LDR circuit using BC547 bipolar junction transistor

 

LDR circuit using BC547 bipolar junction         transistor

Friends, today we will design the LDR circuit, which is made with the help of LDR and BC 547 transistor. With the help of this circuit, we can get the LED automatic ON and OFF. Some components are required to make this circuit. Those components are something like this.

1.   LDR --------------------------- 1pc

2.   BC547 -------------------------1pc

3.   Resistance 22KΩ ------------1pc

4.   Resistance 330Ω -------------1pc

5.   Led 5mm any colour --------1pc

6.   Battery 9VDC ----------------1pc

7.   Connecting wires

 Before making this circuit, it is very important to know some important components. Because the circuit we make, we must first know about the components inside it, what are they doing in our circuit.

1) LDR

As its name suggests, it is a kind of resistor, which depends on the light. As the light decreases or increases, its resistance decreases and increases. Its resistance increases very much when it is dark light. Due to which current flow is not possible. Its resistance decreases when it is bright light. Due to which current flows easily.

 

2) BC547(Bipolar Junction Transistor)

As its name itself suggests, it has 2 junction (N junction and P junction). This transistor consists of 3 terminals (Base, Collector, Emitter). It is an NPN transistor. When positive supply provide to the base of the transistor, due to which the transistor works in ON condition. The direction of current of NPN transistor is emitter from collector. Basically transistor is current control device.  

3) Resistance 

Resistance is a property of conductor that prevents the current flowing in the circuit. The unit of resistance is OHM (Ω). 

Now we make the LDR circuit.

Working

Its working can be done in two ways.

1) If we want the led to be ON at night and led off by day.

2) If we want the led to be ON during the day and led off at night.

So, I will tell you both these ways how we make the circuit according to their working.

1) LED ON in darkness and OFF in brightness 

Diagram construction

As shown in Fig 1, the positive terminal of the battery is connected to a terminal of resistance R1 and resistance R2 and the negative terminal of the battery is directly connected to the ground. The second terminal of Resistance R1 is connected to the base of transistor BC547 (Q2) and terminal 1 of LDR. Terminal 2 of the LDR is directly connected to the ground. The second terminal of Resistance R2 is connected to the anode of the led. Led's cathode is connected to the collector of transistor BC547 (Q2). The emitter of the Transistor BC547 (Q2) is directly connected to the ground.

Fig 1

Working

As you can see in Fig 1 (a) and Fig 1 (b), there is no light in the LDR yet. This means that the resistance of LDR is high and none of it is running current. The transistor BC547 (Q2) will turn ON due to the positive resistance of the battery coming from the positive terminal to voltage resistance R1 and resistance R2 and the transistor BC547 (Q2) getting a positive supply from resistance R1.Our LED will also turn ON as the Transistor BC547 (Q2) becomes ON.

Fig 1(a)

Fig 1(b)

As soon as the light comes in the LDR, the resistance of the LDR will decrease and current flow will start from it. Due to the low resistance in the LDR, the base terminal of the transistor BC547 (Q2) will get ground supply and the transistor BC547 (Q2) will be OFF. Our LED will also be OFF as the Transistor BC547 (Q2) is OFF.

2) LED ON in brightness and OFF in darkness 

Diagram construction

As shown in Fig 2, the positive terminal resistance of the battery is connected to terminal 1 of R4 and LDR2 and the negative terminal of the battery is directly connected to ground. Terminal 2 of LDR2 is connected to the base of transistor BC547 (Q2) and terminal 1 of resistance R3. Terminal 2 of Resistance R3 is directly connected to the ground. The second terminal of Resistance R4 is connected to the anode of the led. Led's cathode is connected to the collector of transistor BC547 (Q2). The emitter of the Transistor BC547 (Q2) is directly connected to the ground.

Fig 2

Working

 As you can see in Fig 2 (a) and Fig 2 (b), there is no light in the LDR yet. This means that the resistance of LDR is high and none of it is running current. Due to no current flow, the transistor BC547 (Q2) will not receive any positive supply and our transistor BC547 (Q2) will remain OFF. Our LED will also remain OFF as the Transistor BC547 (Q2) is OFF. 

Fig 2(a)

Fig 2(b)

As soon as the light comes in the LDR, the resistance of the LDR will decrease and current flow will start from it. Due to current flow in LDR, the base terminal of transistor BC547 (Q2) will get positive supply and transistor BC547 (Q2) will become ON. Our LED will   also turn ON as the Transistor BC547 (Q2) becomes ON.

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Ishan Kumar