user27080: I have a kit of 37 sensors as described here:

http://www.instructables.com/id/Arduino-37-in-1-Sensors-Kit-Explained/?ALLSTEPS
https://tkkrlab.nl/wiki/Arduino_37_sensors

I want to create a simple circuit:

if the light is detected - flash LED
if there is no light - do nothing

Among 37 sensors I don't seem to find anything related to light / brightness / lumens...
Can you please advise me which sensor to get? 

Shall I get a photocell  (Light Dependent Resistor) as described here: https://learn.adafruit.com/adafruit-arduino-lesson-9-sensing-light/parts ? 

user27157: Use a photoresistor aka photocell. They are variable resistors that change there values depending on the intensity of the light source.
It is sensor KY-018 : KY-018 Photo resistor module.
Here is a helpful code:
const int sensor = A0; //declare photocell pin
const int led = 13;

void setup()
{
  pinMode(sensor, INPUT);
  pinMode(led, OUTPUT);
}

void loop()
{
  int val = analogRead(sensor);
  if(val > 512)
  {
    digitalWrite(led, HIGH);
    delay(500);
    digitalWrite(led, LOW);
    delay(500);
  }
}


user19867: Number 23 in the list says photoresistor, corresponding to module KY-018 in your kit, which is what you called photocell (a photocell is another type of sensor to my knowledge).
As the name states, a photoresistor is a device changing it's resistance depending on the amount of light it detects/receive.
A photoresistor usually looks like:

In order to get a meaningful reading from an LDR you need to convert the resistance into a voltage which the Arduino's ADC can read. The simplest method is to use a voltage divider: the junction between the LDR (photoresistor) and R1 is the node to connect your analog pin to get your readings, which will be a voltage that is a proportion of the supply voltage (5V) defined by the ratio of the two resistances.
The KY-018 module mentioned above already provides all the necessary and provides you 3 pins: 5V, ANALOG and GND.

simulate this circuit – Schematic created using CircuitLab

user19409: You are after the "KY-018 Photo resistor module". It is number #23 "Photoresistor" on the instructables.com webpage.

The module contains a Light Dependant Resistor (LDR).
More information on the module can be found at https://tkkrlab.nl/wiki/Arduino_KY-018_Photo_resistor_module.  

27080: <p>I have a kit of 37 sensors as described here:</p>

<ul>
<li><a href="http://www.instructables.com/id/Arduino-37-in-1-Sensors-Kit-Explained/?ALLSTEPS" rel="nofollow">http://www.instructables.com/id/Arduino-37-in-1-Sensors-Kit-Explained/?ALLSTEPS</a></li>
<li><a href="https://tkkrlab.nl/wiki/Arduino_37_sensors" rel="nofollow">https://tkkrlab.nl/wiki/Arduino_37_sensors</a></li>
</ul>

<p>I want to create a simple circuit:</p>

<ul>
<li>if the light is detected - flash LED</li>
<li>if there is no light - do nothing</li>
</ul>

<p>Among 37 sensors I don't seem to find anything related to light / brightness / lumens...</p>

<p><strong>Can you please advise me which sensor to get?</strong> </p>

<hr>

<p>Shall I get a photocell  (Light Dependent Resistor) as described here: <a href="https://learn.adafruit.com/adafruit-arduino-lesson-9-sensing-light/parts" rel="nofollow">https://learn.adafruit.com/adafruit-arduino-lesson-9-sensing-light/parts</a> ? </p>
 27157: <p>Use a photoresistor aka photocell. They are variable resistors that change there values depending on the intensity of the light source.</p>

<p>It is sensor KY-018 : KY-018 Photo resistor module.</p>

<p>Here is a helpful code:</p>

<pre><code>const int sensor = A0; //declare photocell pin
const int led = 13;

void setup()
{
  pinMode(sensor, INPUT);
  pinMode(led, OUTPUT);
}

void loop()
{
  int val = analogRead(sensor);
  if(val &gt; 512)
  {
    digitalWrite(led, HIGH);
    delay(500);
    digitalWrite(led, LOW);
    delay(500);
  }
}
</code></pre>
 19867: <p>Number 23 in the list says <em>photoresistor</em>, corresponding to module KY-018 in your kit, which is what you called <em>photocell</em> (a photocell is another type of sensor to my knowledge).
As the name states, a photoresistor is a device changing it's resistance depending on the amount of light it detects/receive.</p>

<p>A photoresistor usually looks like:
<a href="https://i.stack.imgur.com/N6rE7.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/N6rE7.jpg" alt="enter image description here"></a></p>

<p>In order to get a meaningful reading from an LDR you need to convert the resistance into a voltage which the Arduino's ADC can read. The simplest method is to use a <a href="http://learn.sparkfun.com/tutorials/voltage-dividers" rel="nofollow noreferrer">voltage divider</a>: the junction between the LDR (photoresistor) and R1 is the node to connect your analog pin to get your readings, which will be a voltage that is a proportion of the supply voltage (5V) defined by the ratio of the two resistances.</p>

<p>The KY-018 module mentioned above already provides all the necessary and provides you 3 pins: <code>5V</code>, <code>ANALOG</code> and <code>GND</code>.</p>



<p><img src="https://i.stack.imgur.com/fjOfA.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2ffjOfA.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>
 19409: <p>You are after the "KY-018 Photo resistor module". It is number #23 "Photoresistor" on the instructables.com webpage.</p>

<p><a href="https://i.stack.imgur.com/PEVZ4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/PEVZ4.png" alt="KY-018 Photo resistor module"></a></p>

<p>The module contains a Light Dependant Resistor (LDR).</p>

<p>More information on the module can be found at <a href="https://tkkrlab.nl/wiki/Arduino_KY-018_Photo_resistor_module" rel="nofollow noreferrer">https://tkkrlab.nl/wiki/Arduino_KY-018_Photo_resistor_module</a>.  </p>
