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Presentation on theme: "STREET LIGHTING CIRCUIT Digital Electronics"— Presentation transcript:


2 Description -The circuit diagram present here is that of a street light that automatically switches ON when the night falls and turns OFF when the sun rises. -The circuit uses a Light Dependent Resistor (LDR) to sense the lightLight Dependent Resistor (LDR) 1

3 Tools 1-Transistor(2N2222A). 2-Vcc=6v. 3-Resistors: a- R1=75 b-Re=2.5k c-Ro=1k d-LDR(R2). 4-Light Emitting Diode.(LDR) 2

4 (LDR)Light Dependent Resistor What is LDR? ( Light Dependent Resistor) and symbolizes it short LDR is electrical resistance sensitive to light, changing resistance of about 1,000,000 ohms to zero resistance when light falls on it. Note that the resistor we have its values vary from 300k to 10k when light falls on it. Application Among the most famous applications street lamps, which used to automatic turn them on and off. 3

5 LED(Light Emitting Diode) What is LED? Light Emitting Diode is a light source made of semiconductor materials send light when an electric current passes through it. Characteristics of LED. Consumption of electrical power is low, can be operated with small batteries, a long old, bear the shock, and is small in size. 4

6 Steps: 1.Transistor as a switch. Transistor is used in the saturation region and cut off region. In the saturation region the transistor is in the conduction mode and V ce is nearly equal to 0.In the cut off region the transistor is in the cut off mode and V ce =V cc & I c =I e =0. 2.Voltage divider method. This method allows us to control the base resistance and voltage applied to the base because the base resistance is equal to R 1 //R 2. 5

7 Steps 3.Why do we use R1 at value 75k? Cut off region: Vce=Vcc,Ie=Ic=0,so we assume that R1>R2, so Veq<=0.7v,soR1>=75k Saturation region: Ic=Ie & Vce=0 R1<=162k Comparing the values in the two conditions we find that : Cut off region: R1>=75k Saturation region: R1<=162k So R1s values are between(75k and 162k) We choose R1=75k 6

8 How the circuit works 1- Cut off region. In the daytime,LDR's value is small,where its value is up to 10k,hence when current comes from V cc and because LDR's value is small,current is distributed and current I B is small (low) and V eq is low, so the transistor will be in nonconduction mode (off state) current I e =I c =0 the current will not pass for LED lighting. 7

9 How the circuit works 2-Saturation region At night, LDR's value is big,where its value is up to 300k, hence when current comes from V cc and because LDR's value is big, current does not flow through LDR (open circuit) and all the current will flow toward the transistor, where I B is high and V eq is high, so the transistor is in conduction mode (on state) Current flows and LED lights. 8

10 Summary 1.The street lighting can be controlled by using transistor as switch. 2.Transistor is designed as switch in conduction mode and cut off mode 9

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