Diagram SOlution

 In designing a circuit that can detect light, the most important component is the light sensor. One of the light sensor that we often encounter is LDR (Light Dependent Resistor).

Introducing the Light Sensor

LDR resistance changes with changes in light intensity hit it. In the dark, the LDR resistant is about 10 Mohm and the sunniness of 1 kohm or less. LDR is made from semiconductor materials such as cadmium sulfide. With this material, the light energy that falls cause more load is released or the electric current increased payload. That is, the resistance of the material decreases.
LDR is used to convert light energy into electrical energy. Automatic switches and burglar alarms are a few examples of tools that use the LDR. Because the response to light enough slow, LDR is not used in situations where the light intensity changed drastically.

Characteristics of LDR:
1. At the time not given light, LDR has a very high resistance could reach 10 Mohms
2. But when given a light, the barriers will come down drastically so that the voltage and current can pass through the LDR.

So how do we use the characteristics of the LDR to construct a light detector circuit?
Surely this LDR will be combined with other components. Here is an example circuit with LDR and other components.
 

From the picture above, we can set (varying) the value of R1, R2 and potentiometer P1.

V+ = (RLDR / R1 + RLDR ) 5V

V- = (P1 / R2 + P1) 5V

For example we want to turn on lights automatically when dusk and the lights off automatically when the early morning. Here we can set the reference voltage to be compared with the input voltage of the LDR, we can set these voltages to match the surrounding light intensity.