Quote Originally Posted by Kamikaze47 View Post
I know how to use a transistor as an AND gate, but not sure how to do an OR. I need it to be OR because the output should be high when that output is not selected.
Then use the transistor it 'backwards'...(not backwards like C and E are reversed, just use inverted logic on it)
The output that is 'selected' isn't really 'selected'...it's 'unselected'...that way it idles high.
The other output that is actually 'selected' is really 'unselected'...passing the data thru.

If you use TTL, you'll be 'limited' to TTL voltages and switching levels (.8v low - 2.4v high roughly)
If you use CMOS, you'll be 'unlimited' to roughly 1/3 and 2/3 Vcc for your logic levels.
Depends on what you're driving at the outputs and what you're driving it with at the inputs.

But....your run of the mill 2N2222 will open up all sorts of options as far as voltage levels go. I suppose one of the problems with a transistor MIGHT be switching speeds. Go to fast, and the transistor won't switch fast enough (Early effect, Miller effect, etc). Sometimes it can be 'fixed' with a 'speedup' cap across base and emitter, sometimes not.

Again, depends on the final application.