Variable Resistors

Types

• Potentiometers (turn, slide, ...)
• Thermistors
• Photoresistors
• Force sensing resistors (Interlink)
• Bending resistors (flex sensors)

Reading Variable Resistances with the Arduino

• We can read voltages across a variable resistor using the built-in analog-to-digital converters on the six analog pins of the Arduino board.

• One generic option is to connect a variable resistor (or trim pot) as shown below. You could potentially use a different fixed resistor value in the circuit to adjust the variable voltage range, as described in Sec. 1.2.

• Another option is to scale the values read by the analogRead() function over a desired range and keep the 10 K ohm resistor to minimize power consumption.

  Figure 3: A basic circuit to measure a variable resistance.

  

• Use the analogRead( pin ) routine to obtain a value proportional to the resistance of the sensor.

• The analogRead( pin ) converts voltages from 0 - 5 volts into integer values from 0 to 1023 (or 10-bits of resolution). It takes the system about 0.0001 seconds to perform the operation, so the maximum read rate is about 10000 Hz.

  int ledPin = 13;       // LED pin
  int analogPin = 3;     // potentiometer pin
  int val = 0;           // variable to store the value read
  int threshold = 512;   // threshold 
  
  void setup()
  {
    pinMode(ledPin, OUTPUT);   // sets the digital pin 13 as output
  }
  
  void loop()
  {
    val = analogRead(analogPin);    // read the input pin
    if (val >= threshold) {
      digitalWrite(ledPin, HIGH);   // sets the LED on
    }
    else {
      digitalWrite(ledPin, LOW);    // sets the LED off
    }
  }
  

Force Sensing Resistors

• Force sensing resistors (FSRs) have been a popular sensor option for music controllers.

• They are made up of a resistive material applied to a film, a spacer adhesive, and another film printed with interdigitating contacts.

• The resistive material makes an electrical path between the two sets of conductors on the other film.

• Before a force is applied, the resistance of an FSR is nearly infinite. Once a force is applied, the interdigitating contacts become closer together and a lower resistance results.

• Three basic regions of operation:
  1. initial abrupt change (around 10 grams of force) in resistance
  2. a large region where force is basically proportional to 1/R
  3. a saturation region where additional force produces little additional decrease in resistance
  

• Note that FSRs are not appropriate for accurate measurement of force levels unless calibrated and limited to their more linear functional range.

• FSRs tend to wear out over time, especially when exposed to sweat and dirt.