Heidi Wang's HCDE 439 Physical Computing Page

Requirements

input and output part

actuator output part: e.g. servo motor, LED

part that requires a library: e.g. remote control, capacitive touch, servo motor

>1 library; otherwise, >1 sensor input and 1+ actuator output

if using analogRead(), code that calibrates sensor readings

Schematic

The capacitive surface for the sensor is the end of a wire.

The capacitor and ground for the sensor are the body of the person interacting with the circuit.

Resistance of 1 MOhm was chosen for the capacitive sensor based on the library guidelines for resistor choice to activate on touch contact.

Circuit

Firmware

link to code file

        
/*
  Libraries

  Built with Examples/CapacitiveSensor/CapacitiveSensorSketch and Examples/Servo/Knob

  The servo motor points right when the capacitor is activated
  and left when the capacitor is deactivated.

  The servo motor output is attached to pin 6.
  The capacitor input is attached to pin 4 for send and pin 2 for receive.

  modified 26 Oct 2025
  by Heidi Wang
*/

#include 
#include 

// 1 megaohm resistor between pins 4 & 2,
// pin 2 is sensor pin, add wire, metal capacitive surface
CapacitiveSensor cs_4_2 = CapacitiveSensor(4,2);
// initialize servo object
Servo myServo;
// the number of the write pin for the servo
const int servoPin = 6;

// the threshold for activating the capacitive sensor; change as needed
int threshold = 240;

// the setup function runs one time when the program is started
void setup()
{
  // attach servo object to servo pin
  myServo.attach(servoPin);
  // begin serial communication at 9600 bits/s (baud)
  Serial.begin(9600);
}

// the loop function loops indefinitely
void loop()
{
  // read in the capacitive sensor value over 30 samples of measurement
  long sensorValue = cs_4_2.capacitiveSensor(30);
  // print the sensor value
  Serial.println(sensorValue);

  // rotate the servo depending on whether the sensor value passed the threshold
  if (sensorValue > threshold) {
    // rotate the servo to the right when the sensor is activated
    myServo.write(180);
  } else {
    // rotate the servo to the left when the sensor is deactivated
    myServo.write(0);
  }

  // delay 15ms between reads
  delay(15);
}

Operation

Questions

We can address erroneous readings by only changing the output if the reading was not an error code.

CapacitiveSensor error codes are -1 or -2, and >= 0 otherwise.

            
  // read in the capacitive sensor value over 30 samples of measurement
  long sensorValue

  // do not change the output if the read value was an error code
  if (sensorValue >= 0) {
    // rotate the servo depending on whether the sensor value passed the threshold
    if {} else {}
  }

If the measurement is noisy and deviates from the true measurement by +/- 10%, we can take multiple readings and average them before each adjustment to get a smoother output.

            
  // average 5 readings of the capacitive sensor value over 30 samples of measurement
  long sensorValue = 0;
  for (int i = 0; i < 5; i++) {
    sensorValue = (sensorValue*i + cs_4_2.capacitiveSensor(30))/(i+1);
  }