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Light Activated Switch using Transistor

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LDR circuit diagram

Dark sensor circuit

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

automatic street light circuit diagram


ldr circuit diagram


In this circuit 25 high bright white LEDs are used, each  Row of the series connection there are 5 LEDs in . 5 LEDs in one Row and as this here total 5 series arrangements 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 the high-value resistor is connected between the base of the transistor and +ve supply there is a very small amount of current will pass. This small current will drop with the high negative voltage current which comes through the LED. In this case transistors, Q1 will turn on Because we know that some amount of voltage must be required at the BASE terminal of any transistor to Turn ON to start the supply from Collector to Emitter.  And as a result of 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 light falls on LDR And it gives HIGH resistance. Then no voltage will be dropped with the ground and some voltage remains at the 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 falls directly on it. Give 12 v DC Supply.


SK100 Transistor – 1

BC547 Transistor – 1

Preset 1M – 1

100K resistor – 1

1K resistor – 1

1uf capacitor – 1

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  1. Pingback: How Transistor Works as Switch. NPN and PNP transistor working

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