switch an LED with open drain output

[panfilero]

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

 

[panfilero]

On Tue, 19 Mar 2013 18:00:33 -0700 (PDT),






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

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LED

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|

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gnd







--



John Larkin Highland Technology Inc

www.highlandtechnology.com jlarkin at highlandtechnology dot com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom timing and laser controllers

Photonics and fiberoptic TTL data links

VME analog, thermocouple, LVDT, synchro, tachometer

Multichannel arbitrary waveform generators

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

 

[Spehro Pefhany]

i have a problem, i need to switch an LED on from an open drain output. The 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?

much thanks

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

 

[Artemus]

On Tue, 19 Mar 2013 18:00:33 -0700 (PDT),

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

 

[Artemus]

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

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

 

[panfilero]

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!