Light sensor (LDR sensor)

A light sensor generates an output signal indicating the intensity of light by measuring the radiant energy that exists in a very narrow range of frequencies basically called "light", and which ranges in frequency from "Infrared" to "Visible" up to "Ultraviolet" light spectrum. The light sensor is a passive devices that convert this "light energy" whether visible or in the infrared parts of the spectrum into an electrical signal output. Light sensors are more commonly known as "Photoelectric Devices" or "Photo Sensors" because the convert light energy (photons) into electricity (electrons).

Light dependent resistors (LDR) are very useful especially in light or dark sensor circuit. Normally the resistance of an LDR is high, sometimes as high 1000kΩ, but when they are illuminated with light resistance drops dramatically. There are just two ways of constructing the voltage divider, with the LDR at the top, or with the LDR at the bottom:

In LDR sensor operation, means light must shine into the light dependent resistance for the circuit to be activated. When the light level is low the resistance of the LDR is high. This prevents current from flowing to the base of the transistors. Consequently the lamp does not light. However, when light shines onto the LDR its resistance falls and current flows into the base of the first transistor and then the second transistor and automatically turning the lamp on. The preset resistor can be turned up or down to increase or decrease resistance, in this way it can make the circuit more or less sensitive. During troubleshooting the LDR sensor, i have tabulated all the data of an output voltage value with different resistance value produce by the LDR sensor.

Based on the figure below, the input of the LDR is only 5 volts and the value for the resistor is 10kΩ. Generally, this sensor was constructed based on voltage divider rules and the voltage output is depending on the change in resistance value of the LDR sensor. The input for IC741 has two input pins from one pin from the voltage output and another one is from the ground. Once the voltage enters the IC741, the IC741 will amplify the voltage by multiplying it gain (dB). Output for IC741 will inject to the relay and automatically switch on the lamp.

Figure above shows the circuit of light sensor (LDR circuit)