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# Light dependent frequency variation using LDR

## Summary

This circuit responds to the change in light illumination in the form of varying frequency which can be observed through a flashing LED as output. The idea of this project is to use an LDR in place of a fixed value resistor which in turn generates a particular frequency. The generated frequency corresponds to the light conditions of the ambience.

This concept finds popular application in motor speed control where the output is connected to motor driver circuit. Disco light is another interesting application of the idea where the lighting pattern keeps on changing.

## Description

The circuit of this project uses NE555 to generate a continuous series of pulses in astable mode. The frequency of these pulses is obtained based on values of two resistors, R1 and R2 and by the capacitor, C. The formula for calculating the frequency of the pulses is give as:

f = 1 / ( ln 2(R1+2*R2)C1)

Here, the R1 resistor is replaced with a light dependent resistor (LDR) whose resistance varies in response to the light falling on it. The resistance value drops as the incident light gets brighter and vice-versa. For varying light intensity, a varying resistance is offered by LDR and thus the frequency generated at the output (Pin 3) of NE555 also varies according to the above relation. Therefore the speed with which the LED

would flash will keep on changing depending upon the intensity of light in the surrounding.

For a typical ½ sec pulse, i.e., 2 Hz frequency, the values can be taken as:

R1 (LDR) = 1k-1M

C1 = 1uF

R2 (preset) = 1M

## Components

Resistor

Resistor is a passive component used to control current in a circuit. Its resistance is given by the ratio of voltage applied across its terminals to the current passing through it. Thus a particular value of resistor, for fixed voltage, limits the current through it. They are omnipresent in electronic circuits.

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Capacitor

Capacitor is a passive component used to store charge. The charge (q) stored in a capacitor is the product of its capacitance (C) value and the voltage (V) applied to it. Capacitors offer infinite reactance to zero frequency so they are used for blocking DC components or bypassing the AC signals. The capacitor undergoes through a recursive cycl...

LED

Light emitting diodes (LEDs) are semiconductor light sources. The light emitted...
LDR

An LDR (Light dependent resistor), as its name suggests, offers resistance in response to the ambient light. The resistance decreases as the intensity of incident light increases, and vice versa. In the absence of light, LDR exhibits a resistance of the order of mega-ohms which decreases to few hundred ohms in the presence of light. It can act as a sensor, since a varying voltage drop can be obtained...

555 Timer IC

555 is a very commonly used IC for generating accurate timing pulses. It is an 8pin timer IC and has mainly two modes of operation: monostable and astable. In monostable mode time delay of the pulses can be precisely controlled by an external resistor and a capacitor whereas in astable mode the frequency & duty cycle are controlled by two external resistors and a...

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