Logic Gates Simulator

In this lab you can

• Meet the 7 gates in real-life scenes: a microwave, a doorbell, a night light, a bus warning light, an air conditioner, a stair light and a match game. The scene, the circuit and the truth table always agree.
• Combine gates into ready circuits: a home alarm, a sprinkler, XOR made from AND, OR and NOT, and a circuit that adds 1 + 1.
• Build your own circuit with up to 6 gates and watch its truth table fill in, then save it as a block.
• Solve challenges: name a mystery gate, find the wrong gate, hit a target table, and make every gate from NAND gates only.
• Turn on Guess first to predict ON or OFF before the lamp shows it.

Steps in this lab: 1 Meet the gates · 2 Combine · 3 Build · 4 Challenges

What is a logic gate?

A logic gate is a small circuit that follows one fixed rule to turn its inputs into an output. Every input and output is either 1 (on, high voltage) or 0 (off, low voltage), so a gate works with yes-or-no answers.

Real gates are built from transistors, tiny switches on a chip. (source: Dive into Systems, 5.3 Logic Gates) A modern chip holds billions of them: Apple’s M3 chip has 25 billion transistors. (source: Apple Newsroom, 30 Oct 2023)

You already know the idea from daily life. A microwave runs only when the door is closed and Start is pressed; that rule is an AND gate. In the lab you can tap the door and the button and watch the same rule in a circuit and a truth table.

What is a truth table?

A truth table is a chart that lists every possible set of inputs and the output a gate or circuit gives for each one.

Each new input doubles the rows. A two-input gate has 4 rows (00, 01, 10, 11) and a three-input gate has 8 rows. (source: Lessons In Electric Circuits, Vol. IV, ch. 3 (Kuphaldt)) In the lab, tap any row of the table to set the switches to that row.

The rows always go in counting order, like binary numbers: 00, 01, 10, 11. That way you never miss a case. To see how binary counting works, try the Binary Numbers Lab; to turn words into 0s and 1s, try the Binary Translator.

Which logic gate does what? (all 7 compared)

A small circle on a symbol always means NOT. That is why NAND, NOR and XNOR look like AND, OR and XOR with a circle at the output. In the lab, tap that circle to switch between the pairs.

How do logic gates work?

A logic gate works like a group of switches that decide whether current reaches the output. Two switches in a row (series) make AND: the lamp lights only if both are closed. Two switches side by side (parallel) make OR: either one lights it. You can try both kinds of wiring in the series and parallel circuits lab.

Inside a chip, the switches are transistors, so a gate needs no moving parts. (source: Dive into Systems, 5.3 Logic Gates) The math behind it is Boolean algebra, which works with just two symbols, 1 and 0. Claude Shannon showed in his MIT master’s thesis that this algebra can describe switching circuits; it became the base of digital circuit design. (source: MIT News, 2001)

How do you combine logic gates? (worked example: a home alarm)

You combine logic gates by wiring the output of one gate into an input of the next, so the circuit follows a longer rule.

A home alarm should sound when it is armed (A) AND the door (B) OR the window (C) is open. That rule needs two gates: Alarm = A AND (B OR C). Work it out one gate at a time:

1. Find the middle wire first: B OR C is 1 if the door or the window is open.
2. Then the AND gate: the alarm is 1 only if A is 1 and the middle wire is 1.
3. Three inputs give 8 rows, so check all 8. (source: Lessons In Electric Circuits, Vol. IV, ch. 3 (Kuphaldt))

Result: the alarm sounds in 3 of the 8 rows, all of them with A = 1. Disarmed (A = 0), it stays quiet whatever the door does. Open the home alarm in the lab → The same step also shows XOR built from AND, OR and NOT, and two gates that add 1 + 1 = 10 in binary.

How can you build your own logic circuit?

A logic circuit is a set of gates wired so that switches go in on the left and lamps come out on the right.

In the Build step you get 2 or 3 switches, up to 6 gates in 3 columns and 1 or 2 lamps. Tap a gate, tap a free place, then tap an output dot and an input dot to draw a wire. Wires always run left to right, so a loop cannot form. An input with no wire shows “not connected”, never a silent 0. The truth table of your circuit fills in as you wire, and you can save a finished circuit as a block.

Start building in the lab →

What are the universal gates?

The universal gates are NAND and NOR: with enough of either one alone, you can build every other gate.

“NAND and NOR gates possess a special property: they are universal.”

With NAND gates only, NOT takes 1 gate (feed the same input into both inputs), AND takes 2, OR takes 3 and XOR takes 4. A NAND gate by itself is enough to make any logic circuit. (source: Dive into Systems, 5.3 Logic Gates)

Try the NAND-only challenge →

How can you test what you know about logic gates?

A logic-gate challenge asks you to work backward from a truth table to the gates that make it.

• Mystery gate: test a closed box with two switches; there are 4 rows to try before you name the gate.
• Find the wrong gate: one gate in a circuit is wrong; swap it until every row matches.
• Hit this table: build a circuit for a given table, up to a 3-input “majority vote”.
• NAND only: make NOT, AND, OR and XOR from NAND gates.

Every answer is checked against the full truth table, so any correct circuit counts. Your done marks stay on this device. Open the challenges →

❓ FAQ

Teacher notes

Level: grades 6–8 first (Steps 1–2), grades 9–12 for Build, Challenges and “Go deeper”. Time: one 45-minute lesson, or two with the challenges.

1. Hook (5 min): project the microwave (full screen). Ask: “What has to be true for it to run?” Flip the door and Start. Then tap the little circle on the output and ask why every row of the table flipped.
2. Meet the gates (15 min): turn on Guess first. For each gate, students call ON or OFF before you reveal the lamp; the lab counts right answers.
3. Combine (10 min): home alarm. Students fill in the 8-row table on the worksheet (For teachers → Print worksheet makes it from the board, with the answer key on page 2).
4. Exit task (10 min): the mystery gate challenge, or “Give this to the class” to share a challenge by link or QR code.

Common mix-ups: OR is not “one or the other but not both” (that is XOR); the small circle means NOT; a wire that is not connected is unknown, not 0; rows of a truth table go in binary counting order.

Standards: in the CSTA K–12 Computer Science Standards (2017), the one standard that names logic is 3B-CS-02 (grades 11–12): “Illustrate ways computing systems implement logic, input, and output through hardware components.” (source: CSTA K–12 Computer Science Standards, revised 2017) For grades 6–8 the lab is enrichment that builds toward it.

Sources

• Tony R. Kuphaldt, Lessons In Electric Circuits, Volume IV – Digital, chapter 3: Logic Gates (truth tables, gate rules, gate universality).
• Matthews, Newhall and Webb, Dive into Systems, 5.3 Logic Gates (gates made of transistors; AND, OR, NOT and NAND as complete sets).
• MIT News (2001): Professor Emeritus Claude Shannon, founder of digital communications, dies at 84 (Boolean algebra and switching circuits).
• Apple Newsroom (30 Oct 2023): Apple unveils M3, M3 Pro, and M3 Max (25 billion transistors in the M3 chip).
• CSTA K–12 Computer Science Standards, revised 2017 (standard 3B-CS-02).

Next lab: chain these gates into a 4-bit calculator in the Full Adder Simulator. Related labs: series and parallel circuits, qubits and the Bloch sphere, binary numbers, the binary translator and binary search.