This is a simple light follower application using two CDS photocells.  I included a calibration feature to account for the difference between the photocells and the resistors.  Photocells are mounted as shown using hot glue.  The servo is the standard one that comes with the Sparkfun Arduino inventors kit.

 

/* Sweep
by BARRAGAN <http://barraganstudio.com>
This example code is in the public domain.modified 8 Nov 2013
by Scott Fitzgerald
http://www.arduino.cc/en/Tutorial/Sweep

This coded modified by Wimberleytech LLC and placed into the public domain 31 May 2016

*/

#include <Servo.h>Servo myservo;  // create servo object to control a servo
// twelve servo objects can be created on most boardsint pos = 90;    // variable to store the servo position
int sensorPin0 = A0;    // input from first photosensor
int sensorPin1 = A1;    // input from second photosensor
const int buttonPin = 2;     // the number of the pushbutton pin
int valPin0 = 0;  // variable to store the value coming from the sensor Pin0
int valPin1 = 0;  // variable to store the value coming from the sensor Pin1
int offset = 100;
int buttonState = 1;
int hysteresis = 5;

void setup() {
myservo.attach(9);  // attaches the servo on pin 9 to the servo object
pinMode(buttonPin, INPUT_PULLUP);
Serial.begin(9600);
}

void loop() {
Serial.println(“start loop”);
buttonState = digitalRead(buttonPin);
while(buttonState == LOW)
{
valPin0 = analogRead(sensorPin0);
valPin1 = analogRead(sensorPin1);
if(valPin1 > valPin0)
{
offset = valPin1 – valPin0;  // want to subtract offset from Pin1
}
else if(valPin1 < valPin0)
{
offset = valPin1 – valPin0;  // want to add offset to Pin1
}
Serial.print(“Calibrate: “);
Serial.print(offset);
Serial.println(“buttonState”);
buttonState = digitalRead(buttonPin);
delay(1000);
}

if (analogRead(sensorPin0) < analogRead(sensorPin1)-offset-hysteresis)
{
Serial.println(analogRead(sensorPin0));
Serial.println(analogRead(sensorPin1));
pos +=1;
myservo.write(pos);              // tell servo to go to position in variable ‘pos’
if(pos > 160)
{
pos = 160;
}
delay(50);
}
else if (analogRead(sensorPin0) > analogRead(sensorPin1)-offset+hysteresis)
{
pos-=1;;
myservo.write(pos);              // tell servo to go to position in variable ‘-pos’
if(pos < 10)
{
pos = 10;
}
delay(50);
}

}