Capacitive touch detection circuit (no dedicated chip required)

* This design implements a capacitive touch detection circuit, which only requires capacitors, resistors, and diodes, and the cost is much lower than a dedicated chip.
* The device selection under this design is not optimal. Experiments are required to measure the optimal selection of components under different buttons. Everyone is welcome to try and give feedback.
* The author of the original version of this design is no longer available and is waiting to be claimed. It was released by me after improvement and testing. It is based on the CC-BY-SA 3.0 agreement. You are allowed to change and use it, but please disclose your device selection and debugging results. It is convenient for everyone to summarize and use later. I hope to collect everyone's experimental data and establish a database, and continuously update the selection of touch key components in various environments, through various media, various shapes and structures, and I need your help.
* Each version of this design will try to use components from the [Jiali Tranco Patch Component List] to facilitate everyone's smt.
* This design requires [hundred kHz] level PWM signal as the excitation source. You can use the STM32F031K6 master control in this project to generate PWM signals or connect to other external development boards. Although the output is an analog quantity, in theory it can be directly connected to the ordinary IO port of the microcontroller after configuration.
* The source code and firmware based on mbed os 2 are posted below.
* I am a novice, so I hope you guys can give me some advice.