ECE 2

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LIGHT DETECTOR USING LDR

6/9/2014

INTRODUCTION

A photoresistor or light dependent resistor (LDR) is a resistor whose resistance decreases with increasing incident light intensity; in other words, it exhibits photoconductivity. It can also be referred to as a photoconductor or CdS device, from

"cadmium sulfide," which is the material from which the device is made and that actually exhibits the variation in resistance with light level.

A photo resistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

OBJECTIVES Demonstration of working of light dependent resistor and Here by explaining

application of photocell with two light sensitive circuits. Finally analyzing its advantages and disadvantages in real life.

MATERIALS REQUIRED

A few Jumper WireS 9 Volts Battery 9 Volts Battery Clip Light Dependent Resistor (LDR) Light Emitting Diode (LED) with any color of choice TLC3704 Quad Comparator (only one of its four comparators will be used)

(Alternatively, you can use the single LM311N Comparator with 8pin) 3362P-103-ND 10K Ohms Variable Resistor 1K Ohms (Brown-Black-Red) Resistor X2 330 Ohms (Orange-Orange-Brown) Resistor

Procedure

Insert first transistor Q1-BC547 (NPN) on breadboard (or general PCB) as shown in the circuit diagram 1.

Connect another transistor Q2- BC547 (NPN) on breadboard as in step 1. Connect  wires across emitter pin of both transistors and –ve terminal of battery

(lowest/ bottom row of breadboard.) Connect  a wire across Collector pin of transistor Q1 and Base pin of transistor

Q2. Connect a resistor 1K across positive terminal of battery (topmost row of

breadboard) and Collector pin of transistor Q1. Connect  Light Dependent Resistor (LDR) across positive terminal of battery

(topmost row of breadboard) and base terminal of transistor Q1. insert a resistor- 330 Ohm across base pin of transistor Q1 and negative terminal

of battery (lowest bottom row of breadboard). Connect a resistor 330R across positive terminal of battery (topmost row of

breadboard) and anode terminal of LED (Light emitting diode) & Connect the cathode terminal of LED to Collector pin of transistor Q2.

WORKING

There are two basic circuits using light dependent resistors - the first is activated by darkness, the second is activated by light. The two circuits are very similar and just require an LDR, some standard resistors, a variable resistor (aka potentiometer), and any small signal transistor

In the circuit diagram above, the relay up whenever the LDR is in darkness. The 10K variable resistor is used to fine-tune the level of darkness required before the Relay up. The 10K standard resistor can be changed as required to achieve the desired effect, although any replacement must be at least 1K to protect the transistor from being damaged by excessive current.

FEATURES Advantages

1. Wide spectral response

2. Low cost

3. Wide ambient temperature range

Disadvantages

1. Very in accurate

2. Batch variation can be really large

CIRCUIT DIAGRAM

APPLICATIONS

Inexpensive cadmium sulphide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarm devices, and outdoor clocks.

They are also used in some dynamic compressors together with a small incandescent lamp or light emitting diode to control gain reduction.

LDR is used in light interruption detectors, automatic light circuits, and logarithmic law

photographic light meters.