why do we need modulated carrier for ir tx? (TSOP)

Discussion in 'General Electronics Chat' started by ravichauhan, Mar 3, 2011.

  1. ravichauhan

    Thread Starter New Member

    Mar 3, 2011
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    why cant we simply connect it with a battery with a resistor with a continuous flow? and at receiver side a simply photo diode or transistor can receive it? why we need such complex circuits for modulated tx- rx? do modulation effect distance or something else?
    i am mew to this world, please don't mind for my very basic question.
    ravi
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Your understanding may start by reading the datasheet.

    John
     
  3. bertus

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  4. John P

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    Oct 14, 2008
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    The problem is that the world is full of infrared light, especially daylight. Any attempt that you made to send a signal just by turning an LED on or off would fail because the sensor would already be receiving light from other sources.

    The way to build an optical data link is to use a source with known characteristics, such as a light that's modulated at a known frequency. Then the sensor can have an electronic filter that responds only to that frequency, even if there's a large amount of light in the environment.

    But if you knew that your system would be operating in the dark all the time, then yes, just a simple on-off control would work.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    A good example of this is a TV remote control, which has to compete with the room lighting, and so a carrier system is used to improve resistance to interference.

    On the other hand, some optical fibre systems can use simple binary modulation, because the fibre effectively screens out external interference.
     
  6. ravichauhan

    Thread Starter New Member

    Mar 3, 2011
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    why do we need to modulate ir wave at 38 khz? why don't we just give it battery suply for always on condition and at receiver uses a simple photodiode or transistor to receive it.. pls help, as i m new to this world...
     
  7. wayneh

    Expert

    Sep 9, 2010
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    It's about signal to noise ratio. Having the receiver reject everything that's not near 38K dramatically improves the ratio and allows the receive to be set to be much more sensitive, extending the range of the system. Also, the LED pulses can be brighter than when operated continuously, again extending useful range.
     
  8. jpanhalt

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  9. bertus

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    Hello,

    I merged the posts.
    You have AGAIN hijacked a thread.
    Please keep all in your own thread.

    Bertus
     
  10. Kerim

    Member

    Mar 3, 2011
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    Hi ravichauhan,

    15 years ago, I was new, as you are now, to the world of IR control.
    I tried hard to get a suitable distance and a good sensitivity while sending IR pulses to be received by a photo diode.
    At that time, I was amazed how other IR controls work much better than mine. First I thought that my IR LEDs and the photodiodes are not well matched (not for the same colour wavelength for example). Then I thought that my IR detector circuit is not well designed to compensate the ambient light variation or it may need a notch filter to reject the 50Hz... etc. This took me several months till I discovered the simple trick that the factories follow to let their control units be insensitive to the surrounding lights. The trick is to modulate the transmitted bits on a carrier. To unify the design of the IR detector many companies (if not all) have decided to let the carrier frequency be 38 KHz. Thanks to this standard we can buy now good IR receivers (small 3-pin) at a relatively low price.
    Despite all, the answers you got here should not stop you to test your idea and see its advantage and weakness by yourself. And when after many years you will meet someone asking your question, you will have a lot to tell him :)


    Kerim
     
  11. ravichauhan

    Thread Starter New Member

    Mar 3, 2011
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    thank you all. it will help me in designing good circuits a lot. thank you again..
     
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