Hello. I am planning to add one multiplexer to ALU im building with transistors to be able to choose operations. Now im asking this question, because i only know that multiplexer is the way to go from minecraft(which is not a trustworthy source BTW Razz).

Basically what i did is that i build a ton of not gates and buffergates so that only one output of the multiplexer was high when a certain combination of inputs was on, which is basically translating binary code into whatever you want by setting one cable high. I have drawn a picture of how i think it works.

http://imageshack.us/photo/my-images/4/muxd.jpg/

I dont want to buy it and then learn that it does something completely diffrent, so im asking here.
No, that diagram doesn't look quite right. You have multiple inputs (the outputs from either a buffer or not gate) going into each output (O0 - O7). I think it needs to connect each set of outputs to a three-input AND gate whose output goes to one of the seven outputs.

Here's how I've built demultiplexers:

Code:
I0 --------+------------------------------------------------
           |
           +-|>o---+----------------------------------------
                   |
I1 --------+-------|----------------------------------------
           |       |
           +-|>o---|--+-------------------------------------
                   |  |
I2 --------+-------|--|-------------------------------------
           |       |  |
           +-|>o---|--|--+----------------------------------
                   |  |  |
                   |  |  |
                   |  |  |
                  ---------
                  |       |
                  |       |
                  |       |
                  \       /
                   \     /
                    \___/
                      |
                      O0


(Please excuse the ASCII art)

From this you can tie the other 7 AND gates to the appropriate input or negated input lines.
Ow yeah, i see it now. But it is basically the same thing i have drawn (maybe inverted, i dont know), right? So only one output is going to be on for each combination of outputs, is this correct?
What you're describing here seems to be a line decoder rather than a demultiplexer - I suggest looking at the Wikipedia article for multiplexers to get a better idea of what they are.

The 74HC138 appears to be the component you're after; its datasheet has a logic diagram. The chip enable inputs do allow you to use it as demultiplexer, however.

Edit: The 74HC138 is inverting, which is useful as most chip-select/enable lines are active-low. The 74HC238 is non-inverting if you need active-high outputs.
Thank you! 74HC238 is exactly what i needed.
I have a small follow-up. Its about the usage of the IC that you have recommended me.

The documentation says that it can max. accept 5V. Im running at 3 so its ok. But do i have to put some resistors before inputing the current or is it okay to just plug it in? Same thing with the enable, i just dragged a cable from my 3+ to the leg and its done. Do you think its fine?
As long as the input HIGH state voltage remains in between Vcc+-0.5V the input current is 20mA, so you do not need resistors.
Ha thats probably why it broke when i connected 3 V directly to it. In the documentation there was a line that said :

Parameter Conditions Min Max Unit
CC supply voltage −0.5 +7 V

so i thought it can accept max +7 V. :/
keoni29 wrote:
As long as the input HIGH state voltage remains in between Vcc+-0.5V the input current is 20mA, so you do not need resistors.
..what? That's complete garbage.

As long as the inputs don't exceed rated voltages, it's fine. Further, logic-high is guaranteed to have a threshold about 0.7*Vcc. It's a CMOS device, so you don't need any sort of limiting on inputs (they're high-impedance).
It's what tbe datasheet specifies. Indeed: the current is limited as I said
keoni29 wrote:
It's what tbe datasheet specifies. Indeed: the current is limited as I said
High-impedence means they're already very high resistance, and negligible current flows. Smile I can't think why you'd need current-limiting resistors on a CMOS IC.
  
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