What you are holding

One chip, one screen, no bridge

The part in the middle is an ESP32-C3FH4: a single RISC-V core at up to 160 MHz, Wi-Fi 4 and Bluetooth LE, with 4 MB of flash and 400 kB of SRAM. There is no PSRAM and no second core. One core matters more than it sounds: the radio is serviced from the same core your loop() runs on, so a long delay() is a stretch of time during which nothing is answering the access point.

Two things are on the board that are not on a bare C3 module. A 0.42 inch OLED is soldered to it and permanently wired to two of the GPIOs. And the USB-C socket goes straight to the chip — there is no CH340 or CP2102 between them, because the C3 has a USB peripheral of its own. That is why there is no driver to install on any operating system, and also why the port vanishes every time the chip resets.

Four parts that look like two

Two black buttons sit at the top edge, a millimetre apart, and two LEDs sit either side of the USB socket. In each pair, one is yours and one is not — and the silkscreen naming them is about half a millimetre tall.

The pair that costs people an evening is the buttons. RST restarts the chip and is invisible to your code. BOO is GPIO 9, which the chip reads once at reset to decide whether to run your sketch or wait for an upload, and which is an ordinary input with a pull-up on it for the rest of the time. Holding the wrong one during an upload is the difference between a board that flashes and one that says "Failed to connect".

The size, if you are building it into something

The board is 20.5 mm wide and 25 mm tall, and the two rows of holes are 17.78 mm apart — 700 mil, which is a whole number of tenths, so it drops onto stripboard and across the centre channel of a breadboard without any adaptor. Eight holes down each side at the usual 2.54 mm.

The antenna is a ceramic chip a few millimetres long at the top edge. It is small, and it is the reason a later article in this book is about turning the transmit power down: at full power the radio puts out more than the matching around that antenna can carry, and the board fails to join networks it is sitting next to.

What the box gives you as well

The expansion base is the second board in the bag: a carrier with a power switch, a JST PH 2.0 battery connector, a charger, and a solder pad that connects the battery to an ADC pin so a sketch can read its own battery level. It also decides one thing you cannot undo, which is which way round the C3 board goes into it — that is its own article, and worth reading before the first time rather than after.

Charging needs the switch on

The base has its own charge indicator: LED1 in the charging-circuit notes, connected to the charger's CHRG pin. This is separate from the power and IO8 LEDs on the C3 board. The base's slide switch, SW2, also controls whether the battery is connected to the charging and battery-detection circuit. Plugging in USB is not enough: the switch must be ON to charge the cell.

1. Fit the base in the correct orientation and connect a suitable single-cell battery with the correct polarity.
2. Move SW2 to ON, then connect USB/5 V power. The charge LED stays on steadily while the battery is charging.
3. When charging finishes, the charge LED goes out as CHRG becomes high-impedance.
4. To disconnect the battery for transport or storage, move SW2 to OFF. The battery will not charge in this position, even with USB connected.

With USB/5 V supplied, the indicator behaves as follows:

The supplied charging-circuit notes describe the OFF-state glow as rapid switching of CHRG between low and high-impedance when the charger cannot detect the battery. Your eyes blend the flashes into a dimmer light. On the documented circuit, this behaviour alone is normal and does not indicate a fault; it is not a reduced-current charging mode. An unlit LED by itself is not proof of a full battery either — check that power and the battery are connected and the switch is ON.

Inside the base's charging circuit

The following component references come from the supplied Charging Circuit Instructions (充电电路说明书), based on schematic Schematic2_9, P1, ESP32-C3, V1.0, dated 2025-10-06.

In the notes' description of the switched battery path, Q1's drain connects to battery positive through F2, and its source leads towards SW2. With SW2 ON, contacts 2–3 connect the battery to U13's BAT network and the R10/R11 sensing circuit. With SW2 OFF, the contact moves to pin 1, which is unconnected, opening that path. Pins 4–6 are marked unused.

The two equal divider resistors halve the sensed voltage before it reaches the ADC. The base's BATTERY MONITOR solder jumper must be bridged to read that voltage on GPIO 2; the battery-level article explains the conversion and calibration.

For a custom hardware modification, the notes also suggest monitoring CHRG with an ESP32 GPIO and a 4.7–10 kΩ pull-up to 3.3 V, using its level changes to help identify battery connection status. That needs an added connection and a check of the existing LED circuit's voltage levels before wiring it to a GPIO; it is not a signal already available to a stock sketch.