This example uses an MPR121 capacitive-touch chip to turn wires or copper pads into touch buttons. Touch a pad and the Serial Monitor says which one; hold any pad and the onboard LED lights. It is written for a beginner and is a fun first soldering / wiring project - you can even use pieces of aluminum foil, or (famously) fruit, as the buttons.

How does "capacitive touch" work?

Your body holds a tiny electrical charge. When your finger comes near a pad, it changes how much charge that pad can hold (its capacitance). The MPR121 measures that change on twelve pads at once and tells the ESP32, over I2C, exactly which pads are being touched - no moving parts, no visible buttons. It is the same idea as the buttons on an elevator or a microwave.

What you will need

An ESP32 board with a USB cable.
An MPR121 breakout board (Adafruit, or any generic MPR121).
Six jumper wires, plus a wire (or foil, or a coin) for at least one touch pad.

Part 1 - Wire it up

The chip talks over I2C (two shared data wires) and has an address pin.

MPR121 pin	ESP32 pin	Why
`VIN`/`3V3`	`3V3`	power
`GND`	`GND`	ground
`SDA`	GPIO 21	I2C data
`SCL`	GPIO 22	I2C clock
`ADDR`	`GND`	sets the address to 0x5A

Then run a wire from any pad labelled **0 .. 11** (ELE0..ELE11) to something you will touch - a bare wire end, a square of foil, a drawing pin, a banana.

Part 2 - Flash and touch

Open Mpr121.ino, upload, and open the Serial Monitor at 115200. You should see:

MPR121 ready - touch a pad (ELE0..ELE11)
electrode 0 touched
electrode 0 released
electrode 3 touched

Touch and release a pad and the matching line prints; the onboard LED is on whenever any pad is held. That is a working 12-button touch keypad.

Where this fits

pc_mpr121_read_touched() gives you a 12-bit mask - one bit per electrode - and pc_mpr121_is_touched(mask, e) tests one. From here you can build a real project: a touch keypad that submits a code over HTTP, a set of light switches published over MQTT, or a musical "fruit piano." This is the same "read a cheap breakout, bridge it onto the network" pattern the library uses for the RTC, the radar (example 62), and GPS.

Reading raw values (for tuning)

Besides the on/off mask, pc_mpr121_read_filtered(e) returns electrode e's raw 10-bit capacitance reading. Watching that number climb as your finger approaches is the way to pick better touch / release thresholds (the defaults are 12 / 6). The library builds the whole register bring-up - reset, the NXP filter defaults, thresholds, and the electrode-enable - and you can see those exact bytes in test/test_mpr121.

Troubleshooting

**"MPR121 not found".** Check SDA->21, SCL->22, power, and the ADDR pin. ADDR to GND is address 0x5A; to 3V3 it is 0x5B. An I2C scanner sketch should find the chip.
Touches trigger by themselves, or never trigger. Long pad wires pick up noise. Keep them short to start, and remember the pad senses a nearby finger, not just a hard press.
The LED is inverted (on when idle). Some boards wire the onboard LED active-low. Swap HIGH / LOW in the sketch or use another GPIO.

Build and run (PlatformIO)

The feature lives in the library, so its flag must reach the whole build:

pio ci examples/Drivers/Mpr121 \
  --board esp32dev --lib "." \
  --project-option="build_flags=-DPC_ENABLE_MPR121=1"

(The Arduino IDE reads the flag from build_opt.h beside the sketch automatically.)

How it works (for the curious)

The MPR121 reports a 16-bit status word: bits 0-11 are the twelve electrodes, bit 12 is a combined "proximity" electrode, and bit 15 is an over-current fault flag. pc_mpr121_touched(lo, hi) masks that down to the 12 electrode bits; pc_mpr121_word10() unpacks the chip's 10-bit filtered / baseline readings. Bringing the chip up is a fixed list of register writes (a soft reset, the NXP AN3944 filter and analog-front-end defaults, a touch and release threshold per electrode, then the electrode-configuration register that starts it running); pc_mpr121_build_init() produces that whole list as (register, value) byte pairs, which are verified byte-for-byte on a PC. Only the actual register read/write runs on the ESP32.