It was 2003 when I started college with great enthusiasm and an old Casio calculator from high school that I ended up replacing shortly after with a Texas Instruments TI-86 graphing calculator. At that time it cost me 150 euros, but I spent it because there was no other option and it was going to make my life easier with graphs and matrices. My old TI-86 is already a relic, but those who start engineering this 2026 will spend at least those 150 euros on a more current model like this either this other one from HP.
They have a slightly more modern aesthetic and a color screen, but the essence and prices have barely changed. So to a young developer from Almería an idea has occurred to him: build a professional level scientific and graphing calculator for about 20 euros using open source software. And as its creator, Juan Ramón (alias El-EnderJ), explains, at 15 years old he still doesn’t need it: “I did it simply because of that great injustice.”
A DIY calculator with open source. The project is barely a couple of months old and its premise could not be more ambitious: NumOS (its operating system) runs on the ESP32-S3 microcontroller and aims to break the monopoly of commercial models that cost around 150 euros.
It is not a mobile app or a website: it is a piece of physical hardware that the user assembles and programs from scratch. Knowing how difficult it is for the education system to accept a DIY calculator for exams, El-EnderJ has in mind a “factory-sealed version that is completely legal.” Disclaimer: the final product will use ESP32 S3 N16R8 and a 3.2″ IPS screen.
Grapher app. Via: GitHub
Why is it important. The educational calculator market is controlled by an oligopoly: Texas Instruments, Casio and HP, with devices whose hardware has not been significantly renewed for decades and a price range that has neither changed much over the years nor differs too much from each other.
But the underlying problem is also one of access: this is the case of fantastic free and quality tools such as GeoGebra and Desmos. As El-EnderJ explains: “To use them you must use a mobile phone, a tablet or a laptop, which is completely prohibited in most classrooms. The educational system requires dedicated devices that do not have an internet connection to avoid cheating.” On the other hand, on a technical level it is notable that NeoCalculator integrates a complete CAS engine within such a low budget as manufacturers such as Casio, HP and TI reserve only their high-end models. And be careful, this engine shows the intermediate steps of derivatives, integrals and solving equations.
The eternal? calculator oligopoly. Juan Ramón says that, encouraged by what he saw people doing with graphing calculators (like programming), he looked up the price and was surprised: “I was shocked when I saw that a calculator from more than 30 years ago cost more than 150 euros. So I looked a little more and realized that the cost of producing them is below 20 euros, so you are paying a 130-euro premium.”
Free software has been democratizing tools that were previously either expensive or exclusive for decades, but in hardware everything has been slower. The clearest precedent is NumWorksthe French calculator founded in 2015 that was the first to completely open its source code and allow anyone to modify its operating system. NeoCalculator goes one step further: not only is the software free, but so is the hardware design.
From Shanghai to Almería: the ESP32-S3-BOX-3 chip from Espressif
How it works. The base is the microcontroller ESP32-S3which according to its official documentation incorporates a dual-core Xtensa LX7 processor capable of running at 240 MHz, with 512 KB of internal SRAM, Wi-Fi and Bluetooth 5 connectivity, as well as support for vector instructions aimed at accelerating neural networks and signal processing. It is a chip designed for IoT converted into the brain of a high-performance calculator. El-EnderJ is critical of what it replaces: “The ESP32-S3 is from 2020; the Zilog Z80 of the TI-84 Plus is from 1976. There is a clear difference.”
The mathematical core of the project is not development from scratch, but sophisticated integration. “The biggest challenge has been putting the Giac engine, which is the same one used by the HP Prime, in a chip that has thousands of times less memory than a computer.” In fact, Giac is an open source symbolic calculation engine originally developed at the University of Grenoble and indeed, it is the engine that equips the HP Prime G2. For the graphical interface, the project uses LVGL, an open source embedded graphics library widely used in the industry. Combining hardware SPI with LVGL, NeoCalculator maintains a smooth interface at 60 FPS, which is a demanding performance target for a microcontroller in this price range.
Yes, but. The incipient project of the Almeria developer has important technical and regulatory limitations. The most important is precisely the connectivity of the ESP32-S3, something strictly prohibited in exam contexts. This implies that in its current state NeoCalculator could not be used in official university exams (not the EBAU, which generally restricts graphic models). On the other hand, this fantastic project is still very green: it lacks an integrated physical keyboard and is still pending receipt. OSHWA certificationessential to ensure transparency, the ability to customize or repair each component of the device.
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Cover | Anoushka Puri and El-EnderJ
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