The final lab of the Spring 2025 semester introduced us to real-time operating systems (RTOS) for embedded systems. We ran a series of Arduino sketches on the ESP32S3 that demonstrate essential RTOS capabilities, such as task scheduling, memory allocation, queuing, and multicore operations. The video below demonstrates the functionality of all 7 exercises, and code screenshots for each exercise can be found after the video.

Scheduling LED blinks at different speeds without using the main 

Prioritization of tasks using non-blocking logic. 

Inter-task communication to avoid stack overflows and task conflicts. 

) to control access to shared resources (e.g., global variables) 

Run tasks on multiple cores of the ESP32 to improve efficiency

This project streams live video over WiFi and simultaneously stores static images on the SD card. I was not able to use the code provided in class, so I combined the camera timer functionality from the code provided in class with an example Arduino sketch (Examples/ESP32/Camera/CameraWebServer). 

Week 14 - Real-Time Operating System (RTOS)

This lab entailed the creation of a motor controller using pulse-width modulation (PWM) to change the speed of a DC-motor.

The circuit includes the XIAO ESP32-S3, a small DC motor, one transistor (TIP120), one diode (IN914), two push buttons, one 4.7kΩ resistor, and two 1kΩ resistors (for the buttons). We also used the XIAO expansion board, which allowed us to power the motor using a dedicated LiPo battery, rather than powering it directly from the microcontroller. Our challenge was to use the push buttons to change the motor speed by adjusting the motor's duty cycle via PWM. 

The defining feature of our code was the use of interrupts to handle changes in fan speed. The interrupt routines were instantiated in the 

 function, and allowed us to control the fan speed asynchronously in response to hardware button presses. 

We had some issues getting the circuit to respond to button presses consistently, and the oscilloscope in the video shows quite a bit of noise in the circuit. This may have been due to defective buttons or loose jumper cables, but you can see the basic functionality of the system in the video below. 

The code used for our demo can be downloaded 

Week 13 - Motor Controller 

In this lab, we used the XIAO ESP32-S3 to create a musical keyboard. It seems fitting to describe this device as a 

, since we use the microcontroller to generate a sine wave and then modulate its characteristics using button presses. 

The key design detail of our circuit is the use of the binary weighted method to create a digital-to-analog converter (DAC), which essentially uses a series of resistors to change the voltage of each output pin. For example, the second output of the DAC uses a resistor with twice the resistance of the first output, and the third and fourth outputs follow a similar sequence. The image below shows our initial circuit, which required a bit of debugging. 

, we were able to generate a sine wave using the device that was visible on the oscilloscope. 

With a semi-functional circuit, our next task required using the device to produce four different tones. We used the 

 library to create periodic timers, which ultimately iterates over a 4-bit sine wave at similar frequencies to the musical notes C, D, E, and G.  

Linto and I each created our own keyboards, although his creates arguably better sounds than mine. I realized after creating the circuit that the resistors in my DAC did not use correct ratios of resistor values for each output, which is probably responsible for some of the dissonant sounds the device generates. 

I did not have a breadboard friendly audio output to test the device's sound generation abilities, so created one using a spare 1/4" jack, female DuPont connectors, and a switch. The switch allows me to toggle between mono and stereo by connecting or disconnecting the left and right inputs. 

The images below show my circuit design from a few different angles. 

The first video below demonstrates the basic functionality of the synthesizer. In the second video, I connected the synthesizer to my guitar rig and experimented with different effects. 

James Cole

Week 12 - DAC Synthesizer

Initial circuit design

Generating audio

Demo

Downloads

More posts

Week 14 - Real-Time Operating System (RTOS)

Week 13 - Motor Controller