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INTRODUCTION
LIGHT DEPENDENT RESISTOR | LDR AND WORKING PRINCIPLE OF LDR
What is a Light Dependent Resistor or a Photo Resistor?
A Light Dependent Resistor (LDR) or a photo resistor is a device whose resistivity is a function of the incident electromagnetic radiation. Hence, they are light sensitive devices. They are also called as photo conductors, photo conductive cells or simply photocells. They are made up of semiconductor materials having high resistance. There are many different symbols used to indicate a LDR, one of the most commonly used symbol is shown in the figure below. The arrow indicates light falling on it.
Working Principle of LDR
A Light Dependent Resistor works on the principle of photo conductivity.
Photo conductivity is an optical phenomenon in which the materials conductivity is increased when light is absorbed by the material. When light falls i.e. when the photons fall on the device, the electrons in the valence band of the semiconductor material are excited to the conduction band. These photons in the incident light should have energy greater than the band gap of the semiconductor material to make the electrons jump from the valence band to the conduction band. Hence when light having enough energy strikes on the device, more and more electrons are excited to the conduction band which results in large number of charge carriers. The result of this process is more and more current starts flowing throgh the device when the circuit is closed and hence it is said that the resistance of the device has been decreased. This is the most common working principle of LDR
Characteristics of LDR
LDR’s are light dependent devices whose resistance is decreased when light falls on them and that is increased in the dark. When a Light Dependent Resistor is kept in dark, its resistance is very high. This resistance is called as dark resistance. It can be as high as 1012 Ω and if the device is allowed to absorb light its resistance will be decreased drastically. If a constant voltage is applied to it and intensity of light is increased the current starts increasing. Figure below shows resistance vs. illumination curve for a particular LDR.
Photocells or LDR’s are non linear devices. There sensitivity varies with the wavelength of light incident on them. Some photocells might not at all response to a certain range of wavelengths. Based on the material used different cells have different spectral response curves.
When light is incident on a photocell it usually takes about 8 to 12 ms for the change in resistance to take place, while it takes one or more seconds for the resistance to rise back again to its initial value after removal of light. This phenomenon is called as resistance recovery rate. This property is used in audio compressors. Also, LDR’s are less sensitive than photo diodes and photo transistor. (A photo diode and a photocell (LDR) are not the same, a photo-diode is a p-n junction semiconductor device that converts light to electricity, whereas a photocell is a passive device, there is no p-n junction in this nor it “converts” light to electricity). Types of Light Dependent Resistors: Based on the materials used they are classified as:
• Intrinsic photo resistors (Un doped semiconductor): These are made of pure semiconductor materials such as silicon or germanium. Electrons get excited from valance band to conduction band when photons of enough energy fall on it and number charge carriers is increased.
• Extrinsic photo resistors: These are semiconductor materials doped with impurities which are called as dopants. Theses dopants create new energy bands above the valence band which are filled with electrons. Hence this reduces the band gap and less energy is required in exciting them. Extrinsic photo resistors are generally used for long wavelengths.
Construction of a Photocell
The structure of a light dependent resistor consists of a light sensitive material which is deposited on an insulating substrate such as ceramic. The material is deposited in zigzag pattern in order to obtain the desired resistance & power rating. This zigzag area separates the metal deposited areas into two regions. Then the ohmic contacts are made on the either sides of the area. The resistances of these contacts should be as less as possible to make sure that the resistance mainly changes due to the effect of light only. Materials normally used are cadmium sulphide, cadmium selenide, indium antimonide and cadmium sulphonide. The use of lead and cadmium is avoided as they are harmful to the environment.
Applications of LDR
LDR’s have low cost and simple structure. They are often used as light sensors. They are used when there is a need to detect absences or presences of light like in a camera light meter. Used in street lamps, alarm clock, burglar alarm circuits, light intensity meters, for counting the packages moving on a conveyor belt, etc.
DESCRIPTION
This LDR based alarm can be used as wake up alarm. The Alarm turns on in the morning automatically and turns off only when manually switched off ,so you will have to wake up. It is a simple LDR based light sensor circuit
LDR or Light Dependent Resistor is a kind of variable resistor and its resistance depends on the intensity of light falling on it. In complete dark, its resistance is as high as 1 Meg Ohm, but it reduces to 100 ohms or less in full light. So it is an ideal device to use as light switch. Sensitivity of LDR depends on its size also. Commonly available sizes are 3mm, 5mm, 10mm etc. But costly types with greater sensitivity are also available.
When the circuit is place in dark, near the window, LDR fails to conduct and T1 will not get base bias. So it remains off and buzzer will be silent. When the sunlight falls on LDR, IN the morning, LDR conducts and provides base bias to T1. T1 then conducts to activate the buzzer. Capacitor C1 is provided for the clean switching of T1. T1 will conduct only when C1 is charged fully. This prevents chirping sound during light transition.You can use an easily available Music buzzer to get melody tone.
CONSTRUCTION AND WORKING OF MORNING ALARM USING LDR
This is Morning sun alarm circuit using IC-4011 that we makes in hobby time or toys for my son but really uses. It will have sound alarm when morning like a normal clock alarm with lighting sensor same clock. We use a IC-4011 CMOS only one and a few components also can generate pulsating tone like a general clock.
How This Circuit works.
As Figure 1 This circuit use the IC-4011 quad nand gate as main of circuit, it includes for separate positive logic nand gate on a single silicon chip. In circuit diagram consist of an oscillator circuit in a stable multi vibrator 2 set.
First set include of IC1a and IC1b, then second set include of IC1c and IC1d.
A R1 is LDR (Light Depended Resistor) or photo resistor which is a resistor type. The resistance of a photo resistor changes with the light intensity to strike them.
If no light strikes it.
This resistor R1 may have high resistance rises between 500K to 600K. And if have normal light (Common Fluorescent light.) The resistance will reduce is about 20K – 30K.
On the condition that LDR is getting light, because of its high resistance to more than the value of R2. The voltage at pin 1 of IC1a will lower half of power supply. This cause first oscillator does not work. So the second oscillator also not works. Therefore without sound out of speaker.
When LDR get light its resistance will reduce and voltage drop across it will reduce making voltage across R2 rises unless more than half of the power supply. It cause the first oscillator working to generate the output frequency.
So this frequency can determine with Resistor R1 and Capacitors C1
Frequency output = F = 1/(2.2R1C1)
Which we use value R1 and C1 as circuit above will get frequency about 4 Hz. At the same time the output of IC1b is state “1” will cause the second oscillator (include with IC1c, IC1d and R4, C2) cause have frequency about 900 Hz.