Re: LED & Resistor befuddlement

From: Mike Turco (miketurco_at_yahoo-nospam4me.com)
Date: 06/03/04

  • Next message: Paul Burke: "Re: Bit width?" 
    Date: Thu, 3 Jun 2004 00:07:50 -0700

    "rickman" <spamgoeshere4@yahoo.com> wrote in message
    news:40BEA128.435BADC1@yahoo.com...
    > Mike Turco wrote:
    > >
    > > > > >I have never been able to get an IV curve on LEDs and I have not
    > > > > >measured it myself. Anyone know how low the current must be to get
    the
    > > > > >voltage drop below 1 volt?
    > >
    > > Once a diode is turned on, the voltage remains pretty much the same
    > > regardless of the amount of in-spec current you run through it. If you
    limit
    > > the current too much, the LED won't be bright enough, or won't turn on.
    > >
    > > In general, LED's drop between 1.5 and 2V. As you vary the current
    through
    > > the LED, you will see some variation in the forward voltage, but not
    enough
    > > to make a difference, and I doubt that a reliable way to use these
    devices.
    >
    > I don't think you have read enough of the thread to understand what we
    > are doing.

    I'd better read it from the top.

    > I am trying to use a pull down resistor to pull the IO pin
    > on the MCU to ground when the LED is absent.

    I understand what you're saying. The thing is that the drop across the LED
    is a fixed voltage of about two volts, and that puts the logic level into
    never-never land.

    > When the LED is in place,
    > the pull down resistor needs to be light enough (high enough resistance)
    > to *not* draw any more current than necessary. The pull down resistor
    > is not trying to make the LED light. The goal is to allow the LED to
    > pull the IO pin up to a voltage that will be seen as a 1 on the MCU IO
    > pin.

    OK, lets say you have a 1k pull-up resistor and a 10k across the LED. The
    voltage drop across the 10k resistor is going to be ~ 2V. So, then, you
    change the pull-up to 330 and the 10k to a 100k, but it doesn't make a
    difference? Why? Because the voltage across the LED is always going to be
    pretty much the same -- that's the nature of a diode.

    Another thing I thought about doing is putting two resistors in series under
    the LED. But it just doesn't cut it.

    Last night I was thinking about this problem for just one LED. The fact is
    that I need to implement this into a 4x4 array of these "LED Switches".
    So.... the basic question remains, but its not so many transistors after
    all.

    >
    > I originally assumed that we were talking about TTL levels, but I expect
    > there may be some devices that use CMOS thresholds on the inputs.

    I will probably use a mux on the keypad rather than a controller, so I have
    a choice between TTL & CMOS, but not A/D.

    Mike

  • Next message: Paul Burke: "Re: Bit width?"

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