# ESP32 pinout reference

For the original **ESP32-WROOM-32 / WROVER** devkit, which is what both boards
in this project are. S3, C3, C6 and H2 have different maps entirely — and no
Bluetooth Classic, so they cannot run board A at all.

## Already in use

| Pin | Board | Used for |
| --- | --- | --- |
| GPIO2 | A and B | onboard status LED |
| GPIO16 | A and B | UART2 RX (link) |
| GPIO17 | A and B | UART2 TX (link) |
| GPIO25 | B | LED circuit 1 (Circle) |
| GPIO26 | B | LED circuit 2 (Triangle) |

## Free and safe for output

Use these first. No strapping duties, no boot-time side effects.

```
4   13   14   18   19   21   22   23   27   32   33
```

That is eleven spare pins, which is plenty for more LEDs, a buzzer, servos or
switches. GPIO21 and GPIO22 are the default I2C pins and GPIO18/19/23 the
default SPI pins, but nothing in this project uses either bus, so they are free
here.

## Never use

| Pin | Why |
| --- | --- |
| GPIO6–11 | wired to the on-board SPI flash chip. Touching these bricks the running firmware. |
| GPIO12 | strapping pin (MTDI). Pulled high at boot it sets the flash voltage to 1.8V and the board will not boot. |
| GPIO1, GPIO3 | UART0, the USB serial console. Using them breaks uploads and the serial monitor. |

## Usable, with a catch

| Pin | Catch |
| --- | --- |
| GPIO0 | strapping. Must be high at boot or the board enters flash-download mode. Has a button on most devkits. |
| GPIO2 | strapping, and the onboard LED. Must be low or floating at boot. Fine as an output once running — this project uses it. |
| GPIO5 | strapping. Outputs a PWM burst during boot, so anything attached will twitch at power-on. |
| GPIO15 | strapping. Emits boot log noise unless pulled low. |

## Input only — cannot drive an LED

```
34   35   36   39
```

These have no output drivers and no internal pull-ups. Fine for buttons with an
external pull-up, or for analog input. Connect an LED and nothing happens.

## Current limits

A single pin is comfortable at about **12mA** and hard-limited near **40mA**.
The whole chip should stay under roughly 200mA across all pins at once.

Two or three small LEDs per pin is fine. Anything beyond about 20mA per circuit
— which pre-wired "3V" LEDs sized for a coin cell often draw — wants a small
NPN transistor or a logic-level MOSFET, with the pin driving the base or gate
rather than the LED directly.

## ADC note

ADC2 pins (0, 2, 4, 12–15, 25–27) cannot be read with `analogRead()` while
WiFi is active. Irrelevant to this project, which uses no WiFi and no analog
input, but worth knowing before adding a potentiometer. ADC1 pins (32–39) work
regardless.

---

Created by: Jess Rogerson (yelling commands at Claude.AI)