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Topic: RGB LED HELP! (Read 3441 times) previous topic - next topic

afremont

I can remember seeing just what the OP was talking about years ago, probably like 10 years ago.  They are multicolored LEDs along with a bit of logic that makes them blink, sometimes in different patterns.  You can't do anything but turn them on or off though since they only have power leads.  I don't think you even need a resistor.

And as mike says, photransistors don't need the base lead.  That's what the window is for.  One kinda interesting (I think anyway) thing about phototransistors is that you can use an NPN on top of the load without worrying about supplying the correct voltage to the base.  The light will turn it on anyway.
Experience, it's what you get when you were expecting something else.

Docedison

For as long as I have been in this industry I have wondered about the photo-transistor emitter follower quandary or the absolute requirement of the base bias to exceed the emitter voltage. Only answer I could figure was that it was the energy of the photon that was the deciding factor... Still doesn't quite add up however.

Bob
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Grumpy_Mike


For as long as I have been in this industry I have wondered about the photo-transistor emitter follower quandary or the absolute requirement of the base bias to exceed the emitter voltage. Only answer I could figure was that it was the energy of the photon that was the deciding factor... Still doesn't quite add up however.

Bob


No it is not that.
In an emitter follower the base current is supplied by the same circuit as the transistor is connected to. Therefore the base has to be referenced at 0.7V higher that what you want the emitter to be.
With a photo transistor the photons create the electron hole pairs in the base causing the current to flow. Therefore that current is not subject to the reference voltage of the emitter. In effect the injected current is floating and therefore the voltage the emitter can reach is not limited by the voltage on the base because there is no voltage causing that current flow.

mixania

You should search on Google: "RGB LED fade algorithm".
I tried to experiment with the RGB myself but didn't relise that its so much more difficult than i thought it would be.

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