Woops sorry I worded that wrong. I mean I could just make my own Hex Inverter then getting a crystal on it or buying the IC, but then again if I made my own crystal, I get flexibility.

    So I think I'll go with my first option and use the LC Oscillators. Now all I have to do is get some inductors. =/
     
  12. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    The first glaring error is that there's no path to ground on Q1. If you supplied a choke from the emitter to ground, you MIGHT get a minuscule amount of radiated signal, but there's no visible attempt to match or tune the antenna. Square waves don't propagate through space very well. :)

    Eric
     
  13. ajm113

    Thread Starter Member

    Feb 19, 2011
    176
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    I tried it, but it does seem to work. :( I've removed the NPN so I pretty much connect a 555's output pin and a resistor that connects to ground. Including of course connecting a capacitor between them booth that goes to the antenna. I tried 1.5K and 10K, and it doesn't look like it does much.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    If the antenna is connected through a 0.33uF capacitor to the output of the 555 (a resistor to ground is not needed) and if the 555 is set to a AM radio frequency then it broadcasts silence because it is not modulated by the audio.
     
  15. tom66

    Senior Member

    May 9, 2009
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    This video shows an example using a TL072.

    http://www.youtube.com/watch?v=3I_e7gIyfQg

    I imagine basic AM could be implemented by using a basic transistor volume control. 100-1kohm in series with output. 100-1kohm to ground in series with transistor. Varying base current varies the output amplitude.
     
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