I would like to share this link to the Paul Falstad's "Circuit Simulator Applet".
http://www.falstad.com/circuit/.
This java applet is an electronic circuit simulator. When the applet starts up you will see a simple LRC circuit. The green color indicates positive voltage. The gray color indicates ground. A red color indicates negative voltage. The moving yellow dots indicate current.
To turn a switch on or off, just click on it. If you move the mouse over any component of the circuit, you will see a short description of that component and its current state in the lower right corner of the window. To modify a component, move the mouse over it, click the right mouse button (or control-click if you have a Mac) and select “Edit”.
The "Circuits" menu contains a lot of sample circuits for you to try, including filters, oscillators, amplifiers, logic gates and circuits, phase-locked loops, etc.
If the applet stops responding, try resizing the window. This can happen with Microsoft java; the applet works best with the Java plugin.
You can also check this one
http://www.falstad.com/fourier/. (Fourier Series, i love this one)
Well, i hope this applets are as useful to you as they have been to me.
