Connect with us

switch an LED with open drain output

Discussion in 'Electronic Design' started by panfilero, Mar 20, 2013.

Scroll to continue with content
  1. panfilero

    panfilero Guest

    i have a problem, i need to switch an LED on from an open drain output. Theopen drain output can have no pull up tied to it, it can only be Low and HiZ. I was thinking an n-channel jfet might do it, but then i thought... nope, I can't clear the jfet once it turns on unless I can pull the gate high(I can turn it on by pulling it low, but showing the gate HiZ will do nothing)... is there anything that switches based on a HiZ or Low signal?

    much thanks
     
  2. panfilero

    panfilero Guest

    I mean my circuit has several open drain outputs tied together, their purpose is to override a control signal, basically when everything is running good all my open drains are open and the control signal can do its thing, itsjust tied to a highZ, but if one of my open drains goes low, it pulls thiscontrol signal low and basically disables it.... so tying any of my open drains high defeats the purpose
     
  3. Something like this should work:


    +V +V +V
    | | |
    V -> V -> .-.
    +V - A - B | | RP
    | | | |
    | | | '-' Open drain
    .-. .-. | RB___ |
    | | | | +---|___|- -+---+
    RP | | | | | | |
    '-' '-'RA Q1 |-+ | |-+ Q2
    | | | | |
    +-----+------+--------->|-+ +--->|-+
    .------|-----|------|----. | |
    | |-+ |-+ |-+ | | |
    | | | | | === ===
    | ->|-+ ->|-+ ->|-+ | GND GND
    | | | | |
    | | | | |
    | === === === |
    | GND GND GND |
    | |
    | |
    '------------------------'
    Existing open drain outs



    Add pullup resistors Rp and two n-channel MOSFETs, and
    LED A and/or LED B with series resistors RA and/or RB respectivly.


    Alternatively, you might be able to do this, but the output
    will not be pulled down as low, so it might not work.


    +V
    |
    V ->
    - A
    |
    |
    .-.
    | |
    | |
    '-'RA
    | Schottky diode
    +-----+------+-----|<|------ "open drain"
    .------|-----|------|----.
    | |-+ |-+ |-+ |
    | | | | |
    | ->|-+ ->|-+ ->|-+ |
    | | | | |
    | | | | |
    | === === === |
    | GND GND GND |
    | |
    | |
    '------------------------'
    Existing open drain outs














    Best regards,
    Spehro Pefhany
     
  4. Artemus

    Artemus Guest


    I mean my circuit has several open drain outputs tied together, their purpose is to
    override a control signal, basically when everything is running good all my open
    drains are open and the control signal can do its thing, its just tied to a highZ,
    but if one of my open drains goes low, it pulls this control signal low and basically
    disables it.... so tying any of my open drains high defeats the purpose

    ************

    What's the impedance of the control signal?
    Art
     
  5. Artemus

    Artemus Guest

    Never mind, that isn't going to work.

    How about isolating the control sig from the wire-or bus with a diode?
    Will that still pull the control sufficiently low to disable it?
    Then you are free to pull up the drain(s).
    Art
     
  6. panfilero

    panfilero Guest

    thanks for the tips, using pull-ups on my current open drains for the LEDs and then using that to trigger another FET for my "no pull up" open drain requirement makes sense to me, I think that's a good solution!


    Jim,

    It does pull the CMOS output low, there's a current limiting resistor on the output so nothing should get warm, just a resistor to ground is what the output sees when it's disabled

    much thanks!
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-