LDR Activated Switch System Technology

project circuit

 Light activated solid-state switch, using  LDR , DIAC  &  TRIAC

 to switching the light automatically at night .   


1. Diac ( DB3 ) 400 V , 4 Amp 1
2. Triac ( BT136 ) 400 V , 4 Amp 1
3. L.D.R 6 V 1
4. Resistance (R1) 1.2 K , 1 W 1
5. Resistance (R2) 30 K , 10 W 1
6. Capacitor 0.1 mF , 400 V 1
7. Lamp 230 V , 100 W 1



project circuit



In the presence of sufficient quentity of light flux , the value of resistance of the LDR will become quite low , and therefore maximum current will flow through it. This will prevent to fully charged the capacitor. Since the voltage accross the capacitor will not reach upto the breakover voltage off DIAC. So DIAC will not conduct. Due to no triggering signal TRIAC also in OFF state. So the lamp will not glow.  When the surround become dark, then LDR will con conductive. This will enable the capacitor to get fully charged and the voltage accross it  will become more than the breakover voltage of the DIAC. DIAC send the triggering pulse to the TRIAC gate & the TRIAC will be switched ON. This will complete the path of current flow through the lamp, and the lamp will continue to glow till the TRIAC is ON .


It is the bidirectional triggering diode or diac. Its main function to trigger with a fixed voltage reference. It is made by Silicon.


 Rating :- 

  • Operating Voltage = 400 V
  • Opperating Current = 4 A
  • Breakdown Voltage = 32 V


APPLICATION :-  DB3 DIAC can be used in conjunction with triacs, for simplified GATE control circuits, or as starting element in fluoroscent ballast.


It is a semiconductor device. TRIAC means “TRIode for Alternating Current” . Generaly it is used to switching purpose .



  1. Direct triggering from low power driver (LDR).
  2. High blocking voltage capacity.
  3. Very sensitive gate.


Rating :-

  • Operating Voltage = 400 V
  • Opperating Current = 4 A
  • Gate triggering voltage = 0.7 V
  • Gate triggering current = 5 A



  LDR stands for “Light Dependent Resistor”. It is made from “Cadmium Sulphide”. Few free electrons are present at dark time, but when illuminated the surface of LDR, then free electrons are liberated. Since the LDR become a conductor due to low resistance.


Rating :-

  • Operating Voltage = 6 V
  • Dark Resistance = 500 kohm



 APPLICATION :-  Generally LDR used as light & dark activated switches, like – street light control, over light detector, opto-coupler etc.


When current flow though any materials , encounters an opposing force , similar in respect to mechanical friction . This opposing force create a specified amount of resistance . which device we use to create resistance, called as resistor.

Generally carbon composition resistors are colour coded to indicate their resistance value in ohm unit. All colour bands stands for a digit. The colour bands their value of  resistance and  tolerance.



Rating :-

  • Resistance (R1):-
    • Value of  Resistance  =  1.2 KW
    • Wattage  =  1 W
    • Tolerance  =  ± 5%


  • Resistance (R2):-
    • Value of  Resistance  =  30 KW
    • Wattage  =  10 W
    • Tolerance  =  ± 5%


Capacitors are electronic componants which have the ability of storing electrical energy . Basically all capacitors consist of a parallel facing conductive surface or parallel plates seperated by insulating materials called ‘dielectric’.


Rating :-

  • Value of Capacitance  =  0.1 mF
  • Operating Voltage  =  400 V



LDR Activated Switch System Technology | EndlessEducation
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LDR Activated Switch System Technology | EndlessEducation
This is the engineering project. Light activated solid-state switch, using LDR, DIAC & TRIAC to switching the light automatically at night .
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Kanchan Chatterjee

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