The capacitive sensors of the ESP32

The ESP32 has capacitive sensors that can be used as a touch button. These are 
the famous “TOUCH” pins found on the pinouts. Ten are available on the uPesy 
ESP32 boards.

complete ESP32 pin diagram


Pinout of the uPesy ESP32 Wroom Devkit board



What is it?

Capacitive sensors are widely used to detect the pressure of our fingers, 
especially on touch screens. We can use them on the ESP32 to replace push 
buttons.

Operation of the ESP32 capacitive sensor



Capacitive touch sensors


Capacitive sensors are based on the variation of the capacitance (of a 
capacitor) when the sensor is touched. The ADC (Analog to Digital Converter) 
reads and converts this variation.

Note
It is essential to remember that these capacitive sensors will not be as 
reliable as push buttons, especially for the use we will make of them.

Indeed, many parasitic capacitances come into play and more or less disturb the
measured values.


Parasitic capacitances of the ESP32 capacitive sensor

Parasitic capacitors


It is necessary to consider all these parameters and make a much more rigorous 
and complex electrical circuit to have good measures. We can resume the circuit
to a copper wire to show how it works!


ESP32 wiring diagram of the capacitive sensor


It’s hard to make a simpler circuit 🙂




Use on the ESP32

The code to use the capacitive sensors is straightforward. Only one function is
needed touchRead() . The code to read the capacitive measurement of pin 4 is :

touchRead(4);
//or
touchRead(T0);

Note
The function parameter is either the pin number (here GPIO4) or the number of 
the capacitive sensor associated with the pin (here T0).

The usage is very similar to the analogRead() .

With a short code, the measured values are displayed in the serial monitor:

/*F********************************************************************
*
**********************************************************************/
void 
setup() 
{
    Serial.begin( BAUD );
    delay( 1000 );                     // DELAY TO LAUNCH THE SERIAL MONITOR
    Serial.println( "ESP32 Touch Demo" );
}
/*F********************************************************************
*
**********************************************************************/
void 
loop() 
{
    Serial.println( touchRead( 4 ) );
    delay( 500 );
}


Graph of the ESP32 capacitive sensor response

Waveform displayed in the serial plotter


The hollows correspond to the moment when the wire is touched. A threshold value
must be defined to determine if the cable has been touched. If we are below this
 threshold value, we have touched the button.

Note
The threshold value depends on the material used (wire, length, breadboard) and
should be adjusted.



The code with the threshold is

int capacitiveValue = 100;
int threshold = 20; //Threshold to adjust

/*F********************************************************************
*
**********************************************************************/
void 
setup() 
{
    Serial.begin(115200);
    delay(1000); // Delay to launch the serial monitor
    Serial.println("ESP32 Touch Demo");
}
/*F********************************************************************
*
**********************************************************************/
void 
loop() 
{
    capacitiveValue = touchRead( 4 );
    if( capacitiveValue < threshold )
        Serial.println( "Wire touched" );
    delay( 500 );
}


Wiring on esp32 capacitive sensor prototyping plate



That’s it for the programming! You can also add aluminum foil at the end of the
wire for better sensitivity.