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Current/Voltage drop when LED blinks

Discussion in 'Hobby Electronics' started by Wayne., Jul 16, 2009.

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  1. Wayne.

    Wayne. Guest

    I am doing the traditional "hello world" with an Atmega8.

    The setup: LM7805 with a status LED, Atmega8, and LED with 220ohm
    resistor on PB0.

    When I set PB0 low, the LED illuminates, but the status LED dims for the
    duration.

    I am showing a range of 4.7 to 5.3v, even at rest. I have also changed
    out the voltage regulator; LM7805T, LM7805L, LM317.

    The voltage is far more stable using the LM317, but I still experience
    the dimming of the status LED, even though the voltage is rather
    constant (4.9 to 5.1).

    It is almost like there is a short. I have expanded the circuit to use
    3 Wisher boards: 1 for the voltage regulator, 1 for the blinky, and the
    3rd for the uC.

    The LEDs require a peak of 20ma.

    Any ideas would be appreciated.
     
  2. Andy Wood

    Andy Wood Guest

    What is the input voltage to the regulator? How stable/current
    sensitive is that?

    Does the current drawn by the whole thing increase by the expected
    amount when the LED is on?


    Andy Wood
     
  3. Is the status LED directly across the regulator or another micro output pin?
    If it is directly across the supply then it shouldn't dim, as the LM7805 has
    a low output resistance and can supply ample current.
    If it's on another output pin then the ground of the micro would be suspect.
    Do you have a photo of the actual construction?, this could provide vital
    clues.
    Sounds like it's oscillating perhaps.
    Has your 7805 got proper bypass caps on it?
    What is the 7805 input voltage?
    Looked at it with an oscilloscope?
    Tried a DC bench supply?
    It shouldn't change at all, something is wrong.
    What's a Wisher board?
    What's this "blinky"?, the status LED I presume?
    What's the circuit?
    Most LED's do not need to be driven at 20mA to be usable, 5mA or so is
    usually plenty.

    Dave.
     
  4. Don McKenzie

    Don McKenzie Guest

    if you can scribble down the schematic, scan it, and throw it up on the
    web somewhere, it may help.

    Cheers Don...



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  5. Wayne.

    Wayne. Guest

    It was a dodgy controller. I swapped out the installed atmega8 and
    replaced it with another, and it worked fine.

    I noted that without blinky, the circuit used 50mA: status LED, uC, and
    7805.

    When blinky fired, the current usage jumped to 80mA.

    But in both cases, voltage remained the same.
     
  6. F Murtz

    F Murtz Guest

    Never did find out what blinky does with a wisher board
     
  7. Wayne.

    Wayne. Guest

    Wisher boards are the solderless breadboards.

    The way I had the circuit setup: one board had the voltage regulator
    with the status LED, the seconds had the uC, and the third with the LED
    and resistor for the blinky hello world.
     
  8. Bob Larter

    Bob Larter Guest

    Add two 100nF ceramics to the 7805, one between input & ground, & the
    other between output & ground.
     
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