2-The circuit uses a Light Dependent Resistor (LDR) to sense the light Description1-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 light1
4(LDR)Light Dependent Resistor 3What 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 300kΩ to 10kΩ when light falls on it.ApplicationAmong the most famous applications street lamps, which used to automatic turn them on and off.
5LED(Light Emitting Diode) 4What 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.
62.Voltage divider method. Steps:51.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 Vce is nearly equal to 0.In the cut off region the transistor is in the cut off mode and Vce=Vcc &Ic=Ie=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 R1//R2.
7Steps www.Engg-know.com 6 3.Why do we use R1 at value 75kΩ? Cut off region:Vce=Vcc ,Ie=Ic=0 ,so we assume that R1>R2 , so Veq<=0.7v,soR1>=75kΩSaturation region:Ic=Ie & Vce= R1<=162kΩComparing the values in the two conditions we find that :Cut off region: R1>=75kΩSaturation region: R1<=162kΩSo R1’s values are between(75kΩ and 162kΩ)We choose R1=75kΩ
8How the circuit works 1- Cut off region. www.Engg-know.com 7 In the daytime ,LDR's value is small ,where its value is up to 10kΩ,hence when current comes from Vcc and because LDR's value is small ,current is distributed and current IB is small (low) and Veq is low , so the transistor will be in nonconduction mode (off state) current Ie=Ic=0 the current will not pass for LED lighting.
9How the circuit works 2-Saturation region www.Engg-know.com 8 At night , LDR's value is big ,where its value is up to 300kΩ , hence when current comes from Vcc 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 IB is high and Veq is high, so the transistor is in conduction mode (on state) Current flows and LED lights.
10Summary91.The street lighting can be controlled by using transistor as switch. 2.Transistor is designed as switch in conduction mode and cut off mode