The ADC and the IR Transmitter

Discussion in 'General Electronics Chat' started by gold_sky, Apr 16, 2015.

  1. gold_sky

    Thread Starter New Member

    Apr 11, 2015
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    Hello,

    After i finished my DAC experiment, i began working on ADC stuff.

    1- I am trying to transmit a sound signal using IR LEDs. We know that the sound is analog signal. According to my information, the IR is supposed to get digital (square waves) signals. But, i noticed that some of such types of circuits do not have ADC to convert the analog signal to a digital one!! Does that mean the IR itself has an internal ADC so that we don't bother ourselves building an ADC for that?

    2- Do LEDs only work with pulse generators ? DC?

    thanks.
     
  2. Reloadron

    Active Member

    Jan 15, 2015
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    Not quite sure what you are getting at but...

    If I apply DC to a LED, be it an IR LED or any standard LED I get light be it visible or IR. So what if I modulate the current through the LED? I can do that can't I? What if I vary the current through the LED? The point is I can modulate the LED. I can also turn the LED off and on or pulse the LED. What would those signals look like if I used a photo transistor be it regular or IR to look at my signals?
    LEDs can be digitally pulsed or simply have a constant DC level applied. How about if I apply an AC sine wave? Wouldn't the LED follow the sine wave?
    I can certainly drive a LED with an analog sound (audio) signal) and I can certainly detect that with a photo transistor. Not sure where you want this to figure into DAC or ADC?
    A Google of Infrared Voice Communicator Circuit will bring up dozens of circuit examples.

    Ron
     
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  3. crutschow

    Expert

    Mar 14, 2008
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    An LED can be modulated with an analog signal but digital is generally preferred as it avoids many non-linearity and noise pickup issues in the transmitter and receiver.
    You can easily transmit the signal digitally if you convert the audio to a PWM signal (using a carrier frequency of 50kHz or more) to drive the LED.
    Then you just need a low-pass filter at the receiver end to recover the audio, no ADC.
     
    Last edited: Apr 16, 2015
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  4. gold_sky

    Thread Starter New Member

    Apr 11, 2015
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    Seriously, you explained it very well. So, you are saying that the input is deriving the output. If the input is DC, the LED will be at a DC level (depending on the resistors used so that we dont burn it) and therefore we expect the LED to be ON at that DC level. And if in case the input is a sinusoidal signal, the LED (the output) follows that (alternating). Great!

    The idea of ADC is that I "was" thinking that I "must" modulate the LED "digitally" - i thought the led would not accept analog signals- so that if I have an input of a sine wave for example, this what i would do:
    1- Convert that signal to a digital signal (i was thinking i must modulate the LED digitally).
    2- The conversion process needs an ADC technique.
    3- The LED will follow the digital signal that results form the conversion process.

    However, since you told me that the LED "can" be derived by analog signals, then no need for ADCs.
     
  5. gold_sky

    Thread Starter New Member

    Apr 11, 2015
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    Yeah, i finally realized that it does not have to be a digital signal for the LED.


    I will post a new response in minutes of a new issue i encountered.
     
  6. gold_sky

    Thread Starter New Member

    Apr 11, 2015
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    here are my new issues !!!!!!!!!!!!!!!!



    I am designing two circuits:
    1- A circuit to transmit the signal through an IR LED.
    2- The other circuit receives that signal also through an LED.

    The Goal is this: Put an input from the first circuit and receive that input from the second signal as it is

    - I designed and built the first circuit successfully. Note that my input is a sound wave (analog).
    - I will not use ADCs. I will just try to isolate and buffer my signal from source the input by using a 1:1 transformer and a buffer.

    I did not complete designing the second circuit but i am trying to. I did received a signal using the other LED as a checking process. Now the job is concentrated on filtering and amplifying the received and here are my issues.

    After an amplification, i am going to control the filtering process using a potentiometer.
    The received signal --> amplification--> high pass filter--> buffer--> low pass filter --> hopefully done

    However, i noticed that the high pass filter does not work when i connect it to the buffer!!!! I connected that HPF to the positive terminal of the op amp. when i check the frequency filtering, every thing is passed which is not what i want to achieve at all ( note that i noticed this in the design process using pspice). Why does this happen and any suggestions ?
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,990
    3,226
    Can't really tell what's happening without a schematic of what you are doing as I'm not good at doing a WAG.
     
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