_______________________________________________________________SFE3013BASIC ELECTRONICS________________________________________________________________ASSIGNMENTSIMPLE DARK SENSORNAMEMATRICS NUMBER

ERMAHIJRA BINTI MOHAMAD KALAMD20111048003

MOHD HAKIMI BIN ABD HADID20111048005

NUR AFIFAH BINTI OSMAND20111048006

NURUL ATIYAH BINTI RIPIND20111048011

LECTURER: DR. WAN ZUL ADLI BIN WAN MOKHTARDATE OF SUBMISSION: 27 MAY 2014

CONTENTS

1. INTRODUCTION....2

2. SCHEMATIC CIRCUIT....3

3. DATA MEASUREMENT5

4. EXPLANATION ON HOW THE CIRCUIT WORKS6

5. RELATED PICTURES8

6. BIBLIOGRAPHY..11

INTRODUCTION

Simple dark sensor is an example of electronic circuit that use transistor as a switch. It is practically use in automatic street light. It will automatically switch on lights when the sunset and automatically switch off the lights when the sunrise.Here in this case, the simple dark sensor consists of several electronic components. The electronic components are:1) Two resistors (300 Ohm, 100k Ohm)2) Light dependent resistor (LDR)3) Light emitting diode (LED)4) 9V battery5) Transistor (BC238) The idea is when there is light strike to the LDR then the LED does not glow. Meanwhile if there is no light strike to the LDR, the LED will glow.

SCHEMATIC CIRCUIT

Figure 1 Schematic circuit of dark sensor

Figure 2 Direction of current when the LED does not glow

Figure 3 Direction of current when the LED glows

Hint: Current flows

DATA MEASUREMENT

MEASUREMENTLED DOES NOT GLOWLED GLOWS

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Conclusion

EXPLANATION ON HOW THE CIRCUIT WORKSCase 1 LED does not glowThe dark sensor circuit explains the principle of operation of LDR. LDR is an active resistor which is more conductive (less resistance) when get light means more energy. So it sensitive depends upon the intensity of light falling on it. The resistance of LDR decreases when intensity of light falling on it increases.

When the circuit is close, the current at R2 will flow through either the LDR or through the base of transistor. Current will likely flow through both option. But, when there is the light strike to the LDR, the resistance of LDR will reduce. Since the resistance at LDR is smaller compared to the base of transistor, the LDR have a smaller potential and most of the current will likely to flow through LDR compared to the base of transistor. It doesnt mean that there is no current flow through the base of transistor. There is still current that flow through base transistor but the current that flow is smaller than value of 0.7 V and not enough to switch on the transistor. So there is no current flow through the base-emitter junction. Therefore the transistor is switch off and the LED does not glow.

Case 2 LED glowsThedark sensor circuitexplains the principle of operation of LDR. LDR is a device whose sensitivity depends upon the intensity of light falling on it. The resistance of LDR increases when intensity of light falling on it decreases. In the dark or in an absence of light, LDR exhibits a resistance in the range of mega ohms which is a very higher value of resistance. From the schematic circuit, there are two ways for current at resistor 2, to flow either through the base of transistor or through the LDR. When the intensity of light detected by LDR is lower, its resistance become very high and this make the LDR has a higher potential compared to the base of transistor. Since the nature behaviour of current is that it will flow to highest potential to lower potential, the current at resistor 2, will flow through the base of transistor known as base current, . In this experiment, there is current flow through LDR but it is very small value. So, we will consider that there is no current flow through LDR.

Since the input to the base resistor is greater than of the transistor which is 0.7 V, base currents will flow and the transistor is on. In this case, transistor acts as a switch because when the base current that flow into the transistor is higher than the value, the current will flow through. Otherwise, the transistor will cut off.

RELATED PICTURES

Picture 1. One of the group members soldering the components

Picture 2. Close up of the soldering process

Picture 3. Dark sensor circuit (LED does not glow)

Picture 4. Dark sensor circuit (LED glows)

Picture 5. Close up of the dark sensor circuit

Picture 6. Group members

BIBLIOGRAPHYSagar Sapkota. (2010, November 10). Dark sensor making steps-www.buildcuircuit.com. Retrieved May 6, 2014 from http://www.youtube.com/watch?v=ls3cf2f0H1Q&index=4&list=WLThomas Schwenke. (2012, May 26). Transistor (bipolar) How it works! (Animation). Retrieved May 21, 2014 from, https://www.youtube.com/watch?v=9CrcRabTQ0s&list=WL&index=6Transistor basics. Retrieved May 20, 2014, from http://www.physics.unlv.edu/~bill/PHYS483/transbas.pdfWasim Khan. (n.d.). Automatic street light control system. Retrieved May 20, 2014, from http://www.electricaltechnology.org/2013/04/automatic-street-light-control.html

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