SmartPi Hardware Design & Development

SmartPi Embedded Controller Design

The SmartPi Assistant is an embedded interactive device built around the Raspberry Pi Pico W microcontroller. It integrates a LED matrix panel as the primary visual display, a 3–4 Ω speaker for audio output, and an Adafruit MAX98357 amplifier to drive the speaker with clear sound. The design also accommodates an Adafruit ICS-43434 digital I²S MEMS microphone for capturing voice commands, enabling natural, voice-based interaction. Together, these components form a compact, low-power system capable of displaying dynamic information, responding to spoken inputs, and communicating wirelessly through the Pico W's built-in Wi-Fi—making it a versatile platform for experimenting with edge AI, local sensing, and responsive embedded control.

Why Raspberry Pi Pico W?

The Raspberry Pi Pico W is an ideal choice for the SmartPi Embedded Controller because it combines affordability, performance, and built-in wireless connectivity in a compact form factor. Powered by the RP2040 dual-core ARM Cortex-M0+ processor, it provides sufficient computational power for real-time sensor data acquisition, control logic, and lightweight edge AI tasks, while maintaining low power consumption—critical for continuous operation in field environments. Its Wi-Fi capability enables seamless integration with cloud or local networks for data exchange and remote updates, eliminating the need for external modules. Additionally, the Pico W's rich GPIO interface, extensive community support, and compatibility with MicroPython and C/C++ SDKs make it both versatile and developer-friendly, ensuring rapid prototyping and reliable deployment.

Display Interface: HUB75 LED Matrix Connection

The HUB75 (2×8) connector serves as the standard interface between the Raspberry Pi Pico W and the 64×64 RGB LED matrix panel. It provides a set of 16 data and control lines, including color data inputs (R, G, B), row address lines, latch, clock, and output enable signals, which together allow precise control of the timing and brightness of individual LEDs. The HUB75 interface does not carry any intelligence—it simply maps the Pico's GPIO outputs directly to the LED driver circuitry on the panel. Because the display requires rapid and synchronized data updates to render smooth animations and text, the HUB75 connector offers a convenient and electrically robust way to handle high-speed parallel communication. Using this standardized interface simplifies wiring, ensures compatibility with a wide range of LED panels, and allows the SmartPi Assistant to produce dynamic visual feedback efficiently from the Pico W.

Hardware Implementation: First Prototype

1. I procured a 6RGB full-color LED matrix panel, with 64x64 pixels and 2mm Pitch from Waveshare.com. These are modular pieces that can be daisy chained together to create larger LED matrices. I liked the idea of modular expandable components.

2. I designed a PCB board to interface the LED matrix panel with Raspberry Pi Pico W using the HUB75 standard socket
3. Created the traces in the layout view

There were some crossover issues after the manual trace layout, so I dealt with a couple of crossovers using zero ohm resistors as bridges. There were 3 more crossovers to handle. Instead of opting for a 2-sided board (and the more involved milling process), I discussed with Anthony and came up with making 3 holes through which we'd run wires to connect the HUB75 CLK, LAT, and OE pins from the back of the board, using header pins on the Raspberry Pi Pico.

Added a JST connector for power and a switch to turn the device on/off.

PCB Fabrication Process

4. Created an Eagle 9.x compatible board file (.brd extension) from AutoCAD Fusion
5. Loaded the .brd file into the Bantam software connected to the OtherMill milling machine
6. Taped a single-sided board using double-sided tape, ensuring no overlaps (which would mess up the milling if the board is uneven)
7. Loaded the 1/64" mill bit and started milling the .brd file

8. Clicked "Mill All Visible" with the 1/64" mill bit to mill the finer portions
9. After fine milling, loaded the 1/32" larger mill bit for fast milling of larger areas
10. After completing all traces, used the 1/32" mill bit to drill holes and cut the outline
11. After all this, discovered the board was incomplete—missing the right side Raspberry Pi Pico pins, power points, and the course name text!

11. Anthony exported a .gerber file and loaded its constituent parts for the trace, holes, and outline, and restarted the job
12. This time the milling machine completed the entire board with all pins, traces, holes, outline, and importantly the text with the course and my name on it (except for the JST power pins, which was no big deal)

Assembly and Soldering

13. Started soldering the Raspberry Pi Pico W onto the newly milled PCB. The Adonstar digital magnifier was super useful for precise placement, ensuring the tiny copper pads were centered correctly.

14. Cleaned my soldering iron with a small amount of solder to get good flow, then put a blob of solder on the bottom left pin. Holding the RP Pico W down, I applied the soldering iron and got one pin securely fastened. After that, it was just a matter of completing the rest.

The digital magnifier was not actually useful for soldering itself—it helped more to inspect the solders and ensure there were no bridges between pins.

15. Proceeded to solder the zero ohm resistors, which were super tiny and difficult to keep in place. They would move as the soldering iron approached, getting soldered at weird angles. Heating the trace pad again and ever so slightly moving them back into place was challenging. Got all 3 zero-ohm resistors soldered to bridge underlying traces and tested connectivity—all was fine.
16. Finally soldered the diode from the external 5V Vcc point to VSYS (pin 39), ensuring the tiny (barely visible) arrow mark was in the correct direction. Added the 3 wires needed to avoid trace crossover challenges.

Lessons Learned: PCB Design Mistakes

Despite careful planning, several critical mistakes emerged during the PCB fabrication and assembly process. These errors provided valuable learning experiences about PCB design considerations.

Derisking the Project with Adapter Board

I needed to go to India for 10 days for a meeting and event. I knew the PCB still had power connections and switch to be connected and the HUB75 pins to be tested. It was unlikely all this was going to work in a day. There was also learning the software environment and developing software to test the LED matrix board.

I decided to de-risk the project by using a pre-fabricated RGB Matrix Adapter Board, which I had bought when I ordered the Matrix Board itself. This was a life saver.

1. Installed header pins to the Raspberry Pi Pico so it could fit into the adapter board

2. Pushed the RP Pico W with header pins into the adapter board and connected the HUB75 ribbon connector and power supply JST connector

3. Connected the adapter card to the RGB Matrix
4. Connected the micro USB cable to the Raspberry Pi Pico and the other end to my MacBook

Setting up CircuitPython Environment

I decided to use CircuitPython on the RP Pico with the Mu Editor. Setup instructions:

1. Downloaded the CircuitPython UF2 file
2. Held down the BOOTSEL button, then plugged the Pico into a USB port
3. Released the BOOTSEL button after connecting. It installed as "RPI-RP2"
4. Dragged and dropped the CircuitPython UF2 file onto "RPI-RP2" volume
5. The Pico rebooted, a new disk drive "CIRCUITPY" appeared—done flashing

Initial Test Program

import board
import displayio
import framebufferio
import rgbmatrix
import time

displayio.release_displays()

# HUB75 + Seengreat Pico adapter pin map
rgb_pins = [board.GP2, board.GP3, board.GP4,
            board.GP5, board.GP8, board.GP9]
addr_pins = [board.GP10, board.GP16, board.GP18, board.GP20, board.GP22]
clock_pin = board.GP11
latch_pin = board.GP12
oe_pin = board.GP13

WIDTH = 64
HEIGHT = 64

matrix = rgbmatrix.RGBMatrix(
    width=WIDTH, height=HEIGHT, bit_depth=5,
    rgb_pins=rgb_pins, addr_pins=addr_pins,
    clock_pin=clock_pin, latch_pin=latch_pin,
    output_enable_pin=oe_pin,
    tile=1, serpentine=False, doublebuffer=True
)

display = framebufferio.FramebufferDisplay(matrix, auto_refresh=True)

palette = displayio.Palette(1)
bitmap = displayio.Bitmap(WIDTH, HEIGHT, 1)
tg = displayio.TileGrid(bitmap, pixel_shader=palette)
group = displayio.Group()
group.append(tg)
display.root_group = group

def fill(rgb_hex):
    palette[0] = rgb_hex
    time.sleep(0.5)

while True:
    fill(0xFF0000)  # red
    fill(0x00FF00)  # green
    fill(0x0000FF)  # blue
    fill(0xFFFFFF)  # white
    fill(0x000000)  # black

The test program worked!

Adding Voice Interaction Capabilities

Following insights from the MAS.863 classes on input and output devices, I recognized an opportunity to enhance the SmartPi by adding bidirectional audio capabilities. While the initial design focused solely on visual output through the LED matrix, the addition of audio I/O transforms the device into a truly interactive assistant.

This enhancement enables:

• Audio Output (Speaker): Vocalize displayed messages for situations where visual attention isn't possible
• Audio Input (Microphone): Voice input shifts SmartPi from passive display to interactive assistant supporting a "pull" model

I chose to implement a fully digital audio path using the I²S (Inter-IC Sound) protocol. This eliminates the need for external ADC/DAC converters, reduces noise, simplifies PCB routing, and ensures bit-perfect audio transmission.

Audio Output: Adafruit MAX98357 I²S Amplifier

The Adafruit MAX98357 is a digital-to-analog Class-D audio amplifier that converts I²S digital audio into high-quality analog sound capable of driving a speaker directly. The Raspberry Pi Pico W lacks both a DAC and sufficient output power for a low-impedance speaker. The MAX98357 fills this gap by delivering up to 3 watts of power to a 3–4 Ω speaker.

MAX98357 Connection to Raspberry Pi Pico W

Audio Input: Adafruit ICS-43434 I²S MEMS Microphone

The Adafruit ICS-43434 is a digital I²S MEMS microphone that captures high-quality audio and outputs it as a clean, noise-free digital signal. Unlike analog microphones requiring external amplification, the ICS-43434 performs analog-to-digital conversion internally and transmits audio data directly to the Pico W over I²S.

ICS-43434 Connection to Raspberry Pi Pico W

PCB Version 2: Integrated Audio Design

Building upon lessons from the initial PCB, I started work on an improved version integrating audio capabilities directly into the board. This second iteration incorporates both the speaker amplifier and microphone interfaces alongside the HUB75 LED matrix connection.

Component Integration

• MAX98357 I²S Amplifier: Integrated onto the board, connected to GPIO ports 16–20
• ICS-43434 I²S MEMS Microphone: Added, connected to GPIO ports 22, 26, 27, and 28

Two-Sided PCB Design

The routing complexity increased significantly with audio components. A two-sided PCB board was necessary. The final layout uses three vias to route HUB75 connections to the bottom layer.

Design Rules and Improvements

• Standard trace width: Increased from 16 mil to 20 mil
• Power traces (VBUS/VDD): Widened to 40 mil
• 3.3V power traces: Increased to 30 mil
• Trace spacing: Expanded inter-trace clearance

Milling and Assembling PCB V2

Assembly Process

• Raspberry Pi Pico W: Soldered all 40 pins using wider 20 mil traces
• MAX98357 DAC Amplifier: Soldered header pins for easy replacement
• ICS-43434 Digital Microphone: Soldered header pins for flexibility
• Via Riveting: Installed rivets in three via holes and soldered both sides
• HUB75 Interface: Soldered 16-pin connector socket
• Schottky Diode: Added for reverse current protection
• Parallel Power Connector: Added for LED Matrix high-current requirements

PCB Version 3: Addressing Design Issues

After assembling and testing PCB V2, several design issues became apparent during daily use and testing.

Key Takeaways for PCB V3

• Secure all power connections with proper connectors
• Place Pico at board edge for USB access
• Source actual HUB75 connector with mechanical keying
• Double-check schematic connections before milling
• Route control pins to GPIOs
• Test under real-world conditions

PCB V3: Optimized Trace Routing

After carefully analyzing PCB V2 trace routes, I realized that by leveraging physical pin layouts and adjusting component locations strategically, I could achieve much better trace routing that eliminates crisscrossing traces and potentially eliminates the need for a two-sided PCB.

The key insight: PCB routing is about understanding component pin geometry and placing components where their natural pin arrangement aligns with signals.

Complete Pin Mapping: Pico W to HUB75

PCB V3 Milling Process

Key Improvements in V3 Design

• Single-sided routing: All traces on one side, no vias or bridges needed
• Pico at top edge: Easy USB access and natural routing channels
• Grouped by function: Components grouped by subsystem
• Proper connectors: Actual HUB75 connector for keying
• Shorter power traces: More efficient power distribution
• Signal integrity: Cleaner, more direct paths for critical signals

PCB Design Evolution: Learning Through Iteration

The journey from initial concept to final design demonstrates the iterative nature of hardware development. Each PCB version addressed specific issues discovered in the previous iteration, incorporating lessons about power management, component placement, trace routing, and connector design.

PCB V3: Successfully Assembled and Tested

After addressing all the design issues from previous iterations, PCB V3 was successfully fabricated, assembled, and tested. The improved layout with single-sided routing, proper component placement, and robust power connections proved to be highly reliable.

✅ All Systems Operational: Display, Audio Output, and Wireless Connectivity Working Flawlessly