The Big One

The48ErrCar: ESP32-S3 RC Car Controller Board

A custom-designed PCB that turns an RC car chassis into a Wi-Fi-controlled, sensor-equipped vehicle, driven from a mobile app built in MIT App Inventor.

I designed the board from scratch in KiCad, wrote the firmware in C (ESP-IDF) with the help of AI, hand-soldered every component, and built the control app. This was my first PCB project, so this repository also documents what went wrong and what I learned from it.

Highlights

• Custom 2-layer PCB designed in KiCad, with an ESP32-S3-WROOM-1 at its core
• Two buck converters from a 7.4 V battery pack (2x 18650 in series): one to 3.3 V for logic, one to 5 V for peripherals
• Dual-motor-pair drive through a DRV8833 H-bridge, plus servo steering
• Wi-Fi control: the ESP32 hosts its own access point and a small web server, and the phone app talks to it over HTTP
• Native USB-C programming through the ESP32-S3's built-in USB controller, so no USB-to-serial chip is needed
• Hand-assembled with a soldering iron and heat gun (SMD, 0805/1206 passives, SOIC and HTSSOP packages)

Features

System overview

                    2x 18650 (≅7.4 V)
                           |
                   [3A PTC polyfuse]
                           |
          +----------------+----------------+
          |                |                |
   Buck -> 3.3 V     Buck -> 5 V        Motor supply
   (TPS54331)        (TPS54331)           (7.4 V)
          |                |                |
   ESP32-S3, DHT11,   Servo, pump,       DRV8833
   mic, camera LDO    buzzer, US sensor   -> L / R motor pairs
          |
   ESP32-S3 (Wi-Fi AP + HTTP server)
          ^
          | HTTP over Wi-Fi
          |
   MIT App Inventor app (phone)

Hardware

Power

• Battery: 7.4 V (2x Li-Ion 18650 in series), protected by a 3 A resettable PTC polyfuse.
• 3.3 V rail: TPS54331 buck converter, 10 µH inductor, 22 µF output capacitor. Feedback divider 31.6 kΩ / 10 kΩ gives about 3.33 V. Soft-start, bootstrap and compensation networks are included.
• 5 V rail: a second TPS54331 stage (52.3 kΩ / 10 kΩ divider) for the servo, pump, buzzer and ultrasonic sensor.
• Why buck converters instead of linear regulators: they step the voltage down by fast switching rather than burning the difference as heat, which matters when running from a battery.
• Test points on +7.4 V, +5 V, +3.3 V and GND for easy debugging.

Microcontroller

• ESP32-S3-WROOM-1 (integrated antenna and matching network)
• Reset/boot circuitry: RC delay on EN, pull-ups, and two buttons for reset and download mode
• Local decoupling (22 µF + 100 nF) placed directly beside the module
• USB-C connector wired straight to the S3's native USB D+/D- (CC pull-downs of 5.1 kΩ)

Motor driver and steering

• DRV8833 dual H-bridge drives the left-side and right-side motor pairs (connectors J5 / J4)
• Bulk capacitor on the motor supply to absorb current spikes when starting or stalling
• Current-sense pins tied to ground (full current)
• Servo signal on a 3-pin header

Switched loads

Water pump, buzzer and four lights (front, rear, left and right signals) each use a logic-level N-MOSFET low-side switch with:

• a 10 kΩ pull-down on every gate, so nothing switches on randomly while the ESP32 is booting
• a Schottky flyback diode on inductive loads (pump, buzzer)
• 220 Ω current-limiting resistors on the LEDs

Connectors

Sensors, motors, servo, microphone and lights connect through pin headers and jumper wires rather than being soldered to the board, so they can be mounted wherever they fit on the chassis.

PCB layout	3D render

Firmware

Written in C with ESP-IDF and FreeRTOS (main.c).

• Starts a Wi-Fi access point and an HTTP server on the ESP32 (default address 192.168.4.1)
• Control endpoint /cmd?c=... handles lights, signals, horn, pump, microphone mode and drive commands; /dht returns temperature and humidity
• Drive and steering use LEDC PWM channels
• Dedicated FreeRTOS tasks run in parallel for:
  • turn signals / rear lights
  • reverse safety, using an ultrasonic distance check that stops the motors at 5 cm or closer
  • clap detection, sampling the microphone through the ADC and counting volume spikes
• DHT11 is read with a custom bit-banged driver
• MJPEG camera stream server on /stream (disabled in practice because the camera hardware does not work, see below)

Security note: the access point name and password are defined in main.c. Change them before using the car anywhere other than your desk.

Mobile app

Built in MIT App Inventor (block-based). The app:

• shows an intro screen and checks the connection by pinging the car (/cmd?c=ping)
• enables the Start button only once the car answers, and shows "Connect to Wi-Fi!" if it times out
• provides the control interface for driving, lights, signals, horn, pump, temperature and microphone mode
• includes a credits screen with links to my Instagram, LinkedIn and GitHub

Start	Credits

The App Inventor block screenshots are in MIT/.

Design revisions

V1 (fabricated and assembled)

• Built and soldered by hand. Most of the board works, but it has the issues described below.

V2 (current layout in this repository)

• Corrected the MOSFET footprints (the V1 pinout was wrong, see below)
• Added a 1.5 V LDO (AMS1117-1.5) with 2x 10 µF capacitors for the camera supply
• Added 4.7 kΩ I²C pull-ups on the camera SDA/SCL lines
• Reworked routing around the camera connector

V3 (through-hole version)

• A through-hole variant of the board for easier hand assembly, built after the SMD experience

Schematic	PCB layout	3D render

Known issues

This was my first PCB, and I would rather document the mistakes honestly than hide them:

1. MOSFET pinout reversed. I used an N-MOS symbol and footprint with a gate-source-drain order that doesn't match the actual SOT-23 parts. On the fabricated V1 board, the six transistors had to be fitted upside down on the back of the board to work. Fixed in V2.
2. Camera does not work. I designed for an OV2640, but the module I received from AliExpress turned out to be an OV3660. The pinout is very similar, so the firmware targets the OV3660. I didn't realise the 24-pin FPC connector is numbered 1 to 24 and wired my pin mapping in reverse. I tried a reversing workaround, but the camera never came up. The cause may also be a damaged connector or poor pad contact after rework. I also initially forgot the 1.5 V supply that the camera needs, which V2 adds.
3. Microphone is a spike detector, not an audio input. It only measures volume peaks, which is enough for clap detection but not for recording sound.
4. Layout is not optimal. The routing was rushed. The V2 layout cleans it up, but it has not been fabricated.

What I learned

• Always check pin numbering and orientation against the datasheet and footprint, and verify symbol pin order, not just names
• Add pull-down resistors on MOSFET gates and understand why
• How buck converters work: feedback dividers, soft-start, bootstrap, compensation, inductor and diode placement
• The value of decoupling capacitors and keeping switching components away from the antenna
• Hand-soldering SMD parts with an iron and heat gun is hard, and rewarding
• Firmware design on ESP-IDF with FreeRTOS tasks

Bill of materials (summary)

Off-board: DC motors, servo, DHT11, HC-SR04-style ultrasonic sensor, microphone module, water pump, LEDs, 2x 18650 cells.

Repository structure

.
|-- docs/      images, gifs, documentation
|-- Through-hole/     KiCad project, schematic, layout V2
|-- MIT/           MIT App Inventor project pictures, videos, logo
|-- The48ErrCar.apk      Apk to download the app
|-- firmware.c/          ESP-32 Code
|-- hardware.c/          KiCad project, schematic, layout

Assembly

The board was assembled entirely by hand with a soldering iron and a heat gun.

Status

The board, firmware and app work together. The car was not assembled onto a chassis: my focus was the PCB design, schematic and firmware, so mounting is left as a DIY step.

Author

Iustin George: GitHub · LinkedIn · Instagram