Digital Logic Gate Simulator Community

Key ideas

• Every logic gate is just a rule for turning one or two 0/1 inputs into a single 0/1 output, and those simple rules are the building blocks of every digital computer.
• AND is 1 only when both inputs are 1; OR is 1 when either input is 1; NOT simply flips its input.
• NAND and NOR are AND and OR with the output inverted, and either one by itself is a universal gate: any Boolean circuit, including AND, OR and NOT, can be rebuilt using only NAND gates (or only NOR gates).
• XOR outputs 1 only when its two inputs differ, which is exactly the rule for adding two binary digits without carrying, and XNOR is 1 only when its inputs match, which is how circuits test two bits for equality.
• A truth table lists the output for every possible combination of inputs, and for n inputs there are always 2^n rows, so a circuit with 3 inputs has 8 rows and one with 4 inputs has 16.
• Combinational circuits like the ones you build here have no memory: the outputs depend only on the current inputs, not on what happened before, which is what makes it safe to recompute the whole circuit from scratch every time an input changes.
• Every circuit you wire also has a Boolean algebra expression, shown live above the truth table (for example Sum = A⊕B), and for circuits with 2 or 3 input pins a Karnaugh map groups the truth table into a grid that makes it easier to simplify the expression by eye.

Things to try

• Load the Half Adder preset, then flip A and B and check the Sum and Carry against the live truth table and the Boolean expression shown above it.
• Load the Full Adder preset and verify that A=1, B=1, Cin=1 gives Sum=1 and Cout=1, the same as 1+1+1=11 in binary.
• Load the 2-to-1 Multiplexer preset, flip the select line S, and see how it switches which of I0 or I1 reaches the output Y.
• Turn on the NAND-only challenge and build a NOT gate from a single NAND (tie both its inputs to the same input pin), then build an AND gate from two NAND gates.
• With 2 or 3 input pins wired to one output, open the Karnaugh map under the truth table and compare its grid to the Boolean expression readout.
• In the quick-learn panel at the top, switch between AND, XOR and XNOR with the same A and B values and notice which ones agree and which ones disagree.

Where this is used

Every processor, memory chip and embedded controller is built from millions of gates like these; digital electronics engineers use exactly this kind of gate-level design, truth tables and adder circuits to build calculators, CPUs, traffic light controllers and the arithmetic units inside every computer and smartphone.

How to ask a question people can answer

A good thread on the Digital Logic Gate Simulator saves everyone time. Start with a title that names the exact problem, such as the input you used, the result you expected and the result you got. A title like “Not working” is hard to answer, while a title with the specifics gets replies quickly.

• Say what you tried. List the steps you followed and any settings you changed, so replies do not repeat what you already did.
• Share a small example. A short sample input and the output you saw is worth more than a long description. Remove anything private first.
• Mention your setup. Your device, browser and whether you are on a phone or a computer can explain many odd results.
• One question per thread. Separate topics get clearer answers and are easier to find later with the search box above.
• Come back and reply. If a suggestion worked, say so and upvote it, and if you found the fix yourself, post it for the next person.

Community rules

Be kind, stay on topic and keep personal data out of your posts. Do not post adult, hateful or illegal content, advertising or repeated links. Every post is shown with its author’s username and date, and posts that break these rules can be hidden by moderators. This space is for help with the Digital Logic Gate Simulator and related tasks, and it is separate from the sitewide discussion forum where broader topics live.