Light dependent frequency variation using LDR

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.

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