Light Dependent Resistor (LDR)-Construction and Working

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Optoelectronics devices are the devices that alters their electrical characteristics, in the presence of visible or invisible light. The various types are light dependent resistor (LDR), photodiode and phototransistor.

Construction of Light Dependent Resistor (LDR)

Light dependent resistor (LDR) are made from cadmium sulphide which contains no or very few free electrons when not illuminated. At this time its resistance is quite high. When it absorbs light, electrons are liberates inside the LDR (Light Dependent Resistor). Hence the conductivity of the material increases with increase in intensity of light.

Cadmium sulphide (CdS) is, therefore, a photoconductor. The approximate relationship between the resistance and illumination is

where E is illumination in lux, R is resistance in ohms, A and α are constants. The value of α depends upon the cadmium sulphide used. It also depends on the manufacturing processes.

Light Dependent Resistor
Light Dependent Resistor (LDR)

The basic symbol for LDR is shown in figure above. The Light Dependent Resistor (LDR) consists of a pair of metal film contacts separated by a snake like track of cadmium sulphide film. It is design to provide the maximum possible contact area with the two metal films.

The structure of Light Dependent Resistor (LDR) is housed in a clear plastic or resin case. It is to provide free access to external light. Practical Light Dependent Resistors (LDRs) are readliy available in a wide variety of sizes and package styles. The most popular size of Light Depenedent Resistor (LDR) having a face diameter of roughly 10 mm.

Some Important Terminologies of (LDR)

Spectral Response: The resistors are only light dependent over a limited range of wave-lengths. Light Dependent resistor (LDRs) have their maximum response at about 680 nm.

Temperature Dependency: The electrons are excited not only by photons but also by thermal agitation. Therefore the dark resistance is not infinite at normal temperatures. It increases with increase in ambient temperature. And it can be reduced by cooling the device. The temperature can also affect the resistance under illumination. At practical illumination levels and normal ambient temperatures the temperature coefficient is, however, very small and can be neglected.

Recovery Rate: When an Light Dependent Resistor (LDR) is brought from a certain illuminating level into total darkness. Then the resistance does not increase immediately to the dark value. The recovery rate is specified in k-ohm/second and for current LDR types. And it is more than 200 k-ohm/sec (during the first 20 seconds starting at a light level of 1,000 lux). The recovery rate is very much greater in the reverse direction. For example going from darkness to illumination level of 300 lux. It only takes less than 10 ms to reach a resistance which corresponds with a light level of 400 lux.

LDRs are inexpensive, sensitive, and they are readily available devices. They have very good power and voltage handling capabilities. It is very similar to those of a conventional resistor. Their only significant defect is that they are fairly low acting, taking tens or hundreds of milli-seconds to respond to sudden changes in light level.

Application of Light Dependent Resistor (LDR)

Practical applications of Light dependent Resistor (LDR) include light and dark activated switches and alarms, light beam alarms and reflective smoke alarms etc.

Over Light detector
Over Light detector

An overlight detector is there in figure above. The LDR is very similar to variable resistor whose resistance decreases with the increase in light intensity. When the light falls on an Light Dependent Resistor (LDR) having normal intensity. Then its resistance is large enough and the voltage across R1, is insufficient to trigger the SCR. However, when light falling on LDR is of large intensity. Then the resistance of Light Dependent Resistor (LDR) falls and voltage drop across R1, becomes large enough to trigger the SCR.

Consequently, the buzzer sounds the alarm. Now, if the strong light disappears, the buzzer continues to sound the alarm. The reason behind this is once the SCR is fired, the gate looses all control. And the buzzer sounds the alarm.

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