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Light detector using LDR

The LED turns ON when light is incident on the LDR . This project is widely used in security systems .

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In the following circuit ,
CASE 1:
When light is not incident on the LDR , it acts as a very high resistance . Due to which the voltage at the base of the transistor is too LOW to turn the transistor ON.
No current flows through the emitter collector jn of the transistor and so NO CURRENT FLOWS THROUGH THE LED.

CASE 2:
When light is incident on the LDR , it starts acting as a low resistance . Now , the voltage at the base is HIGH enough to turn the transistor ON.
The transistor starts working and current flows through the emitter collector junction .
Hence , the current flows through the led .

The pot or potentiometer is used to vary the resistance . So to find a cut-off point where the transistor just starts working .

Peace :-)

8012534324_c64fb60f08.jpg

The dark detector .

JPEG Image - 25.54 kB - 10/26/2016 at 11:39

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ldr.JPG

The schematic .

JPEG Image - 289.49 kB - 10/22/2016 at 02:48

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  • 1 × LDR Resistor Networks / Thick Film Networks
  • 1 × BC 547 Discrete Semiconductors / Transistors, MOSFETs, FETs, IGBTs
  • 1 × LED Fiber Optics / Emitters
  • 1 × RESISTOR 220R current limiting resistance
  • 1 × POTENTIOMETER 100K

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Akshay Srivastava wrote 10/26/2016 at 11:53 point

I have uploaded the circuit of a DARK DETECTOR in files , you might wanna check it out .

In this circuit when light is incident on the LDR , it act as a LOW resistance . So the maximum current flowing through the potentiometer (variable resistance ) flows through the LDR and minimum flows through the base of the transistor . This is because of the property of current to flow in the direction of minimum resistance. Hence , base is turned OFF and the transistor acts as an open switch letting no current to pass through the LED .

While in the other case , when light is NOT incident on the LDR , it acts as a very HIGH resistance and so maximum the current passes through the base of the transistor. This turns the base ON and transistor acts as a short switch letting current pass through the LED .

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agamergirl wrote 10/25/2016 at 20:19 point

How might one do the opposite? As in, when low light or no light is detected, then power the device?

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