I have to give a presentation on digital logic and I want to show some pretty animations and stuff. What software would you recommend (MacOS or Linux) that I can use to make some simulations of simple logic gates; maybe a 4-bit adder?
This is not a very sophisticated audience, so I want to make something where it would be clear to the layman what’s going on. Also it will help me review the subject. 
Free software is nice but I’m willing to shell out a few bucks. Not a lot of bucks, though.
Google for a free verilog or VHDL simulator. I don’t know if there are any, but I’d be surprised if there weren’t. The one I use at work runs >$10k per license, but for this you don’t need anything that fancy. (In fact, I don’t see why you need much more than powerpoint for what you’re doing. You can figure out the logic states for a 4-bit adder on paper, no?)
Would this be handy? http://logic.ly/
Yeah, I could just make an animation, but I would prefer something that actually “works” so I can demonstrate how it can be extended by adding new bits, or playing around with components in response to audience questions. (These are high schoolers.)
That looks really cool!
OK, this is officially awesome! Using the free demo I was able to make a one bit full adder in just a few minutes, and it actually works! (Not the most efficient design but it’s the easiest one I could remember.)
This will be $30 well spent. Thanks.
nice!
opencores.org is a good place to go, and here is something about a free Verilog simulator. However I think logic.ly sounds better as a teaching tool.
I like tkGate, myself. It’s fully graphical.
That looks pretty good too, thanks.
Now that it seems the OP has been asked and answered, I’d like to make a very minor hijack if I could.
I’m in the process of getting ready to teach arcade game repair theory to a bunch of hobbyists and I could really use a logic simulation package which includes standard 74 series logic. I realise I could probably re-purpose one of the above tools and build my own 74LS parts, but I’d prefer to get on with writing the tutorials and building the simulations instead of chaining flip-flops and other gates together to make an LS139 
I used something like this many years ago at university, but my google-fu has let me down.
thanks
t.
trmatthe, will this do what you need?: http://www.gamedev.net/community/forums/topic.asp?topic_id=488374
My Google search was: “74 series” logic simulator
My “WTF” is that you are teaching them, and are using software to simulate the things. Why not give them actual 7400 TTLs, and a breadboard. That’s how I learned, and I spent no more on those parts than the software to simulate them costs. Plus, you need a several hundred dollar computer to run the software. This just isn’t adding up, to me…
I have 45 minutes. And I already have a laptop and they have a projector.
45 minutes not enough time to teach everyone how to set up and use a breadboard and wire a bunch of chips. The purpose of the presentation is to introduce the concepts of digital logic and show an example of what can be done.
I was going to suggest this circuit simulator which is better for analog circuits but it has some digital stuff, too. Menu: Circuits->Combinational Logic for examples of what you were looking for. You click on the 1s and 0s to change their states. Right-click to select and place additional components.
I’d like to know if there was a Macintosh one for simulating basic electronic components.
