A simple but interesting experiment: with the use of simple mathematical functions it is possible to generate waveforms such as square, saw and sine wave and still apply a low pass filter on the square wave, as I demonstrate in the video. This application is known as Numerically Controlled Oscillator (NCO), which is the basis of DDS AD9850 for example. Of course, for practical use it would be necessary to use a higher processing power MCU.I am using the IDE connected to Arduino Uno. The graph plot is done by the IDE's serial monitor (serial plotter).

An Arduino Uno is a "digital" beast; its pins only know 5V (HIGH) or 0V (LOW). It does not have a true DAC (Digital-to-Analog Converter). This project forces you to bend the hardware architecture to your will, utilizing a custom-built Resistor Ladder (R-2R network) alongside raw C++ Register manipulation to mathematically forge smooth, undulating acoustic Sine Waves out of pure digital noise.

The Fast-Port Manipulation Trap

You cannot use digitalWrite() to build high-frequency AC audio waves. It requires 50 clock cycles to execute.

1. The R-2R Ladder Code: We build a physical matrix of resistors across exactly 8 identical pins (Pins 0 to 7. This is known collectively as PORTD). By firing specific combinations of pins (e.g., 01101001), the resistors physically combine the voltages to output specific analog voltages!
2. The Output Register: Instead of using 8 slow digitalWrite commands, the program writes directly to the CPU's absolute memory matrix using binary math! PORTD = 0b10000000; // This fires Pin 7 instantly in a single blazing microsecond clock cycle!

Modulating the Sine Matrix (PROGMEM)

A sine wave requires drawing a sweeping curve physically out of electricity.

• You must create a lookup table (#define TABLE_SIZE 256) containing the pre-calculated geometry of a circle mapped to voltage outputs (0 to 255).

const unsigned char sineTable[256] PROGMEM = { 128, 131, 134, 137, 140... }

• In the primary loop(), you use a timer interrupt running at a high frequency.
• Every interrupt, the Arduino fetches the next integer from the PROGMEM table and instantly throws it out PORTD!
• If you hook up an oscilloscope to the output of the resistor ladder, you will see a smooth, mathematically-pure AC Sine Wave curve—something a digital Uno was never fundamentally designed to do!

Advanced Electrical Synthesizer Kit

• Arduino Uno/Nano (The ATmega328P architecture is required for the specific PORTD register manipulation!).
• At least 16 highly accurate resistors (Usually 10k Ohm and 20k Ohm) to build the sprawling R-2R Digital-to-Analog converting ladder array.
• An Oscilloscope (Absolutely necessary to visually prove that your C++ code is generating the correct mathematical curvature).

Below are images of plotted waveforms:

Video:

Connections:

Instructions:

Connect Arduino Uno to PC or Notebook via USB cable, choose one of the functions to be plotted (SAW, SQR, FILTER and SINE), load the sketch in Arduino Uno, go to the Tools menu and open the Serial Plotter.

Enjoy - Julio Cesar - JCR