Arduino MOSFET touch switch

Question

I am trying to make a touch switch using an Arduino and a MOSFET. I have managed to get an LED to turn on when the pad is touched and it seems to work well (fully on when I touch it and fully off when i don't). However, when I try to replace the LED for an Arduino pin pulled high (internally) I run into problems. Sometimes the switch is read as low when I touch the pad, but other times it just becomes unresponsive. Could anyone see where i am going wrong? Thanks.

Answer 1

Your approach is deficient in several ways.

First, when you touch the pad with finger, you have huge AC signal coming in, 50/60Hz. When you release your finger, the gate charge can be left in any phase, positive, or negative. That's why your MCU reads low sometimes. More, huge AC amplitude likely stress the MOSFET input beyond voltage specifications, and some latchup occurs.

Second, it is very likely that your FET will be eventually destroyed by ESD.

To implement the idea of touch contactless buttons for various controls in electronic and household equipment, the Electronic Industry developed a special class of devices called touch sensors. They are not simple devices. Several semiconductor companies offer ICs that provide this function reliably. In many cases the touch sensor functionality is included in MCU directly. To get an idea, check this list on Digi-Key. This is a sample, directly for your application,

Answer 2

In case anyone is interested, I have created a solution to my problem using Arduino code. So far it seems to work flawlessly. I have added a 5k resistor in series with the gate, to reduce the likelihood of harming the gate.

#define input_pin A0
#define led_pin 3

int input_data;
unsigned long timer;
bool setTimer;
bool touched = false;
bool first = true;
bool toggle = true;


void setup() {
  pinMode(input_pin, INPUT_PULLUP);
  pinMode(led_pin, OUTPUT);
  Serial.begin(9600);
}
void loop() {

  digitalWrite(led_pin, toggle);


  if (digitalRead(input_pin) == LOW) {
    touched = true; //set the variable on the first low which is read
    setTimer = true;
  }

  if (touched == true && first == true) { //only allow it to happen once
    first = false;
    toggle = !toggle;
    Serial.println(toggle);
  }

  if (setTimer == true) {
    timer = millis();
    setTimer = false;
  }

  if (millis() - timer > 100) {
    first = true;
    touched = false;
  }
}