An op-amp simulator with a scope on the output
Mokxi is a free op-amp simulator that runs in your browser, with nothing to install and no account needed. Wire an op-amp on a real breadboard, drive it from a signal generator and watch the output on a two-channel scope. Pick a 741, a TL072, an LM358 or a rail-to-rail part, and it has the gain-bandwidth, slew rate and output swing from its datasheet, so it clips and rolls off where the real one does.
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What the running circuit above shows
Gain from two resistors, and the supply that limits it.
A signal generator puts a 1 kHz sine on IN+, 0.5 V either side of 0.8 V. The output goes back to IN- through a 20 k resistor, and IN- goes to ground through 10 k, so the gain is 1 + 20 k / 10 k = 3. On the scope the output is the same sine, the right way up, three times as tall: 0.9 V to 3.9 V. The engine gives 0.901 V and 3.899 V.
Open it and turn the generator up to 1 V. Three times that asks the output for -0.6 V to 5.4 V, which a single 5 V supply cannot give, and the tops and bottoms of the trace are sliced flat. That is clipping, on the scope, the way it looks on a bench.
Four op-amp models, from their datasheets
Typical-column numbers. Every one can be overridden in the properties panel.
The rail-to-rail part is an idealized op-amp with a 10 MHz gain-bandwidth and a 25 mA output limit, not a part number. The others behave like their datasheets: a 741 on a 5 V supply only swings from 1.5 V to 3.5 V, and an LM358 can reach ground but not the top rail. An input offset, in millivolts, is multiplied by the circuit’s gain along with the signal, and a negative-rail setting lets a textbook split-supply circuit swing below ground.
Frequency response, sweeps and SPICE
More than a scope: a Bode plot, a DC sweep and an ngspice deck.
Analysis, in the editor’s more menu, draws a Bode plot of any circuit: gain in decibels and phase against a log frequency axis, with the -3 dB corner marked. An amplifier’s bandwidth is its gain-bandwidth product over its noise gain. A 741 in a non-inverting gain of 11 is 3 dB down at 90.85 kHz in the engine, against 90.91 kHz for 1 MHz / 11, and as a unity-gain buffer the same part is 3 dB down at 1.000 MHz.
The same sheet sweeps any value you can set (a resistor, a supply, the generator’s offset) and plots a node against it, with a CSV of every point. And Save, then Download SPICE, writes the circuit as an ngspice deck, with each op-amp as a subcircuit carrying its model’s numbers.
What the model leaves out
Said plainly, so a result is never taken for more than it is.
The op-amp is a single-pole macromodel, not a transistor-level device. It has an open-loop gain with one dominant pole, a gain-bandwidth product, a slew rate, headroom short of each rail, an input offset, an output resistance and a current limit. It has no input bias current, no input resistance, no second pole (so no loss of phase margin) and no noise. The inputs draw no current at all.
That is the right model for gain, clipping, active filters and comparators, which is most of what a class or a hobby project builds. For noise or stability margins, run the circuit as a SPICE deck; for a manufacturer’s full model of one chip, use LTspice or ngspice.
Three steps from a blank tab to a running circuit.
Open the editor
It runs in your browser tab. Nothing to install, and you need no account to start.
Your first projectWire it and write the code
Drag parts onto a breadboard, run each wire from pin to pin, and write the sketch beside it.
WiringPress Run
Your sketch compiles in the tab and runs on a model of the chip, with the Serial Monitor beside it.
Run and serial
Open a project and press Run.
Every one runs in the editor exactly as it is drawn, ready to change.
What makers say about Mokxi.
“The AI checker helps check and correct my circuits, and it has way more components than Tinkercad. As a student I really appreciate the tool.”
“I’d been looking for a real online electronic simulator that was intuitive and complete when I found Mokxi. I was able to quickly import a Fritzing file and my Arduino sketch and start testing. Mokxi has all of the makings for the best online Arduino simulator I have been able to find!”
“I’m using Mokxi to develop and simulate an STM32-based four-wheel-drive robot car before building the actual hardware. It allows me to experiment with DC motor control, PWM, RPM measurement using photointerrupters, and different acceleration, braking, and reversing strategies, helping me understand and refine the system before implementing it in real life.”
Questions, answered.
Is it free?
Yes. Every op-amp model, the signal generator, the scope and the frequency response are on the free plan, with no card.
Do I need an account?
No. The circuit runs the moment the page does. An account is only for saving a project to the cloud and sharing it by link.
Can I simulate a 741 op-amp?
Yes. Pick the 741 model and it has the datasheet’s 1 MHz gain-bandwidth, 0.5 V/us slew rate and 1.5 V of headroom from each rail.
Can I build an op-amp with a negative supply?
Yes. Set the op-amp’s negative property to the negative rail in volts, and the output can swing that far below ground, the way a split-supply textbook circuit is drawn.
Does it work on a Chromebook?
Yes. It runs in Chrome with nothing to install.
Do I need an account at all?
No. The editor, the templates, the gallery, the compiler and Run all work without signing in. Your browser keeps your work as you go. An account saves your projects online, so they follow you to any computer. It also gives you a link to share.
Which boards are supported?
Forty-five, all on the free plan. Arduino: the Uno R3, Uno R4 Minima and Uno R4 WiFi, Nano, Nano ESP32, Nano RP2040 Connect, Mega 2560, Leonardo, Micro, the Pro Mini in its 5 V and 3.3 V builds, and the SparkFun Pro Micro. ESP32: the classic DevKit V1, the ESP32-S3, the ESP32-C3 and C6 DevKits, the Espressif DevKitC V4, the Freenove ESP32-WROVER, the Wemos LOLIN32 Lite, the Wemos D1 R32, the Cheap Yellow Display, the TTGO T-Display, the ESP32-C3 SuperMini and the Seeed XIAO ESP32-C3, C6 and S3. ESP8266: the NodeMCU, the Wemos D1 mini and the ESP-01S. Also the Raspberry Pi Pico and Pico W, the Seeed XIAO RP2040, the Waveshare RP2040-Zero, the Seeed XIAO SAMD21, the WCH CH32V003, the STM32F411 Black Pill, the STM32 Nucleo-F411RE and the STM32F103 Blue Pill, the BBC micro:bit V2, and the ATtiny85 and the ATmega328P in its 16 MHz and 8 MHz builds, which plug straight into the breadboard as bare chips. Your own code compiles in the browser for all of them. Then the Raspberry Pi 4, Pi 5 and Zero 2 W computers, which run your Python with gpiozero and RPi.GPIO, with nothing to compile. Each board page lists what it can and can’t do. For example, the WiFi boards have pretend WiFi, not a real radio, and the page says so.
Is my sketch really compiled in the browser?
Yes. A real compiler (clang, built for the web) runs in your browser. It downloads once, and your browser keeps a copy. After that, every build happens on your own computer. There is no build server and no waiting in line, so there are no build minutes to run out of.
Which browsers work?
Any up-to-date Chrome, Edge, Firefox or Safari, on a computer, a phone or a tablet. They all support WebAssembly, which Mokxi needs, and they have for years.
Does it work on a phone?
Yes. The editor works with touch, the buttons are big enough to tap, and the layout fits a small screen. The simulation runs on the phone itself.
Can I try Pro before I pay?
Yes. Every account can try Pro free for 7 days, once. It takes a card, nothing is charged for those days, and if you cancel before they end you pay nothing. It starts when you choose it.
Can I cancel anytime?
Yes, in one press from your account page. Your plan runs to the end of what you paid for.
Do you offer refunds?
Yes, once per account. Write to us within 14 days of a payment and we refund all of it. Lifetime Pro is refunded in full within 14 days of buying it.
Is there a student discount?
Yes. Students get Pro at half price with a school email.
Do students need accounts?
No. The simulator works signed out, so your class starts from one link with nothing to sign up for. An account only adds cloud saving and sharing, and a student can make one later, or never.
Does it work on a Chromebook?
Yes. Everything runs on the machine in front of the student, so a Chromebook works as well as a lab PC.
What does it cost for a class?
Nothing, if your students only need to open and run circuits: the whole simulator is on the free plan. The class tools (a roster, starter projects, auto-graded checks, the gradebook) come with Pro and Classroom. Classroom is $9.99 a student a year, from 5 students, so at least $49.95 for the year. Your own seat is free, and from 50 students each one costs less. Your students get Pro’s room while they are in your classes. Pay by card, invoice or purchase order. Pro on its own runs classes for up to 10 students in all. To try the class tools with real students first, start the free 30-day trial from your class page: one class of up to 20 students, no card.
Is student work private?
Yes. A student’s projects in a class are always private: they cannot be opened by link or put in the gallery. One exception, and only for reading: a teacher may open, and never write, a project owned by somebody in a class they teach, which is what a card on the class wall does. What we store about a student, who processes it, how a school has it deleted, and how we handle COPPA consent for students under 13 and FERPA as a school official are all on the student data privacy page at /privacy/students.
Is Mokxi AI free?
Every signed-in account gets questions every day: 5 on a free account, 50 on the Pro trial, 100 on Pro and the other paid plans, and 25 on Lifetime Pro.
Does Mokxi AI work with ESP32 and Raspberry Pi Pico sketches?
Yes. It knows which board the project is on and answers for that board, from an Uno to an ESP32, an ESP8266, a Pico or an STM32.
Can Mokxi AI change my code without asking?
No. It proposes a change as a card. Accept is yours, and only appears once the changed code compiles; Ctrl+Z undoes it.
What does Mokxi AI send with a question?
Your project text, the matching help articles and the last six messages of the chat, never your name or account. "What is sent" shows exactly what went.
Latest arrivals in Mokxi.
More simulators
Each one opens on a circuit that is already running.
Build an amplifier and scope it
Open the editor, drop a part on the breadboard and press Run. Nothing to install, and no account needed.