Build your own
The Raspberry Pi Pico is an amazing device that has a huge range of possibilities. This particular project focuses on using it as a capacitive touch board, and in the spirit of open-source this page lets you know how you can turn any of your pico’s GPIO pins into a touch sensor.
All you need, is a rpi pico and one 1M Ohm resistor for each pin you intend to use.
The resistor needs to go between the GPIO Pin and the GND pin. The leg of this connection between the pin and resistor is the part senses the capacitance.
From here all you need to do is rinse and repeat for however many sensors you’d like before coding.
All the information about coding is up on the github page for this project. You will need to use Circuitpython as this uses the native touchio library.
For the nitty gritty of how this works, it is explained in the touchio documentation:
// This is a capacitive touch sensing routine using a single digital
// pin. The pin should be connected to the sensing pad, and to ground
// via a 1Mohm or thereabout drain resistor. When a reading is taken,
// the pin's capacitance is charged by setting it to a digital output
// 'high' for a few microseconds, and then it is changed to a high
// impedance input. We measure how long it takes to discharge through
// the resistor (around 50us), using a busy-waiting loop, and average
// over N_SAMPLES cycles to reduce the effects of noise.