Here a basic electronics circuit simulation. It free, you can "buit" basic circuits, so you see what the circuit does and you can download the app. it is a Java App, the con is can't save & print your work. Here the link.http://www.falstad.com/circuit/
I hope it help you and it help others. That app help me out to figure things out.
If you want to pay for a better electronics circuit simulation, here the site. http://www.new-wave-concepts.com/
. Anyway, check it out. I have Circuit Wizard Pro, it is expensive, but it is worth the investment. I made a few projects using that software.
I wish a Arduino simulator was available. ( I got VBB, not working right )