What is a Light-Dependent Resistor (LDR)?

An LDR, short for Light Dependent Resistor, is a kind of resistor that works on the principle of photoconductivity. It implies that its resistance changes depending on how much light falls on the device. The resistance of an LDR reduces as more light falls on it and vice-versa. Common uses for LDRs are light sensors, light meters, automatic streetlight control systems, etc. where sensitivity toward illumination is required.

Construction of an LDR 

LDRs can be made from cadmium sulfide (CdS), lead sulfide, lead selenide, indium antimonide or cadmium selenide which are photosensitive semiconductor materials. To ensure that modern LDRs do not harm the environment, lead and cadmium materials are usually not used for their construction.

A Light Dependent Resistor (LDR) typically consists of light-sensitive material mounted on an insulating substrate made of ceramic acting as a base. The material is shaped into a zigzag so that it has a specific power rating and resistance. The zigzag arrangement separates the metal-coated parts into two separate areas which optimizes device performance.

Each side of these zones has ohmic contacts with as little contact resistance as possible so changes in resistance are mainly caused by variations in brightness. For electrical connections to be made, two metal contacts are positioned at either end of the zigzag formation so that they can be integrated into various circuits.

LDRs have a transparent cover over the top of the light-sensitive substance. The covering protects the substance while still allowing light through so that the LDR can work properly by picking up light from its environment.

They are available in sizes that include but are not limited to 5 mm (about 0.2 in), 8 mm (about 0.31 in), 12 mm (about 0.47 in), and 25 mm (about 0.98 in) and are often referred to as ‘light sensors’.

Working Principle of LDR 

A Light Dependent Resistor (LDR) operates using photoconductivity as its working principle. This is the characteristic of certain substances where they change their electrical resistance when light falls on them. Light energy on an LDR's surface causes electrons within the semiconductor material to move. The electrons are excited by this energy so much that they can jump from their usual place in what is called the valence band up to an area known as the conduction band leaving holes behind into which other nearby atoms' electrons may fall if there are any vacancies available there thus creating positive charge centers. This process also increases the number of electric charge carriers that were initially immobile around atoms until this time — which means more charge carriers become free or liberated hence resulting in higher electrical conductivity levels observed within these materials; therefore their resistance becomes very low.

Light intensity and resistance in a Light Dependent Resistor (LDR) are inversely proportional. In the absence of light, the semiconductor material of the LDR has fewer free charge carriers, resulting in high resistance. When the intensity of the light increases, more electrons are excited, which significantly reduces the resistance. This inverse relationship between light intensity and resistance makes LDRs ideal for applications where light sensitivity is crucial, such as in light meters, automatic streetlights, and various light-sensing devices.

Contrasting Light Sensors: Photodiode vs. LDR 

A photodiode and a Light Dependent Resistor (LDR) are both light-sensitive devices, but they operate on different principles and have distinct characteristics. A photodiode is a semiconductor device that converts light into an electrical current. It operates in reverse bias, where the current flow is proportional to the light intensity. Photodiodes are known for their fast response times and high sensitivity, making them suitable for applications requiring precise light measurements, such as optical communication and light sensing in various electronic devices.

On the other hand, an LDR operates based on photoconductivity, where its resistance decreases with increasing light intensity. Unlike photodiodes, LDRs do not generate current but instead vary their resistance in response to light levels. LDRs are generally slower in response time compared to photodiodes and are often used in applications where speed is not critical, such as in light meters, automatic street lighting, and simple light-sensing circuits.

What does an LDR Circuit look like?

In addition to an LDR, a Light Dependent Resistor (LDR) circuit has components like a relay, Darlington pair, diode, and resistors that are used to control the illumination control mechanism based on ambient light conditions.

To power the LDR circuit, a DC voltage is necessary, sourced either from a bridge rectifier circuit or a battery. The bridge rectifier transforms the AC supply into DC, with a step-down transformer lowering the voltage from 230 V to 12 V. Diodes configured in a bridge configuration convert AC to DC. Further regulation reduces the 12 V DC to 6 V DC, serving as the primary power source for the circuit.

In daylight, the LDR exhibits a low resistance, of around 100 Ω, allowing current to pass through it to the ground. This low resistance path prevents sufficient current from flowing to the relay coil, thus keeping the light source switched off. Conversely, at nighttime, the LDR's resistance surges to approximately 20 MΩ. This prevents current from flowing through the LDR, directing it through the alternative low-resistance route, and amplifying the base voltage of the Darlington pair beyond 1.4 V. Triggering the Darlington pair energizes the relay coil, illuminating the light source in darkness.

Characteristics of LDR

Light-dependent resistors (LDRs) exhibit highly responsive behavior to varying light conditions, with resistance decreasing significantly as light intensity increases, often dropping below 1K ohms in strong light. Conversely, in darkness, LDRs exhibit increased resistance, known as dark resistance.

Characterized by non-linearity, LDR sensitivity varies across different wavelengths of light. While some photocells are insensitive to specific wavelengths due to material composition, LDRs typically demonstrate rapid changes in resistance when exposed to light, with a recovery rate of 8 to 12 milliseconds upon light removal.

In comparison to phototransistors and photodiodes, however, LDRs display lower responsiveness. While photodiodes feature a PN-junction semiconductor for light-to-electricity conversion, LDRs lack this junction, functioning passively to convert light into electricity.

Applications: 

Light Dependent Resistors (LDRs) are utilized in diverse applications due to their ability to detect changes in light intensity. They serve as light sensors in electronic devices, control systems for automatic lighting, and exposure regulators in cameras. LDRs contribute to energy efficiency in solar energy systems, enhance security in surveillance setups, and aid in weather monitoring. They find applications in industrial automation, consumer electronics, traffic management, and agriculture. With their reliability and versatility, LDRs play a crucial role in optimizing processes, conserving energy, and ensuring safety across various industries.