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One type of simple sensor is a photoresistor. It changes its electrical conductivity (resistance) depending on the incidence of brightness and can therefore also be called a brightness sensor. To connect the sensor we use Many sensors also act as a variable resistor, and unfortunately our arduino isn't designed to measure resistance; we need to convert this resistance to a variation in voltage.  To  do this, we need a so-called voltage divider circuit.

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For our photoresistor there is the following circuit: 

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Example-Code

For the code, let's output the values read from Arduino via the Serial Monitor (see: Serial).

#define BRIGHTNESS A0
void setup()
{
  Serial.begin(9600);
}
void loop()
{
  int reading = analogRead(BRIGHTNESS); // We get the sensor value here
  Serial.println(reading); // We send the message to serial 
}

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