The diodes are connected in the form of a bridge used to alter the AC voltage into DC. The voltage regulator is used to change the 12v DC-6v DC, and then, this DC voltage is supplied to the entire circuit. A v AC supply for both the bridge rectifier and the load is to be kept continuously for continuous operation of the light sensor circuit. This is due to the resistance offered by the light-dependent resistor in the daytime or when the light falls on the LDR, then it is less compared to the resistance of the remaining part of the sensor circuit.
We are alert of the principle of current, that the flow of current always flows in the path of low resistance. Therefore, the relay coil does not get sufficient supplies to get strengthened.
Hence, the light is switched off in the daylight. Thus, due to the high resistance of the resistor , the flow of current is low or almost zero. Now, the flow of current through the low-resistance lane such that it increases the base voltage of the Darlington pair to reach more than 1. As the Darlington pair transistor is triggered, the relay coil acquires enough supply to get energized, and hence, the light switches in the nighttime.
Devices that are made with different materials will respond in a different way to the wavelengths of light, which means that the various components can be employed for a variety of applications. But once working with IR, we should take care to evade heat build-up caused due to the heating effect of the radiation.
Latency is the main aspect associated with an LDR which means the time taken to respond to the components for any changes. So, this feature is particularly significant for designing a circuit. It takes a visible amount of time from any changes within a light level before the LDR attains its last value for the new level of light. So for this reason, the light-dependent resistor is not a better choice wherever there is a reasonably quick changing of light values.
Once the light changes occur over a certain time period then they are above sufficient. The recovery rate of resistance is nothing but the rate at which point the resistance changes. Usually, The LDR responds in a few tens of milli secs once the light is given after complete darkness, however, once the light is removed, then it can take up to a second.
The LDR specifications mainly include maximum power dissipation, maximum operating voltage, peak wavelength, dark resistance, etc. The values of these specifications mentioned below.
The light-dependent resistor is very responsive to light. When the light is stronger, then the resistance is lower which means, when the light intensity increases then the value of resistance for the LDR will be decreased drastically to below 1K.
When the light drops on LDR, the resistance will be decreased and when the resistor is placed in the dark then the resistance will be increased which is called dark resistance. If any device absorbs light then its resistance will be reduced radically. Some kinds of photocells are not at all sensitive to a specific range of wavelengths because it depends on the used material. In this way it follows the same convention used for photodiode and phototransistor circuit symbols where arrows are used to show the light falling on these components.
It is relatively easy to understand the basics of how an LDR works without delving into complicated explanations. It is first necessary to understand that an electrical current consists of the movement of electrons within a material. Good conductors have a large number of free electrons that can drift in a given direction under the action of a potential difference. Insulators with a high resistance have very few free electrons, and therefore it is hard to make the them move and hence a current to flow.
An LDR or photoresistor is made any semiconductor material with a high resistance. It has a high resistance because there are very few electrons that are free and able to move - the vast majority of the electrons are locked into the crystal lattice and unable to move. Therefore in this state there is a high LDR resistance.
As light falls on the semiconductor, the light photons are absorbed by the semiconductor lattice and some of their energy is transferred to the electrons. This gives some of them sufficient energy to break free from the crystal lattice so that they can then conduct electricity. This results in a lowering of the resistance of the semiconductor and hence the overall LDR resistance. The process is progressive, and as more light shines on the LDR semiconductor, so more electrons are released to conduct electricity and the resistance falls further.
Structurally the photoresistor is a light sensitive resistor that has a horizontal body that is exposed to light. The active semiconductor region is normally deposited onto a semi-insulating substrate and the active region is normally lightly doped. In many discrete photoresistor devices, an interdigital pattern is used to increase the area of the photoresistor that is exposed to light.
The pattern is cut in the metallisation on the surface of the active area and this lets the light through. The two metallise areas act as the two contacts for the resistor. This area has to be made relatively large because the resistance of the contact to the active area needs to be minimised.
This type of structure is widely used for many small photoresistors or light dependent resistors that are seen. The interdigital pattern is quite recognisable. Each material gives different properties in terms of the wavelength of sensitivity, etc. In view of the environmental concerns of using Cadmium, this material is not used for product in Europe.
Regardless of the type of light dependent resistor or photoresistor, both types exhibit an increase in conductivity or fall in resistance with increasing levels of incident light. The sensitivity of photoresistors is shown to vary with the wavelength of the light that is impacting the sensitive area of the device.
The effect is very marked and it is found that if the wavelength is outside a given range then there is no noticeable effect. Devices made from different materials respond differently to light of different wavelengths, and this means that the different electronics components can be used for different applications. It is also found that extrinsic photoresists tend to be more sensitive to longer wavelength light and can be used for infrared. However when working with infrared, care must be taken to avoid heat build-up caused but he elating effect of the radiation.
One important aspect associated with photoresistors or light dependent resistors is that of the latency, or the time taken for the electronic component to respond to any changes.
This aspect can be particularly important for a circuit design. However when the light changes take place over a period of time they are more than adequate. It works on the principle of photoconductivity whenever the light on its photoconductive material falls it absorbs its energy and the electrons of that photoconductive material that is in the valence band get excited and go to the conduction band and thus increases the conductivity as per the increase in light intensity.
Also, the energy in incident light should be greater than the bandgap gap energy so that the electrons from the valence band got excited and go to conduction band. The LDR has the highest resistance in dark around Ohm and this resistance decreases with the increase in Light. Photodiodes give quick response and are used where needed to detect quick response on and off like in optical communication, optoisolators.
The photodiodes are semiconductor devices and work on PN junctions. The photodiode works on the principle of converting the light energy into electric energy while the LDR is a resistance and its resistance decreases with the increase in light intensity. This type of photoresistor is made with pure semiconductors without any doping. This kind of photoresistor uses pure semiconductors like silicon and germanium. This type of photoresistor uses the doped semiconductor; this means some impurities are mixed with the semiconductor to make this photoresistor.
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