LIGHT DEPENDENT RESISTORS

LDRs or Light Dependent Resistors are very useful especially in light/dark sensor circuits. Normally the resistance of an LDR is very high, sometimes as high as 1000 000 ohms, but when they are illuminated with light resistance drops dramatically.

What is a Light Dependent Resistor?

A light dependent resistor is a small, round semiconductor. Light dependent resistors are used to re-charge a light during different changes in the light, or they are made to turn a light on during certain changes in lights. One of the most common uses for light dependent resistors is in traffic lights. The light dependent resistor controls a built in heater inside the traffic light, and causes it to recharge over night so that the light never dies. Other common places to find light dependent resistors are in: infrared detectors, clocks and security alarms.

Identification

A light dependent resistor is shaped like a quarter. They are small, and can be nearly any size. Other names for light dependent resistors are: photoconductors, photo resistor, or a CdS cell. There are black lines on one side of the light dependent resistor. The overall color of a light dependent resistor is gold. Usually other electrical components are attached to the light dependent resistor by metal tubes soldered to the sides of the light dependent resistor.

Function

The main purpose of a light dependent resistor is to change the brightness of a light in different weather conditions. This can easily be explained with the use of a watch. Some watches start to glow in the dark so that it is possible to see the time without having to press any buttons. It is the light dependent resistor that allows the watch to know when it has gotten dark, and change the emissions level of the light at that time. Traffic lights use this principle as well but their lights have to be brighter in the day time.

Considerations

Light dependent resistors have become very useful to the world. Without them lights would have to be on all the time, or they would have to be manually adjusted. A light dependent resistor saves money and time for any creation that needs a change in light. Another feature of the light dependent resistor is that it can be programmed to turn on with changes in movements. This is an extremely useful feature that many security systems employ. Security would be harder without light dependent resistors.

Expert Insight

It is possible to build a light dependent resistor into an existing light circuit. There are many electrical plans that outline how to install one. Usually the sign for a light dependent resistor on these plans is marked by a rectangle with two arrows pointing down to it. This shows the placement of the light dependent resistor in the circuit so that it will work properly. Usually only an electrician can build new circuits, however.

Benefits

There are many great benefits to light dependent resistors. They allow less power to be used in many different kinds of lights. They help lights last much longer. They can be trigged by several different kinds of triggers, which is very useful for motion lights and security systems. They are also very useful in watches and cars so that the lights can turn on automatically when it becomes dark. There are a lot of things that light dependent resistors can do.

Measure Light Intensity using Light Dependent Resistor (LDR)

The resistance of the Light Dependent Resistor (LDR) varies according

to the amount of light that falls on it. The relationship between the resistance

RL and light intensity Lux for a typical LDR is

RL = 500 / Lux Kohm

With the LDR connected to 5V through a 3.3K resistor, the output

voltage of the LDR is

Vo = 5*RL / (RL+3.3)

Reworking the equation, we obtain the light intensity

Lux = (2500/Vo - 500)/3.3

555 timer IC

The 555 timer IC is an amazingly simple yet versatile device. It has been around now for many years and has been reworked into a number of different technologies. The two primary versions today are the original bipolar design and the more recent CMOS equivalent. These differences primarily affect the amount of power they require and their maximum frequency of operation; they are pin-compatible and functionally interchangeable.

This page contains only a description of the 555 timer IC itself. Functional circuits and a few of the very wide range of its possible applications will be covered in additional pages in this category.

The figure to the right shows the functional block diagram of the 555 timer IC. The IC is available in either an 8-pin round TO3-style can or an 8-pin mini-DIP package. In either case, the pin connections are as follows:

1. Ground.  2. Trigger input.  3. Output.  4. Reset input.  5. Control voltage.  6. Threshhold input.  7. Discharge.  8. +VCC. +5 to +15 volts in normal use. 

The operation of the 555 timer revolves around the three resistors that form a voltage divider across the power supply, and the two

comparators connected to this voltage divider. The IC is quiescent so long as the trigger input (pin 2) remains at +VCC and the threshhold input (pin 6) is at ground. Assume the reset input (pin 4) is also at +VCC and therefore inactive, and that the control voltage input (pin 5) is unconnected. Under these conditions, the output (pin 3) is at ground and the discharge transistor (pin 7) is turned on, thus grounding whatever is connected to this pin.

The three resistors in the voltage divider all have the same value (5K in the bipolar version of this IC), so the comparator reference voltages are 1/3 and 2/3 of the supply voltage, whatever that may be. The control voltage input at pin 5 can directly affect this relationship, although most of the time this pin is unused.

The internal flip-flop changes state when the trigger input at pin 2 is pulled down below +VCC/3. When this occurs, the output (pin 3) changes state to +VCC and the discharge transistor (pin 7) is turned off. The trigger input can now return to +VCC; it will not affect the state of the IC.

However, if the threshhold input (pin 6) is now raised above (2/3)+VCC, the output will return to ground and the discharge transistor will be turned on again. When the threshhold input returns to ground, the IC will remain in this state, which was the original state when we started this analysis.

The easiest way to allow the threshhold voltage (pin 6) to gradually rise to (2/3)+VCC is to connect it to a capacitor being allowed to charge through a resistor. In this way we can adjust the R and C values for almost any time interval we might want.

The 555 can operate in either monostable or astable mode, depending on the connections to and the arrangement of the external components. Thus, it can either produce a single pulse when triggered, or it can produce a continuous pulse train as long as it remains powered.

light-emitting diode ( LED )

(pronounced /ˌɛl.iːˈdiː/ [1] ) is a semiconductor light source. LEDs are used as indicator lamps in many devices, and are increasingly used for lighting. Introduced as a practical electronic component in 1962,[2] early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet and infrared wavelengths, with very high brightness.

The LED is based on the semiconductor diode. When a diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is usually small in area (less than 1 mm2), and integrated optical components are used to shape its radiation pattern and assist in reflection.[3]

LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. However, they are relatively expensive and require more precise current and heat management than traditional light sources. Current LED products for general lighting are more expensive to buy than fluorescent lamp sources of comparable output.

They also enjoy use in applications as diverse as replacements for traditional light sources in automotive lighting (particularly indicators) and in traffic signals. Airbus uses LED lighting in their A320 Enhanced since 2007, and Boeing plans its use in the 787. The compact size of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are useful in advanced communications technology

ELECTRONIC LETTER BOX

INTRODUCTION

The ELECTRONIC LETTER BOX is a very useful gadget for homes and offices the circuit uses single Monolithic -555 in dual in line plastic package and a few external components.The main advantage of the letter box is we can find out the presence and absence of letter inside the box without opening the box.

PARTS LISTINTEGRATED CIRCUIT IC1---555DIODES D1-Red L.E.D, D2-Green LEDRESISTORS R1-220E/.25W, R2-220E/.25WPRESET VR1-100KE

BULB B1-9V BULBLIGHT DEPENDED RESITOR (LDR)BATTERY BI- 9V

WORKING

The L.D.R. is the most important part of this circuit.The Resistance of L.D.R.is miniumWhen the light falling on is maximum and its resistance is maximum when the lightfalling on is minimum.LD.R is connected to pin no2 of C-555.When the letter is insidethe letter box,the intencity of light falling on L.D.R.is minmium, so resistance is high , the voltage across the threshold pin no 6 goes above 2/3 VCC. The internal flip-flop resets and the output

goes low and the red L.E.D is In the forwared biased condition emitting red light.In the absence of letter in the letter box, light falling on L.D.R. is maximum so resistance is low and the voltage across the threshold pin goes below 2/3 VCC. The internal flip-flop sets and its output goes high which forwared bias the green L.E.D. emitting green light.

ELECTRONIC

LETTER BOX

MADE BY

ATUL KUMAR (914003)SUPERVISED BY

ANCHAL SRIVASTAVA

2009- 2010