switch an LED with open drain output

open 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?

How much current does the LED need? What can the port sink?

What do you mean by "The open drain output can have no pull up tied to it." If that's literally true, it can't be done.

Absent that, here are two ways:

Vcc------R------led------port

or

Vcc-------R------+-------port | | LED | | | gnd

The open drain output can have no pull up tied to it, it can only be Low an d 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 h igh (I can turn it on by pulling it low, but showing the gate HiZ will do n othing)... is there anything that switches based on a HiZ or Low signal?

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I mean my circuit has several open drain outputs tied together, their purpo se is to override a control signal, basically when everything is running go od 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 d rains high defeats the purpose

open 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?

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 +-----+------+-----||-+ ->|-+ ->|-+ | | | | | | | | | | | | === === === | | GND GND GND | | | | | '------------------------' Existing open drain outs

Best regards, Spehro Pefhany

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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

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purpose

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

open 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?

Guess what an open drain output does with no pull-up...

Initially it might be called a "wanderer" >:-}

Pull it low

Then go to hi-Z... it stays low ;-) ...Jim Thompson

Put the pullup and LED on the open drains, and use a separate MOSFET (e.g. a 2N7002) to ground the control signal. You'll need another inverter stage in between, or perhaps you can effectively move the open drains to the P-channel side and have the resistor and LED pull down instead of up.

You can usually do that on MCU GPIO pins.

Cheers

Phil Hobbs

open 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?

If

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

HOW does it over-ride the control? Show a schematic. If you're trying to over-ride a conventional CMOS output by pulling it low, you might not release the magic smoke, but something IS likely to get warm. ...Jim Thompson

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!