The circuit diagram of a LDR is shown below. When the light intensity is low, then the resistance of the LDR is high. This stops the current flow to the base terminal of the transistor. So, the LED does not light. So current flows onto the base of the first transistor and then the second transistor. Consequently the LED lights. Here, a preset resistor is used to turn up or down to increase or decrease the resistance. Light dependent resistors have a low cost and simple structure.
These resistors are frequently used as light sensors. These resistors are mainly used when there is a need to sense the absence and presence of the light such as burglar alarm circuits, alarm clock, light intensity meters, etc. LDR resistors mainly involves in various electrical and electronic projects. For better understanding of this concept, here we are explaining some real time projects where the LDR resistors are used.
This security system controlled by an electronic eye project is based on photo sensing arrangement. The proposed system uses a stage ripple carry binary counter to sense the intensity of light using LDR. This project is very useful to deter burglars from shopping malls, banks and jewelry shops, etc.
This project uses a light dependent resistor. So small change in voltage of pin 2 is enough to change the output state of pin 3 from 1 to 0 and 0 to 1. The output has only two states high and low and cannot remain in any intermediate stage. It is power by 9V battery for portable use.
The circuit is economical in power consumption. To detect the present robber we have used LDR and a source of light. An LDR can also be used in a burglar alarm circuit for e.
When the safe is closed it is dark and so the resistance of the LDR is high. If it is opened and the LDR exposed to light the resistance drops allowing current to flow which can then trigger a relay circuit which rings an alarm. This configuration is useful when the current draw at the output has to be changed by light sensitivity.
Such configuration is quite useful in activating relays at the output junction. Pull-down configuration — In this configuration, the LDR sensor is connected between the VCC and output junction and a pull-down resistor is connected between the output junction and ground.
In such configuration, the voltage is first dropped by the LDR and then the voltage is dropped across pull-down resistor and output. When no light is falling on LDR, it has higher resistance and more voltage is dropped across it.
Therefore, a lower voltage so a LOW logic is left to drop at the output junction. When light falls on the LDR, its resistance is reduced and a lower voltage is dropped across it. This leaves a higher voltage so a HIGH logic dropped at the output junction.
This configuration is useful when voltage drop at the output junction has to be changed by light sensitivity. The pull-down resistor used in the circuit has 10K ohm value. Due to pull-down configuration of LDR sensors, when the light is not falling on it, a LOW logic is latched at the data pins of HT12E and same is transmitted as the respective data bit over the RF system.
In the alarm circuit, the transmitted 4-bit data is detected by the RF receiver and passed serially to the HT12D decoder. The LEDs are used to show a visual simulation of the alarm system.
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