PaperFlux prints a summary of my GitHub activity on a 58 mm thermal receipt, once a week: total stars, followers, a 7-day commit histogram, top languages, and most-starred repositories. No screen, no notification, no button, a strip of paper comes out on its own, like a bank statement for open-source work.

The project spans four disciplines: a custom electronic board, embedded firmware, a web service in production, and a 3D-printed enclosure.

Repo: github.com/albanpetit/paperflux (MIT, full source: ECAD, firmware, web service, enclosure)

Requirements

• A single plug. Runs off one USB-C charger, no dedicated printer power supply.
• Autonomous. Connects to Wi-Fi, knows the date, prints on schedule with no intervention.
• Reliable. A reboot or power loss must never trigger a reprint; a failure must not waste paper in a loop.
• Simple on the board side. The receipt layout is computed server-side, the board just downloads an image and prints it.

System overview

Two parts talk over HTTPS: a web service that builds the receipt image, and a board that downloads and prints it.

1. Power, the board negotiates 9 V (5 V fallback) over USB-C PD, powering both the electronics and the printer.
2. Scheduling, firmware syncs over NTP and checks every minute whether a receipt is due (weekly by default).
3. Rendering, the board calls the web service, which pulls GitHub stats, lays out an HTML template in headless Chromium, and converts it to a 1-bit PNG exactly as wide as the print head.
4. Printing, firmware decodes the PNG on the fly and streams it to the printer line by line.

Choosing the components

The core constraint: one USB-C plug has to power a thermal head, which draws far more than 5 V/500 mA, hence USB Power Delivery.

Built-in Wi-Fi, native USB (flash + console, no USB-serial chip)
Reads charger profiles over I²C, requests voltage, switches output
Back-to-back N-channel MOSFETs
2 A fixed-output buck, few external parts
For the buck converter
MCU at 3.3 V, PD controller's I²C side at 5 V
Protects D+, D−, CC1, CC2
USB 2.0, enough for PD + data
58 mm thermal, TTL interface, 384-dot width

9 V rather than 5 V roughly halves current for the same power, right when the head draws the most.

Schematic

Power path: the AP33772S negotiates over CC1/CC2, senses current through a 5 mΩ shunt, watches temperature via an NTC, and closes two back-to-back MOSFETs once voltage is set. That switched output (VCC) feeds the printer directly and the 3.3 V buck. Important consequence: the ESP32-C3 is powered through the PD switch, so it dies with the printer if a protection trips.

Logic side: the ESP32-C3's native USB goes straight to the connector (PD, flashing, and console share one port). I²C runs from the MCU through the PCA9306 level shifter to the AP33772S. ESD protection sits right at the connector.

Three status LEDs (VBUS present, PD negotiation status, 3.3 V rail present) give an instant visual diagnostic.

PCB

33.55 × 50.05 mm, 4-layer stack (top routing / GND plane / 3.3 V plane / bottom jumpers), ENIG finish for the AP33772S's 0.5 mm-pitch WQFN-24. Manufactured by AISLER; v0.2 fixed a connector issue and wrong reset-circuit resistor values from v0.1. An interactive BOM (InteractiveHtmlBom) made hand assembly manageable on a dense 0402-heavy board.

Key design details

• Buck regulator: fixed-output AP63203, FB tied straight to the 3.3 V rail (no divider), EN tied to input so it starts as soon as the switch closes. Tight placement and a continuous ground plane keep the switching loop away from the Wi-Fi antenna.
• Power switch: back-to-back MOSFETs block current in both directions when off; the PD controller sits upstream (stays powered) while the ESP32-C3 sits downstream (powers off with the printer). A 100 Ω divider lets firmware...