Light Activated Switch using Transistor

This is a light controlled LED light circuit, Light activated switch ,which fully depend on Light . LDR is used in this for sensing the DARK  and LIGHT. LDR is a Light Dependent Resistor which resistance is increases as darkness and its resistance is Decreases when Light falling on it.



In this circuit 25 high bright white LED used, each  Row of series connection there are 5 LEDs in . 5 LEDs in one Row and as this  here total 5 series arrangent are connected and in one series only 5 LED connected.


During Daytime, Simply light falls on the LDR and it offers a LOW Resistance. When the resistance of LDR is LOW , It means its conductivity is HIGH. In high conductivity LDR will pass the current of negative voltage on the base of transistor BC457.  Because of high value resistor are connected between the base of transistor and +ve supply there are very small amount of current will pass. which small current will droped with the high negative voltage current which come through LED. In this case transistors Q1 will turn on Because we know that some amount of voltage must be required at BASE terminal of any transistor to Turn ON to start the supply from Collector to Emitter .  And as a result  Q2  also do not conduct supply and then there are no any conductivity occurs between LEDs  and all LED are in OFF and do not Glow.

During Night Time No any light falls on LDR And it gives HIGH resistance. Then no voltage will dropped with ground and some voltage remain at BASE of transistor Q1 . In this condition Q1 will Turn ON and then this transistor also turned on the transistor Q2. And conductivity starts and all LEDs GLOW.

Assemble the circuit in any general purpose pcb and LDR should set as the light fall directly on it. Give 12 v DC Supply.

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Hi, My name is Aman bharti, I am interested in making and study of Electronics & circuit diagram. I like to share my knowledge and all ideas with people which i get from My Experiment & from different sources. I try to provide any circuit details deeply with test result as possible as. You can contact me on- Email – [email protected]

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