Light operated relay using LDR & SCR

Explain working of light operated relay using LDR & SCR

• Principle:

                      When light is emitted on LDR then internal resistance of LDR is decrease.

     We can control high voltage supply.

• Circuit diagram:

          

• Construction:

                       The circuit consists of one SCR and a load which is connected with SCR .instead of load we can use relay to which either AC&DC load can be connected a variable resistance and LDR are connected in series and the combination is connected to the gate of the SCR AC supply is given to circuit.

• Working:

                       Now as we know internal resistance of the LDR is high. when light is not falls or emitted .at that timer when supply is given and sufficient light is not available  to the LDR the resistance of the, LDR is high so a very small current flow through the LDR and to gate of SCR which is enable to turn on SCR. so the SCR is off and load current does not flow .to turn on SCR ,gate pulse is required to this high current is required to flow through LDR to gate .when light is falls on LDR, the applications of the LDR becomes low ,so large current flows in to the gate of the SCR as a result the SCR is on and load current is states to flow since it if is a DC supply than commutation circuit is required to turn off supply and if SCR is working on AC supply. the SCR is automatically turned off after  +ve half cycle so in the next cycle again light should falls on the SCR to trigger the SCR thus the LDR can be used to trigger the SCR .

                       Now, when relay is connected instead of load then when light is emitted on LDR than SCR turned on and relay is energized due to large current flow through relay.

• Application:

•   To light switch.

•   Missile system.
•   Light operated alarm.
•   Camera exposure setting.
•   Street light controller.