The xLogicCircuits Applet

The applet on this page lets you build simulated logic circuits from AND, OR, and NOT gates and see how they behave. This is not a serious circuit design tool. It's an educational tool for learning the basics about logic circuits. In particular, only the "logical" behavior is simulated, not the electrical components from which real gates are made. The xLogicCircuits applet is one of several applets written by David Eck for use with his introductory computer science textbook The Most Complex Machine. (They can also be used independently of the book.) For a list of other applets and for lab worksheets that use the applets, see the index page.

The applet is set to load some sample circuits. You should see an XOR circuit with two inputs on the left, some gates and wires that compute the exclusive or of the inputs, and an output on the right. (The definition of XOR is that the output is ON if one of the inputs is ON and the other is OFF.) To run a circuit, you first have to turn the power on, using the "Power" checkbox at the bottom of the applet. Then you can click on the circuit's inputs to turn them on and off. The signals from the inputs propagate along the wires through the circuits.

More instructions and more details are given on this page below the applet.

Note, added June 28, 2011: Tom Arnall has used xLogicCircuits to build an entire (simple) computer! The link will take you to a page where you can run the applet and read about how it works.

(Java not available.)

Building Circuits

You can build circuits by dragging components from the scrolling pallette on the left of the applet. This pallette always contains at least the six standard components: NOT gate, OR gate, AND gate, Input, Output, and "Tack." The gates are components that do computations. An AND gate, for example, has two inputs and one output. It turns its output on if both of its inputs are on. An OR gate also has two inputs. It turns its output on if either one of its inputs is on (or if both are on). The NOT gate has one input and one output. It turns its output on if and only if its input is off. Inputs and Outputs can be placed anywhere along the outer boundary of the circuit. An Input represents an input for the circuit as a whole. When the power is on, you can turn Inputs on and off manually by clicking on them. An Output represents a value computed by the circuit. The values of Outputs cannot be set manually. A "Tack" is simply an attachment point for wires. You might want to use Tacks to help make your circuits neater.

Once some components have been dragged onto the circuit board, you can draw wires to connect them. Every wire leads from a source to a destination. To draw a wire, you have to click the mouse first on the source. Then hold the mouse button down while you move the mouse to the destination. When you release the mouse button, the wire will be added to the circuit. If you release the mouse button when the mouse is not over a valid destination, no wire will be drawn. Circuit Inputs are valid sources for wires. So are Tacks. So are the outputs of gates. Valid destinations include circuit Outputs, inputs of gates, and Tacks. You can draw as many wires as you want from a source, but you can only draw one wire to a destination. (This makes sense because when the circuit is running, a destination takes its value from the single wire that leads to it. On the other hand, the value of a source can be sent to any number of wires that lead from it.)

When you are drawing wires, you'll notice some helpful visual clues. Sources generally have a purple sort of color. Destinations are green. A Tack, which can be both a source and a destination is green until a line has been drawn to that Tack; then the Tack turns purple. When you are drawing a wire and you move the mouse over a legal destination, the component will be hilited. (A hilited gate is draw in blue, a hilited Output is enclosed in a blue rectangle, and a hilited Tack is enclosed in a blue circle.) Note that some components, such as gates, have multiple inputs. When you move the mouse over a component with several available inputs, the wire will jump to the nearest one. Make sure that you get the wire attached to the one you want.

The gates in the pallette can be rotated into four different orientations. Just click on the little red, curved arrow on the left, above the gate. Once a gate has been dragged onto the circuit board, it can't be rotated. However, it can be resized. When the gate is hilited, a box appears around it with small drag handles in each corner. Drag one of these handles to change the size of a gate. (To hilite a gate, or any component or wire, just click on it.

Any component on the circuit board can be moved around. Just drag it by holding down the right mouse button. Alternatively (if you have a one-button mouse, for example) you can hold down the control key to drag a component.

Whenever a wire or component is hilited, you can delete it by clicking on the "Delete" button. If you delete a component that has some wires attached, those wires will also be deleted. There is an "Undo" button that you can use to get back something that you delete by mistake. (Note that the "Undo" button can only undo one operation.)

You can edit a circuit even while the power is on.


In addition to the six standard components, the scrolling component pallette can also contain circuits that have been built from these basic components and iconified using the applet's "Iconify" button. These iconified circuits can be used as components in building other, more complex circuits. You can drag them onto the circuit board, just like other components. The applet on this page should have loaded three such iconified circuits: "4 Bit Adder," "Clock," and "One Bit Mem."

To see inside an iconified subcircuit, hilite it and then click on the "Enlarge" button. The circuit will expand to fill the circuit board. (You can also just double click on the subcircuit. However, not all browsers respond to double clicks.) When you expand a circuit icon from the pallette, its icon is removed from the pallette. The circuit that was on the board previously is iconified and moved to the pallette, unless it is empty -- in which case, it is discarded. There is only one circuit on the board at any given time. You can edit the circuit while it is on the circuit board, and the changes you make will be permanent even after you re-iconify the circuit.

If the circuit on the board containes a subcircuit, you can enlarge that circuit to see what is inside it. This does not remove the main circuit from the board -- it just lets you see an enlarged part of it. When you shrink the subcircuit back down to its original size, the main circuit is still there. For an example, Enlarge the "4 Bit Adder" sample circuit from the pallette. You'll see that it contains four copies of a subcircuit called "Adder." If you hilite one of the "Adder" circuits and Enlarge it, you can see what's inside it. You'll also see a big red message, "Enlarged from 4 Bit Adder," to remind you that what you are looking at is a part of a larger circuit.

It is important to understand that when you drag a subcircuit onto the circuit board, what you get is a copy of the circuit from the pallette. If you edit the original circuit on the pallette, it doesn't affect any copies that have been made. If you edit one of the copies, it doesn't affect the other copies. Note in particular that it's easy to get two circuits that have the same name but that are different internally. (My advice, of course, is don't do it!)

By the way, a circuit can have multiple outputs. When you draw a wire from a subcircuit to another component, make sure that the wire starts from the output that you want.

Other Features

The "Clear" button can be used to clear all the components from the circuit board. If several circuits are stacked up on the board, it only affects the one on top. The Clear button does not remove circuits from the pallette. (If the circuit you Clear is an enlarged subcircuit that is part of another circuit, the Clear button does not remove the circuit Inputs and Outputs. This is on the theory that they are probably connected to other components in the containing circuit, and you probably don't want to delete all the connecting wires. However, you can still delete the Inputs and Outputs individually, if you insist.) Note: You can use the "Undo" button to undo a Clear, provided that you do so before you do any other operation.

The "Save" and "Load" buttons are for working with files. (Hoewever, it is likely that your browser's security policy will prevent you from using them.) The Save command saves the entire state of the applet, including all the circuits in the pallette and on the circuit board. When you Load a file, everything in the applet is thrown away and replaced by the data in the file. Note that the Load command is undoable. Pressing the Undo button immediately after loading a file will restore the previous data.

Above the applet you'll also find a text-input box labeled "Title:". This is name of the circuit currently visible on the circuit board. You can change the name by editing the contents of this text-input box.

At the left of the strip of controls below the applet is a Speed pop-up menu. This menu affects the speed at which signals propagate through the circuit. You might want to use the slower speeds to watch what is happening in more detail. (At the Fast speed, it is possible in some browsers that the circuit will not be drawn correctly. For example, the "Clock" sample circuit should blink very quickly when the speed is set to Fast, but your browser might not allow applets to draw to the screen quickly enough to show this.)

Another interface note: You should be able to insert a tack into the middle of an existing wire by double-clicking on the wire. If you double-click and hold the mouse down on the second click, you can drag the tack to a different position. (Again, I note that some browsers might not support double-clicks.)

And one more: It is not possible to draw a wire directly from an output of a gate or circuit to an input of the same gate or subcircuit. However, there is nothing to stop you from making loops out of several components, as is done in two of the sample circuits.

About the Sample Circuits

The sample circuits loaded by the applet on this page are all fairly standard examples of logic circuits. The XOR circuit, which is visible on the circuit board when the applet starts up, is discussed above. The "4 Bit Adder" is a circuit that can be used to add two 4-bit binary numbers. When the numbers are put on the eight input wires at the top of the circuit, the sum -- which can have five bits -- appears on the five output wires on the bottom and left of the circuit. (The output on the left is the "carry" bit.)

The "Clock" circuit has one input and one output. If you turn on the power, it will start turning its output wire on and off. If you turn on the input, however, the clock will stop "ticking." It is worthwhile to watch this example with the speed set to Moderate or Slow. (You can slow down the rate of ticking by inserting more Tacks into the loop. This is because of the way signals are propagated through circuits. The simulated circuits in this applet use "discrete time." At each discrete moment of time, each component in a circuit gets the values from its input wires, and computes values for each of its output wires. Real circuits don't work this way. Nevertheless, the logical behavior of most simulated circuits is -- with a few exceptions -- the same as the behavior of real circuits.)

The "One Bit Mem" circuit is a memory circuit that remembers the value of one bit. To store a bit (ON or OFF), put the value to be stored on the lower of the two Inputs on the left of the circuit. Turn the upper Input ON; wait a while, until the circuit settles down; and turn the upper Input OFF. The value that you've stored will appear on the output wire. This value will remain stored in the circuit as long as the upper Input is OFF. (If you don't wait long enough before turning the upper Input OFF, the value will not be properly stored. In fact, strange things can happen that are different from what would happen in a real, electrical circuit.)

David Eck (, August 1997