Electronics > Beginners

Questions about parts selection and things I don't understand

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AvariCe:
Hello!
I have an idea for a project that will hopefully lead me to design my first pcb. It will be an attiny13a which will send a pwm signal to a 4 to 16 demultiplexer which will then fade in/fade out an LED out of 16 at random. All of this will be powered by a 3v coin cell battery. The whole idea is to make self contained christmas ornaments.

And I've hit a wall due to not understanding a few things:
* From what I understand (which, again, is not very much), I will actually be able to feed a pwm signal to the demultiplexer and have it work as intended. Is that correct?
* On the datasheet of the demultiplexer, there isn't a figure for power consumption(unless I missed it), only a figure for total power dissipation of 300 mW. Does that mean it will continuously draw 300 mW? As you might  have guessed by this question, I can't understand power dissipation.
* I can't decide about what leds I shoud use. As this will powered by a meek battery, I don't want the leds to draw a lot of power (hence the fade in/out shenanigans) but I want them to be fairly to somewhat bright as well. From your experience, what mcd's should I be looking at and at what is the point I should compromise?Lastly, I don't have much experience designing stuff like this or choosing parts, so any help or suggestion is welcome.
Thanks for your time.

ledtester:
1. It should work. If you look at the rise and fall times (t_r and t_f) you'll see they are in the nanosecond range. All of the digital ICs are designed to be used in circuits which operate in the megahertz and faster range.

2. The 300mw is a limit - that's the maximum power the chip can safely dissipate without being damaged. At 3V 300mw is 100mA which is a lot of current for a CMOS logic chip.

Power consumption of CMOS logic chips (4000-series, 74HC/HCT) is very, very low - in the micro-watt range.

3. Generally a higher mcd means a brighter and more efficient led which is desirable if you want to conserve power. You also want to choose an LED for its appearance which includes how it's encased - clear vs. diffused. You might just have buy a bunch of different ones and try them out.

You should consider using a micro with a few more IO pins and using charlieplexing:

https://www.reddit.com/r/explainlikeimfive/comments/47097a/eli5_what_is_charlieplexing_and_how_does_it_work/

If you have 5 free IO pins you can charlieplex 20 leds.

Update: Realized you might even be able to use the attiny13a. If only one LED will be illuminated at a time, just write a custom routine for each LED which will fade it in and out via charlieplexing.

AvariCe:
Thanks a lot! Also appropriate username :D. I had no idea about charlieplexing, will look into it now!

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