DIY USB-C PD Tools: Power Meter & Wi-Fi Logger

I am currently thinking about a new device, that would succeed the PD logger. The idea is not only to log VBUS voltage and current, but also to log and decode the PD negotiation between the USB-C power supply and the connected cable and device. I would need to decode the data on the CC and VCONN lines.

I am interested in obtaining the cable e-marker data, the power supply capabilities, and the consumer device selection.

What I have in mind is a pcb with SMD electronics, including an on-board ESP32-S3 controller, an interface to a 2.8 inch resistive touch display (I need more space for meaningful data display), and of course the analog electronics for VBUS voltage and current measurement and for CC and VCONN signal processing. 

I would design a circuit that can be controlled by the touch display. However, the very same board would also work without the display, but could be controlled by a Wifi web interface, instead - similarly to the one I have for the PD logger.

Does this make sense. Do you have any thoughts on such a device? 

I am currently thinking about a new device, that would succeed the PD logger. The idea is not only to log VBUS voltage and current, but also to log and decode the PD negotiation between the USB-C power supply and the connected cable and device. I would need to decode the data on the CC and VCONN lines.

I am interested in obtaining the cable e-marker data, the power supply capabilities, and the consumer device selection.

What I have in mind is a pcb with SMD electronics, including an on-board ESP32-S3 controller, an interface to a 2.8 inch resistive touch display (I need more space for meaningful data display), and of course the analog electronics for VBUS voltage and current measurement and for CC and VCONN signal processing. 

I would design a circuit that can be controlled by the touch display. However, the very same board would also work without the display, but could be controlled by a Wifi web interface, instead - similarly to the one I have for the PD logger.

Does this make sense. Do you have any thoughts on such a device? 

Sometimes it’s easier to see a project in action than just read about it. I recorded a detailed video that walks through the design, assembly, and usage of both devices: the OLED Power Meter and the Wi-Fi PD Logger.

In the video you’ll see:

• how the PCBs and 3D-printed cases fit together,
• how the Power Meter shows live values on its display,
• how the PD Logger serves a web interface and logs data,
• and some example measurements while charging real devices.

👉 Check out the full walkthrough in the embedded YouTube video below.

Once assembled and flashed, the PD Logger is very straightforward to use.

1. Connect it between a USB-C charger and the device you want to power. In this example, I tested it while charging a tablet.
2. As soon as the charger negotiates with the load, the PD Logger powers up and joins the local Wi-Fi network.
3. Open a browser and go to pd-logger.local (or the device’s IP address). You’ll see a simple web page that shows the current voltage, current, and power. The values update automatically every 5 seconds.

But the Logger can do more than live readings. All measurements are stored locally for several hours. By opening the /graphics page in your browser, you get interactive plots for voltage, current, and power over time.

This makes it easy to analyze charger stability, load behavior, and even spot small fluctuations. The Y-axis auto-scales, so sometimes the variations look dramatic—but a closer look at the scale often reveals that the changes are tiny.

Finally, the Logger also provides a CSV export button, so you can download the data and analyze it further in tools like Excel or Python.

With this setup, any USB-C charger becomes a transparent, measurable power source—and all you need to access the data is a web browser.

The enclosures for both devices were designed in Fusion 360. The design is intentionally simple: two shells and a small insert, which together hold the electronics securely in place.

I wanted the cases to be not only functional but also practical for repeated use. Instead of using fragile plastic clips, the design relies on M2 screws to close the enclosure. To make this possible, M2 nuts are embedded during the print. The print is paused at just the right height, the nuts are inserted into the cavities, and then the print continues. This creates durable threads that can handle many assembly cycles.

The models are included in the GitHub repository and can be 3D-printed on any FDM printer. PLA works fine, but PETG is more robust if you expect frequent handling.

This approach keeps the assembly straightforward: slide in the electronics, close the case with four screws, and the device is ready to use.

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.

When working with USB-C Power Delivery, the CC (Configuration Channel) and VCONN pins are critical. They are used during negotiation to select the voltage and current profile that the charger provides. Without them, PD simply won’t work.

There’s a catch, though:

• A standard USB-C cable only forwards one of the two CC lines, depending on how the plug is inserted.
• The USB-C specification doesn’t require both CC pins to be passed through. That means if you just connect a charger through an ordinary cable, you can’t expose both CC/VCONN signals to your circuit.

For devices like my Power Meter and PD Logger, this is a problem. Both gadgets need to make the CC/VCONN lines available at their output port, so the downstream device can negotiate properly with the charger.

The solution: instead of relying on a regular cable, I mount a male USB-C plug directly on a small daughter PCB that connects straight into my circuit. This way, both CC/VCONN lines from the charger side are routed to the output connector. The result is full compliance with USB-C PD negotiation, regardless of plug orientation.

This small design choice turned out to be essential for making the tools reliable and universally compatible with different chargers.

I started this project because I often use USB-C chargers in my maker builds. They are compact, powerful, and everywhere. But I quickly realized one big limitation: I had no clear way to measure what was really going on.

• Which voltage profile did the charger negotiate?
• How much current was actually flowing?
• Was the power stable over time?

Commercial USB-C testers exist, but most are closed-source and not easily hackable. So I decided to make my own tools.

In the end, I built two different devices:

• The OLED Power Meter – simple, compact, and with instant readout of voltage, current, and power.
• The Wi-Fi PD Logger – no display, but logs data for hours and serves live plots in a browser.

Both share the same core: a PCB with USB-C connectors, a shunt resistor, and an INA219 measurement chip. The Power Meter is based on an STM32 microcontroller, while the Logger uses an ESP-01s with Wi-Fi.

What surprised me the most was how quickly these tools became part of my everyday bench setup. I use the Power Meter when I just want a quick look, and the Logger when I want to study a charger’s behavior over time.

All the resources are available in the GitHub repository: schematics, PCB files, firmware, and 3D-printable cases. If you’ve got a USB-C charger and a 3D printer, you can build your own too.

Next, I’ll add some detailed build logs with photos and sketches from the design process. Stay tuned!