Hi Chip,
I apologize. Maybe this post is partly responsible for the misunderstanding?
Gents thank you for your comments.
You all have a valid point in your own way.
One thing you all overlooked that this is a chaser.
Think of a very compact 4017 counter with 24 outputs and it all makes sense.
This implies that you're only going to light one LED at a time. That's ok and I guess it's an interesting design but you don't really need 15 pins to do that.
If you have another application for this circuit in mind state your application and let's talk about it. i.e.: Light up all 24 LEDs at one time.
As it is the circuit does 3/24=1/8 with 25ma LEDs, no sink transistors.
Now you're talking about lighting 3 LEDs at a time, one in each group of eight, with a 1/8th duty cycle. No transistors required. That's a viable design. I like it. You might get about 2.5-ma "average" current per LED which is surprisingly bright.
Decrease the series resistors, and use lower mA LEDs (SMD) and you can light up all at the same time if you electrically supply it well.(LEDs of 5-7mA)
Now you're talking about lighting up to 12 LEDs at a time, up to four in each group of eight, at 50% duty cycle. No transistors required. You might get about 2.5-ma "average" current per LED which is surprisingly bright. You may perceive a difference in brightness depending on the number of LEDs lighted at one time in each group.
You should get the same "average" current per LED when lighting 3 of 24 LEDs @ 12.5% duty cycle as you do when lighting 12 of 24 LEDs @ 50% duty cycle.
Most LEDs (check your specs) can drive high current pulses for a short time. (I have 5mm LEDs that can pulse 120mA for a short duration-ms).
You lost me. I know about LED "pulse" or "peak" current specs'. Why are you mentioning it for this direct I/O design?
The rest of the brightness issues (so minor after above changes) can be made in software to compensate.
Your post seemed incoherent (to me). You said to consider the design as a pseudo 4017 with 24 outputs (1 LED at a time) then you talk about lighting all the LEDs. I guess I assumed this was a type of "learning exercise" better suited for a blog.
Mike (Pommie) is right. We should encourage you in this wonderful new adventure and so I hope you accept my sincere apology. Please let me know if I can help you in any way.
Cheerful regards, Mike