help with 38khz ir transmitter using multivibrator

Discussion in 'The Projects Forum' started by aruna1, Jul 23, 2009.

  1. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    hello
    i was wandering can i use simple multivibrator circuit to generate 38khz signal to use as a IR transmitter for TSOP1738?

    i did some readings and calculations and came up with resistor and transistor values that gives 38khz.
    but since i dont have a oscilloscope i don't know whether this circuit will work.

    and most importantly all 38khz IR transmitter circuits on internet are based on NE555.i wonder why nobody use a multivibrator?

    calculations are based on article
    http://en.wikipedia.org/wiki/Astable_multivibrator

    any help would be great
    thanks

    here is my circuit diagram[​IMG]
     
    Last edited: Jul 23, 2009
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    The NE555 is a multivibrator that requires only a few external components, has an easily predictable frequency, and is almost guaranteed to work.

    Considering your lack of testing equipment, I wonder why you would not want to used such a fool proof approach initially?

    John
     
  3. millwood

    Guest

    yes, your design will work.

    no, you don't need the two diodes.

    ne555: it is just cheaper for them and has more current capability so it can drive a led directly. and newer ones don't even use a 555 timer.
     
  4. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    actually NE555 requires minimum 4.5v.but i need a circuit that runs on 3v.thats why i used multivibrator
     
  5. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    -thank you
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    The datasheet for the TSOP1738 lists a minimum supply voltage of 4.5V. So, your 3-V circuit will not work with that device.

    John
     
  7. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    tsop for the reciever(runs on mains current)

    multivibrator for the transmitter(battery operated)
     
  8. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    Get the CMOS version of the 555, such as LMC555. It will work down to around 1.5V.
     
  9. Arm_n_Legs

    Active Member

    Mar 7, 2007
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    If there is a uC in your circuit, you can use it to generate the 38KHz signal. I find it very simply to generate a 38 KHz square wave on uC (such as the C8051F930) with a Programmable Counter Array (PCA). Only two lines of code is required.
     
  10. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    ok guys i made it.it can generate 38khz.i tested it with HRM3800 infrared reciver.here is the diagram
    [​IMG]


    but now my problem is its working distance is very very low.only about 2cm:cry:

    can someone help me to amplify its working distance?
     
  11. millwood

    Guest

    what's the voltage drop of the IR diode? if it is higher than 3v, you are out of luck.

    potential solutions would include running at higher voltage (more batteries or a boost-up converter), running Q1/Q2 richer, or using a dedicated driver, etc.
     
  12. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    1) You could be off frequency, but not so far off that that the detector doesn't respond when close.
    2) Your detector reduces sensitivity when presented with cw at 38 KHz. You need to modulate your signal as described in the TSOP datasheet.
    3) And of course, your IR emission may be very weak. What is the dispersion of your IRED?

    John
     
  13. aruna1

    Thread Starter Active Member

    Dec 20, 2008
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    its about 1.85v
     
  14. millwood

    Guest

    wouldn't you then need a resistor to limit the current going through that led?
     
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