Chapter 10 Touch Sensor

ESP32 offers up to 10 capacitive touch GPIO, and as you can see from the previous section, mechanical switches are prone to jitter that must be eliminated when used, which is not the case with ESP32’s built-in touch sensor. In addition, on the service life, the touch switch also has advantages that mechanical switch is completely incomparable.

Project 10.1 Read Touch Sensor

This project reads the value of the touch sensor and prints it out.

Component List

ESP32-WROVER x1	GPIO Extension Board x1
Breadboard x1
Jumper M/M x1

Related knowledge

Touch sensor

ESP32’s touch sensor supports up to 10 GPIO channels as capacitive touch pins. Each pin can be used separately as an independent touch switch or be combined to produce multiple touch points. The following table is a list of available touch pins on ESP32.

Name of touch sensing signal	Functions of pins	GPIO number
T0	GPIO4	GPIO4
T1	GPIO0	GPIO0
T2	GPIO2	GPIO2
T3	MTDO	GPIO15
T4	MTCK	GPIO13
T5	MTDI	GPIO12
T6	MTMS	GPIO14
T7	GPIO27	GPIO27
T8	32K_XN	GPIO33
T9	32K_XP	GPIO32

The touch pin number is already defined in ESP32’s code base. For example, in the code, you can use T0 to represent GPIO4.

The electrical signals generated by touch are analog data, which are converted by an internal ADC converter. You may have noticed that all touch pins have ADC functionality.

The hardware connection method is shown in the following figure.

Circuit

Schematic diagram

Hardware connection

If you need any support, please contact us via: support@freenove.com

Sketch

Sketch_10.1_TouchRead

Download the code to ESP32-WROVER, open the serial monitor, and set the baud rate to 115200. As shown in the following figure.

Touched by hands, the value of the touch sensor will change. The closer the value is to zero, the more obviously the touch action will be detected. The value detected by the sensor may be different in different environments or when different people touch it. The code is very simple, just look at Reference.

Reference

uint16_t touchRead(uint8_t pin);

Read touch sensor value. (values close to 0 mean touch detected)

Project 10.2 Touch Lamp

In this project, we will use ESP32’s touch sensor to create a touch switch lamp.

Component List

ESP32-WROVER x1		GPIO Extension Board x1
Breadboard x1
LED x1	Resistor 220Ω x1	Jumper M/M x3

Circuit

Schematic diagram

Hardware connection

If you need any support, please contact us via: support@freenove.com

Sketch

Sketch_10.2_TouchLamp

Download the code to ESP32-WROVER, open the serial monitor, and set the baud rate to 115200. As shown in the following figure,

With a touch pad, the state of the LED changes with each touch, and the detection state of the touch sensor is printed in the serial monitor.

The following is the program code:

/**********************************************************************
  Filename    : TouchLamp
  Description : Control led by touch button.
  Auther      : www.freenove.com
  Modification: 2024/06/18
**********************************************************************/
#define PIN_LED   2
#define PRESS_VAL 14	  //Set a threshold to judge touch
#define RELEASE_VAL 25	//Set a threshold to judge release

bool isProcessed = false;
void setup() {
  Serial.begin(115200);
  pinMode(PIN_LED, OUTPUT);
}
void loop() {
  if (touchRead(T0) < PRESS_VAL) {
    if (!isProcessed) {
      isProcessed = true;
      Serial.println("Touch detected! ");
      reverseGPIO(PIN_LED);
    }
  }

  if (touchRead(T0) > RELEASE_VAL) {
    if (isProcessed) {
      isProcessed = false;
      Serial.println("Released! ");
    }
  }
}

void reverseGPIO(int pin) {
  digitalWrite(pin, !digitalRead(pin));
}

The closer the return value of the function touchRead() is to 0, the more obviously the touch is detected. This is not a fixed value, so you need to define a threshold that is considered valid (when the value of the sensor is less than this threshold). Similarly, a threshold value is to be defined in the release state, and a value in between is considered an invalid disturbance value.

#define PRESS_VAL 14	  //Set a threshold to judge touch
#define RELEASE_VAL 25	//Set a threshold to judge release

In loop(), first determine whether the touch was detected. If yes, print some messages, flip the state of the LED, and set the flag bit isProcessed to true to avoid repeating the program after the touch was successful.

if (touchRead(T0) < PRESS_VAL) {
  if (!isProcessed) {
    isProcessed = true;
    Serial.println("Touch detected! ");
    reverseGPIO(PIN_LED);
  }
}

It then determines if the touch key is released, and if so, prints some messages and sets the isProcessed to false to avoid repeating the process after the touch release and to prepare for the next touch probe.

if (touchRead(T0) > RELEASE_VAL) {
  if (isProcessed) {
    isProcessed = false;
    Serial.println("Released! ");
  }
}