USB question

Discussion in 'The Projects Forum' started by Vanush, Oct 14, 2010.

  1. Vanush

    Thread Starter Active Member

    Apr 19, 2008
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    Hi

    I want view the signal emitted from an IR remote control on an oscilloscope. I have a 3-pin IR receiver but no power supply. Is it possible to take the USB cable from the computer, cut it and put a resistor in between it and Vcc on the diode to power the diode? Note I dont have any 78xx voltage regulator.. but IR receiver needs 4.5V/3mA minimum
     
  2. wayneh

    Expert

    Sep 9, 2010
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    USB power would be plenty for the IR receiver module. Just be doubly sure of the pinouts - you don't want to fry your computer.
     
  3. t06afre

    AAC Fanatic!

    May 11, 2009
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    Without usb negotiation, the max current will be 100mA. But this should cover your application. If you have an USB hub. It would be an good idea to use this to test things out. If something goes wrong. You will most likely fry circuits in the USB hub, and not the PC. The cost of an USB hub is far less the cost of a PC
     
  4. Vanush

    Thread Starter Active Member

    Apr 19, 2008
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    What resistor do i need to get 3mA for the diode.
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
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    Ohms law will tell you. ;)
     
  6. wayneh

    Expert

    Sep 9, 2010
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    To be quick and conservative, assume the entire voltage drop is across the resistor. That will start you with a R value - computed using Ohm's law - that is too high and a current too low, because in fact some of the total voltage drop will be across the LED. Then move R down in increments until you get the current (or brightness) you want. LEDs vary, but a voltage drop of 1-3 volts is normal.
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
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    If you dont know the forward voltage of the LED, you are gonna have a problem.

    If you have it, subtract it from 5v.

    Now divide that by .003 The answer is the value in ohms for the resistor that will get you 3ma after the diode drop and the 5v of the USB.
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    The OP is asking for help with a 3-pin IR receiver not a IR tranciever. At least as far as I can read ;) But to cut some time Vanush. Post the which 3-pin IR receiver you are using
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    I read "power the diode".. I was thinking he was trying to power the IR diode...

    jeez. Yes, the part number would be nice.
     
  10. Vanush

    Thread Starter Active Member

    Apr 19, 2008
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