A gas sensor simulator: an MQ-2 leak alarm on an Arduino

Yes. The circuit on this page, every part in it and the editor it opens in are on the free plan, with no card.

No. The circuit runs the moment the page does, and the editor opens without one. A free account is only for saving a project to the cloud and sharing it by link.

Its heater is still cold. The reading starts high and falls as it warms, which takes about 20 seconds here. Wait before you trust it, as the sketch in this circuit does.

It is a voltage that rises with gas concentration, read as a count from 0 to 1023. In this circuit clean air reads 11, 800 ppm reads 109 and 5000 ppm reads 226. It is not ppm; turning it into ppm takes the datasheet curve and a calibration.

Use two thresholds. This sketch starts the alarm at a reading of 100 and only clears it under 80, so a reading that wobbles around 100 cannot toggle the buzzer.

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.

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.

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.

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.

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.

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Yes, in one press from your account page. Your plan runs to the end of what you paid for.

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.

Yes. Students get Pro at half price with a school email.

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.

Yes. Everything runs on the machine in front of the student, so a Chromebook works as well as a lab PC.

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.

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.

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.

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.

No. It proposes a change as a card. Accept is yours, and only appears once the changed code compiles; Ctrl+Z undoes it.

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.