Playing with the capacitive touch inputs on the ESP32. -- Based on the Espressif ESP32 TouchRead example code.
Adding in de-glitching and digital filtering of the reported values and dynamic baseling tracking.
In this example I use Touch0 ('T0' in the code) which mapped to GPIO 4.
Keywords: ESP32, touchRead, capacitive touch sensor, de-glitch
The capacitive touch features of the ESP32 seem like a very cool way for users to interact with devices without needing mechanical switches.
However, the touchRead function in the ESP Arduino API seems to be susceptible to noise. A number of people have reported 'glitching' in the signal. (Sharp, single-point spikes in the data.)
As an experiment to test this, I connected a piece of copper tape to one of the capacitive touch inputs on` my module and started with the TouchRead example from Espressif. I saw much the same thing. That said, the magnitude of the glitching behavior is quite dependent upon the geometry of the copper tape pad. (I suspect that a bit of hardware filtering might eliminate much of the issue.)
To solve this in software, I added a simple de-glitching feature and provided the option to smooth the data to reduce noise.
Noise is filtered using a simple exponential smoothing function where the contribution of a new data point to the current value is determined by a numerical factor ranging from 0 to 1.0. The closer the numerical factor is to 1.0, the less a given point contributes.
Curves obtained from the SerialPlotter in the Arduino IDE. (Offset for clarity)
- Red Trace: Raw data from touchRead function
- Green Trace: Data after de-glitching
- Blue Trace: Data after smoothing
Touching the copper results in the excursion in the data.
Feel free to adapt / use as you will.
Regards,
John.