LED pull up configuration

PIN >> Resistor ( say 330 ohms) >> LED >> Ground.

That will light led when pin goes high. Resistor used to limit current thru LED.

You have to make sure the pin achieves a logic 1 level for lock detect pin with the extra load.


Regards, Dana.

Some LEDs won't light up from 3V (blue, for sure -
green, maybe marginal). If you need less forward
current than the I/O can source, then no prob. If you
are too limited then consider a common emitter NPN
with base driven via a few-K resistor from I/O, and
collector to the LED with its anode to +5V (or +3 if
adequate) and a series limiting resistor to set the current.
You could also use inverters, and if necessary parallel
them for current, perhaps cheaper and smaller on-PCB.

dolgaleb said:

I have a synthesizer that sends 3V to a LOCK DETECT pin when the signal is locked. I want to connect an LED to this pin to have a visual indication that the synthesizer is locked. I was told I can't connect an LED to this pin because the 3 V is just a signal and that I need to use a pull up resistor. How do I implement this?

Click to expand...

This may be true. YOu don’t give us any specs. What’s the output-high current of the LOCK signal?

I'm not sure that voltage drop chart is relevant now. Certainly in the case of 'super bright' LEDs, the Vf is fairly constant across the color range as they all produce near UV from the die itself and then different phosphors to convert it to the hues needed.

I use blue and white 'super bright' LEDs successfully from 3.3V supplies. I just checked some 5mm blue and white LEDs from a CC source at 20mA and they both clamped at 3.05V.

Brian.

No shortage of conflicting charts ....



Regards, Dana.

Both are correct - in their own way.

The color of light emitted from the die itself depends upon it's chemical composition and that in turn decides the V/I characteristics. The newer designs of LEDs, in particular the brightest ones produce visible light using a different method, instead of the die itself producing the visible color, it excites a phosphor material that in turn produces the light. Different phosphor chemistry decides which color is produced.

It's akin to fluorescent tubes, it the paint inside the glass that produces the light, not the gas inside.

If you think about al the masses of small modules and devices on the market these days, mostly running on 3.3V or less, they wouldn't be able to drive LEDs with the voltage available according to the charts but clearly they can.

Brian.

Wow. This all has pretty much nothing to do with the OP's original question: how to drive an LED with an (up to this point) undefined output. The only question, as far as I can see, is:

How much current can the output source at 3V?

Once that is known, picking an LED (or a drive transistor, if required) is pretty straight forward.

A representative ARM processor, looks like driving high not a
good choice for Blue LED on 3.3V I/O.




Regards, Dana.