LDRs

What is an LDR?

An LDR stands for Light-Dependent Resistor. It is a special type of resistor whose resistance changes depending on the intensity of light falling on it. When the light level changes, the resistance of the LDR changes, which affects the current flowing through a circuit.

LDRs are commonly used in circuits that need to detect light levels, such as automatic lighting systems or light sensors.

How LDRs work

LDRs are made from semiconductor materials. Their key property is that their resistance decreases as the light intensity increases. This happens because more light photons hitting the semiconductor free up more charge carriers, allowing current to flow more easily.

In darkness or low light, the resistance of an LDR is very high, meaning very little current can flow. As the light gets brighter, the resistance drops, allowing more current to pass.

This property makes LDRs useful in applications like automatic street lighting, where the lights turn on at dusk (when resistance is high) and off at dawn (when resistance is low).

For instance, if an LDR has a resistance of 10 k 3 in darkness and 1 k 3 in bright light, the current through the circuit will increase significantly when the light level rises.

LDR characteristics

The change in resistance of an LDR is non-linear, meaning it does not decrease at a constant rate as light intensity increases. Instead, the resistance drops sharply at low light levels and more slowly at higher light levels.

LDRs are sensitive to visible light, making them ideal for detecting changes in daylight or artificial lighting.

Because of their sensitivity, LDRs are often used in light-sensitive switches that automatically turn devices on or off depending on the ambient light.

For example, a security system might use an LDR to detect if a light is switched on or off in a room, triggering an alarm if unexpected changes occur.

Practical uses of LDRs

LDRs have many practical applications, including:

• Street lighting control: LDRs detect when it gets dark outside and automatically switch street lights on, saving energy by turning them off during daylight.
• Camera light meters: LDRs measure the brightness of a scene to help cameras adjust exposure settings for clear photos.
• Security systems: LDRs can detect changes in light levels, such as a door opening or a light being switched on, triggering alarms or notifications.

Example: Calculating current change with an LDR

Example: Suppose an LDR is connected in a simple circuit with a 9 V battery and a fixed resistor of 1 k 3 in series. In darkness, the LDR resistance is 10 k 3, and in bright light, it drops to 1 k 3. Calculate the current in the circuit in both conditions.

First, find the total resistance in each case:

In darkness: $R_{\text{total}} = 10\,000\,\Omega + 1\,000\,\Omega = 11\,000\,\Omega$

In bright light: $R_{\text{total}} = 1\,000\,\Omega + 1\,000\,\Omega = 2\,000\,\Omega$

Using Ohms law, $I = \frac{V}{R}$:

In darkness: $I = \frac{9}{11\,000} = 0.00082\,\text{A} = 0.82\,\text{mA}$

In bright light: $I = \frac{9}{2\,000} = 0.0045\,\text{A} = 4.5\,\text{mA}$

So, the current increases significantly when the light level rises, showing how the LDR changes the circuit behaviour.