How does an ldr

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. It is not uncommon for the values of resistance of an LDR or photoresistor to be several megohms in darkness and then to fall to a few hundred ohms in bright light. The sensitivity of light dependent resistors or photoresistors also varies with the wavelength of the incident light.

LDRs are made from semiconductor materials to enable them to have their light sensitive properties. Many materials can be used, but one popular material for these photoresistors is cadmium sulphide, CdS, although the use of these cells is now restricted in Europe because of environmental issues with the use of cadmium. Similarly cadmium CdSe is also restricted. Other materials that can be used include lead sulphide, PbS and indium antimonide, InSb. Although a semiconductor material is used for these photoresistors, they are purely passive devices because they do not possess a PN junction, and this separates them from other photodetectors like photodiodes and phototransistors.

The LDR symbol used in electronic circuits is based around the resistor circuit symbol, but shows the light, in the form of arrows shining on it.

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. Majority of street lights, outdoor lights, and a number of indoor home appliances are typically operated and maintained manually in many occasions. This is not only risky, however additionally it leads to wastage of power with the negligence of personnel or uncommon circumstances in controlling these electrical appliances ON and OFF.

This article discusses in brief about what is a light dependent resistor, how to make a light dependent resistor circuit and its applications. An LDR or light dependent resistor is also known as photo resistor , photocell, photoconductor. It is a one type of resistor whose resistance varies depending on the amount of light falling on its surface. When the light falls on the resistor, then the resistance changes.

These resistors are often used in many circuits where it is required to sense the presence of light. These resistors have a variety of functions and resistance. A typical light dependent resistor has a resistance in the darkness of 1MOhm, and in the brightness a resistance of a couple of KOhm. This resistor works on the principle of photo conductivity. It is nothing but, when the light falls on its surface, then the material conductivity reduces and also the electrons in the valence band of the device are excited to the conduction band.

These photons in the incident light must have energy greater than the band gap of the semiconductor material. This makes the electrons to jump from the valence band to conduction. These devices depend on the light, when light falls on the LDR then the resistance decreases, and increases in the dark.

When a LDR is kept in the dark place, its resistance is high and, when the LDR is kept in the light its resistance will decrease. The figure below shows the curve between resistance Vs illumination curve for a particular light dependent resistor. The passive component is basically a resistor whose resistance value decreases when the intensity of light decreases. This optoelectronic device is mostly used in light varying sensor circuit, and light and dark activated switching circuits.

Some of its applications include camera light meters, street lights, clock radios, light beam alarms, reflective smoke alarms, and outdoor clocks. Automatic Street Light Circuit. Simple Fire Alarm Circuit. Light Activated Switch Circuit. Night Security Light.