Here’s a high-level schematic of how the measuring devices are built. At the core, the design is simple but effective:

• USB-C plug and receptacle

  The device sits in-line between the charger and the load. All essential signals are passed through so the load can negotiate properly with the charger.

• CC and VCONN lines

  These are carried from input to output to maintain proper Power Delivery negotiation.

• VBUS shunt resistor

  A small 50 mΩ resistor is inserted into the VBUS path. By measuring the voltage drop across it, we can calculate the current flowing to the load.

• Step-down converter

  Converts the charger voltage (5–20 V) into 3.3 V, powering the electronics regardless of the PD profile.

• INA219 measurement chip

  Handles accurate voltage and current measurement over I²C.

• Microcontroller

  Either an STM32 (for the OLED Power Meter) or an ESP-01s Wi-Fi module (for the PD Logger).

• Optional display

  The OLED Power Meter uses a small I²C display to show live data. The PD Logger skips the display and instead serves a web interface.

This modular architecture allowed me to build two different tools from the same foundation—one optimized for quick bench checks, the other for long-term monitoring and logging.