TICKER: LED matrix widgets via WiFi with a PocketBeagle

Inspiration

In an age where messages and media are shared instantly and young adults spend hours before screens, retro-style low-resolution displays caught my attention as a medium to stay connected and informed but not overwhelmed and distracted. The company Tidbyt makes an intriguing app-enabled display in this style, though with a high price.

In this project, I design a self-contained LED matrix with WiFi, button inputs, and a temperature and humidity sensor, in addition to an open-source software framework to run multiple widgets and allow users to design their own. The widgets for this project specifically include a simple clock, an indoor temperature and relative humidity monitor, and a Spotify "currently playing" display with album artwork and artist information (shown in the image below and as the cover image for this project). Additional widgets that could be coded in the future include a stock market tracker, a long-distance message display, an outdoor weather forecast, a list of live sports scores, and a quote-of-the-day display, etc.

Video

Hardware

Please note that all images and diagrams shown in this project are available in high resolution at the "ticker" GitHub repository (also see below).

1. Laser Cut Housing

The included "Housing Laser Cut" Adobe Illustrator file (laser_cut.ai) can be used to engrave and cut 5 mm (3/16 in) thick wood or acrylic for the LED matrix and sensor housing. The dimensions for this frame are based on the dimensions of the 4 mm pitch LED matrix from Adafruit, included as the "LED Matrix Dimensions" PDF file (led_matrix_4mm.pdf). For different material thicknesses or pitch sizes, this laser-cut file would need to be modified to suit the new dimensions.

The laser cut settings using 5 mm (3/16 in) thick wood with an Epilog Fusion Pro are shown below:

• Engraving: 600 DPI (resolution), 70.0% speed, 80.0% power
• Cutting (Vector): 15.0% speed, 100.0% power, 10.0% frequency

The housing can be assembled using wood glue or superglue and the LED matrix can be glued or screwed into the frame as shown in the images below. Be careful to note the upright orientation of the display. It is helpful to add the buttons, power port, and power switch before assembling the frame; the 5 user buttons can be screwed or glued to the top frame piece, and the power switch and port can be glued to the back frame piece, as shown in the images below. The back frame of the TICKER device should not be glued to allow access to the internal space for adding or maintaining the electronics (see below).

2. LED Matrix

The wiring for the 64x32 LED matrix from Adafruit is shown in the images below. A table of the specific connections is also included as the "Wiring Chart" PDF file (wiring_chart.pdf). The LED matrix is divided into 2 halves; the upper half pixels are controlled via the R1, G1, and B1 color pins (for red, green, and blue channels), while the lower half pixels are controlled via the R2, G2, and B2 color pins. The A, B, C, D, clock (CLK), and latch (LAT) pins assist with writing colors to specific pixels (rows and columns) on the matrix using shift registers. Lastly, the output enable (OE) pin is used to temporarily turn off all LEDs on the display. This article by Big Mess o' Wires offers a good explanation of the mechanics of writing data to the LED matrix, as well as this more technical article by Glen Atkins.

Logic level shifters need to be used to convert the 3.3 V logic of the PocketBeagle GPIO pins to the 5 V logic of the LED matrix. The wiring for these is also shown in the images below and in the "Wiring Chart" PDF file (wiring_chart.pdf). Overall, each GPIO pin of the PocketBeagle used to drive the LED matrix passes into an "A" column pin on the level shifter board, its signal is converted to 5 V logic and passes out through the corresponding "B" column pin, and that level shifter pin is then wired to the appropriate LED matrix pin.

The LED matrix is powered using the 5V/4A power supply on a separate set of power pins, which should be wired to the power port (female 2.1 mm DC jack) on the housing. The power switch on the housing can be wired in series with the positive LED matrix wire and the power port if desired. Two additional wires should also be connected to the 5 V LED matrix power pins to power the PocketBeagle via the USB adapter (see below).

3. WiFi USB Adapter

The WiFi USB adapter wiring is shown below and in the "Wiring Chart" PDF file (wiring_chart.pdf) included with this project. The breakout module should be connected to both the actual WiFi adapter and the 5 V DC power supply via the 5 V power pins on the LED matrix (see above) as detailed in the included "Wiring Chart" PDF file (wiring_chart.pdf). Ensure that the PocketBeagle USB1 host port is enabled by soldering or connecting the ID pin to the GND pin and the pin VBUS to the VIN pin, as shown below and in the included "PocketBeagle Pinout Diagram" PDF file (pocketbeagle_pinout_diagram.pdf) or "Wiring Chart" PDF file (wiring_chart.pdf).

4. Push Buttons

Each of the 5 momentary push buttons should be wired according to the circuit diagram shown below and as detailed in the "Wiring Chart" PDF file (wiring_chart.pdf). It is most efficient to do this on a single breadboard using 3.3 V and GND voltage rails. 1k to 10k Ohm resistors can be used.

5. AHT10 Temperature/Humidity Sensor

The AHT10 temperature/humidity sensor uses the I2C protocol for digital communication. The sensor should be wired to I2C-supported pins on the PocketBeagle (PB) as shown in the circuit diagram below and as detailed in the "Wiring Chart" PDF file (wiring_chart.pdf). Two 1k Ohm resistors must be used to pull up the SDA and SCL lines. The sensor can be glued to the housing of the device in a place with good ventilation to get an accurate reading.

Final Hardware

The final device hardware is shown and annotated in the images below. Zip ties and ribbon jumper cables can be used to keep the wiring less messy. The breadboards were glued directly to supports on the back of the LED matrix panel. As previously stated, all images and diagrams shown in this project are available in high resolution at the "ticker" GitHub repository (also see below).

Software

Please see the linked "ticker" GitHub repository (also see below) for all of the software files as well as installation, setup, and running instructions and acknowledgements.

Operation

Assuming the hardware and software have been set up according to the documentation, the PocketBeagle should power on when the device is powered on. The PocketBeagle startup process takes about 90 seconds. After the device successfully connects to a pre-defined network (this should happen automatically), the TICKER application can be run either via the Cloud9 terminal with "sudo python3..." or on boot using "crontab"—see the software documentation—to illuminate the LED matrix.

Users can directly interact with the display via the 5 push buttons:

• The "<" button cycles backward to the previous widget.
• The "•" button performs an action for the current widget if the current widget supports an action input. For the provided Spotify "currently playing" widget, when pressed one time, this button pauses/plays the current track; when pressed two times, this button skips to the next queued track; when pressed three times, this button skips to the previous track.
• The ">" button cycles forward to the next widget.
• The "–" button reduces the brightness of the LED matrix.
• The "+" button increases the brightness of the LED matrix.

Because the device is WiFi-enabled, users can also interact with the display through software APIs. For example, with the Spotify "currently playing" widget, the display will automatically switch between tracks if the user changes tracks on their local device (i.e., phone or computer).

The entire device can be shut off without issue by powering down the system (i.e., via the power switch or power cable).

Challenges

There were numerous challenges in running this 64x32 LED matrix using a PocketBeagle. This matrix operates by rapidly refreshing the screen with a new image to display; to do this without egregious latency, the PocketBeagle's PRU (programmable real-time unit) was employed to allow the computer to simultaneously update the display, handle user inputs, and retrieve new information (i.e., from the internet). Without using the PRU, the display flashes a mess of colors. The display driver can still be further optimized to reduce the "ghosting" effect that occurs on some images in the current setup.

Furthermore, the 5 V logic used by the display necessitated the use of 2 logic voltage shifting chips from the 3.3 V logic from the PocketBeagle to 5 V. While this improved the performance of the display (i.e., reduced flickering), it complicated the internal wiring.

The integration of the buttons, various chips, and software into a standalone housing was probably the greatest challenge of this project. Handling the large number of specific pin connections to the level shifters and PocketBeagle, especially within the limited space of the housing, was particularly difficult. In addition, extensive software testing was performed to ensure that the device would be robust enough to handle unexpected user inputs without intervention to make the system wireless (save for the DC power wire). The final layout of the components and the use of ribbon jumper cables were chosen to reduce the mess of wiring required to connect the display, chips, and buttons to the PocketBeagle.

Future Improvements

As previously mentioned, both the wiring can be improved (i.e., via a cape or PCB) and the display driver can be better optimized to reduce the ghosting and flickering that still occurs intermittently on the display. Furthermore, multithreading can be used to allow the device to accept user inputs through the buttons while simultaneously updating the display; currently, the display lags when a button is pressed. The adjustment of the display brightness should be improved or calibrated for the LED matrix since the current software causes the pixel colors to shift, especially at low brightness levels. Lastly, new widgets can be created using the provided software framework (as described in the introduction to this project), and the appearance (i.e., fonts, colors, sizing, etc.) of the existing widgets can be modified to improve readability and visibility.

Altogether, this was an incredibly valuable project for me to learn about embedded systems, and I hope this documentation is useful for aspiring engineers! I also wanted to thank Professor Erik Welsh in particular for his immense support on this project, particularly with wiring and initially running the LED matrix.

Notes

• Project completed 14 November 2023 for the ENGI 301 Introduction to Practical Electrical Engineering (Fall 2023) course with Professor Erik Welsh at Rice University.
• Much of the prototyping for this device was done at the Oshman Engineering Design Kitchen at Rice University in Houston, Texas.