Smartsocket

ESP32-C3 smart USB-C charge controller

A small adapter that sits between a USB-C charger and a phone and stops charging at a battery level you choose, for example 80 %, then starts again at a lower one, for example 40 %. The phone reports its battery level over Wi-Fi, and the adapter does the switching. I designed the circuit and the board, wrote the firmware and the companion apps, and assembled, soldered and tested it in October 2026.

How it works

To the charger the adapter looks like a simple 5 V device, and to the phone it looks like a simple 5 V charger. Stopping the charge is the same as pulling the plug out, so every phone already knows how to handle it.

• Power path. A P channel MOSFET (SI4435) switches the 5 V VBUS line, driven through a 2N7000 by the ESP32-C3. Up to 3 A at 5 V, nothing above 5 V anywhere on the board.
• USB-C signalling. The firmware reads the charger's CC pins to learn whether it offers basic current, 1.5 A or 3 A, then advertises the matching limit to the phone by switching one of three CC resistors.
• Phone detection. An analog input on the phone side CC line tells the controller whether something is plugged in, even while the output is off.
• State. The allowed or stopped state is kept in flash, so it survives a restart. A red LED shows the state, and the BOOT button gives "charge to 100 % once", WPS pairing and a reset.

The full circuit table, pin map and decision rules are in DESIGN.md.

Firmware

C++ for the Seeed XIAO ESP32-C3 in the Arduino environment (firmware/smart_socket/).

• Wireless control over Wi-Fi, with a web page served by the adapter and a JSON API.
• Setup over Bluetooth from the Android app, by WPS, or through a setup network.
• Firmware updates over the air, through Wi-Fi or Bluetooth, with an MD5 check before restarting into the new program.
• Each adapter answers at its own smartsocket-xxxxxx.local address, so several can share one network.
• Upper and lower limits, a charge timer, charging hours, a heat cut out and the last 20 charging runs.

The decision rules are mirrored in Python in tools/logic.py and covered by 62 scenario tests. tools/test_device.py checks a real adapter feature by feature over Wi-Fi.

Before building, copy firmware/smart_socket/secrets.example.h to secrets.h and set your own update password.

Apps

• Android (app-android/): finds every adapter on the network, sets one up over Bluetooth, and reports the phone's battery level in the background.
• Windows (app-windows/): every adapter in one list with limits, timer, history and program upload.
• iPhone: two Shortcuts automations calling http://smartsocket.local/api/battery?level=…, described in PHONE-SETUP.md.

Board

Rev 0.2, 51.2 x 24 mm, two layers, made by PCBWay. The charger plugs into one end and the phone cable leaves from the other. Schematic and board are generated by scripts in kicad/ and checked with ERC, DRC and a netlist comparison.

A 3D printed case is drafted in OpenSCAD in case/.

Limits of version 1

• 5 V only, up to 15 W. Laptops need 20 V and USB Power Delivery negotiation, which is a job for version 2.
• Charge only: the data wires are not connected.
• Needs a charger with a USB-C socket.

Tools

ESP32-C3, C++, KiCad, Python, Java for Android, C# for Windows, OpenSCAD.