February 7, 2023 / 88 /

What is a photoresistor and what is it used for?

Light is a form of electromagnetic radiation that is visible to the human eye. Light travels in straight lines and has both wave-like and particle-like properties. The effect of light can cause changes in the behavior of electrons.

It can also transfer energy to electrons and cause them to become more mobile or to change their state. This is what makes a photoresistor function.

What is a Photoresistor?

A photoresistor, also known as a light-dependent resistor (LDR), is a type of electronic component that can be used to detect light.

It is made of a material that changes its electrical resistance based on the amount of light it receives. When light shines on the photoresistor, its resistance decreases, allowing more electrical current to flow through it. When there is less light, the resistance increases and less current flows. This change in resistance can be measured and used to determine the level of light.

How does a photoresistor look like?

Symbol of a photoresistor

The symbol of a photoresistor denotes the combination of a resistor and light rays or photons.

How photoresistor works?

A photoresistor works on the principle of photoconductivity. Photoconductivity is a phenomenon where the electrical conductivity of a material increases when exposed to light. In the case of a photoresistor, it is made of a material (such as cadmium sulfide or cadmium selenide) that changes its resistance when exposed to light.

When light hits the photoresistor, it creates electron-hole pairs in the material, which facilitates the passage of electric current through the material. This lowers the resistance of the photoresistor and allows more current to flow through it. As the level of light decreases, the number of electron-hole pairs decreases and the resistance of the photoresistor increases, which reduces the flow of current.

In a circuit, the photoresistor is usually connected in series with a fixed resistor, and the voltage source is connected to two resistors. The resistance of the photoresistor and the fixed resistor, form a voltage divider that generates a voltage proportional to the light level.

The voltage across the photoresistor can be measured and used to drive other components in the circuit, such as LEDs or buzzers, to perform different functions depending on the light level.

Types of photoresistors

There are two main types of photoresistors:

1. Cadmium Sulfide (CdS) Photoresistor: This is the most common type of photoresistor. It is made of a cadmium sulfide material that makes it resistant to light.
   CdS photoresistors are relatively inexpensive, easy to use, and widely available, making them a popular choice for many light sensing applications.
   However, they can be affected by temperature changes and aging, which can reduce their accuracy over time.
2. Metal Oxide (MOX) Photoresistor: This type of photoresistor is made of a metal oxide material, such as vanadium oxide or indium tin oxide, that changes resistance when exposed to light.
   MOX photoresistors are more sensitive to light than CdS photoresistors and are not as affected by temperature changes or aging.
   They are usually more expensive than CdS photoresistors, but are more accurate and have a longer lifetime.

There are other types of photoresistors, such as phototransistors, which are a type of bipolar transistor that uses light to control its current flow, and photodiodes, which are a type of diode that generates a current when exposed to light.

However, this type of photoresistor is less commonly used in photodetection applications than CdS and MOX photoresistors.

Applications of photoresistors

Photoresistors have a wide range of applications because they can detect changes in light levels. Some common applications include:

1. Light Dimmer Switches: Light dimmers use photoresistors to control the brightness of lamps and lights. When the light level changes, the resistance of the light resistor changes, which can be used to adjust the brightness of the lamp.
2. Automatic streetlights: Photoresistors are used in automatic streetlights to turn the light on and off according to the ambient light level. As the light level decreases, the resistance of the photoresistor increases, which triggers the illumination of the streetlight.
3. Camera Flashes: Camera flashes use photoresistors to determine ambient light levels and adjust the flash accordingly. The light resistor is used to measure the light level in the room, and the camera flash will fire if the light level is too low.
4. Security systems: Some security systems use photoresistors to detect changes in light intensity and trigger an alarm. For example, when the security system is armed and the light level changes (e.g., when a window is broken), the resistance of the photoresistor changes, triggering an alarm.
5. Robotics: Photoresistors can be used in robots to detect changes in light levels and navigate the robot. For example, the robot can use photoresistors to detect when it has entered a dark room and adjust its behavior accordingly.

Other Applications: Photoresistors are also used in many other applications, including temperature, humidity, and gas detection. They are also used in the automotive industry to detect changes in light levels and control functions such as headlights and dashboard lighting.

Beginners projects using photoresistor

Here are a list of beginners-friendly projects you can work on using photoresistor:

1. Light Activated LED: Connect a photoresistor and LED to the circuit and turn on the LED when the light level changes.
2. Light-Sensitive Night Light: Create a night light that turns on when the room gets dark and turns off when there is light.
3. Light Sensitive Alarm: Create an alarm that goes off when the light level changes.
4. Light Sensitive Music Player: Connect a light resistor to the music player and let the music play when the light level changes.
5. Light-Sensitive Fan: Build a fan that turns on when the light level decreases and turns off when it increases.
6. Light-Sensitive Lamp Dimming: Connect a photoresistor to the lamp and use it to adjust the brightness of the lamp.
7. Photosensitive Camera Trigger: Connect a photoresistor to the camera and use it to trigger the camera’s flash.
8. Light Sensitive Plant Watering System: Create a plant watering system that turns on when light levels decrease and turns off when light levels increase.
9. Photosensitive Temperature Indicator: Connect a photoresistor and a temperature sensor to the circuit and use them to display the temperature on the LED screen.
10. Light Sensitive Weather Station: Build a weather station that measures light and displays current weather conditions.
11. Photosensitive Thermometer: Connect a photoresistor and a thermometer to the circuit and use them to display the temperature on the LED screen.
12. Light-Sensitive Light Meter: Build a light meter that measures the light level and displays the measurement result on an LED screen.
13. Photosensitive Humidity Sensor: Connect a photoresistor and a humidity sensor to the circuit and use them to display the humidity level on the LED screen.
14. Photosensitive Gas Detector: Connect a photoresistor and a gas sensor to the circuit and use them to detect the presence of gas.
15. Light-Sensitive Robot: Build a robot that uses photoresistors to navigate and react to changes in light levels.

Advantages and disadvantages of photoresistors

Advantages of photoresistors:

1. Sensitivity: Photoresistors are very sensitive to light and can react very quickly to changes in light intensity.
2. Cost-effective: Photoresistors are relatively inexpensive compared to other photosensitive technologies, making them an affordable solution for many applications.
3. Versatility: Photoresistors can be used in many applications, including light sensors, street light controls and automatic lighting systems.
4. Low power consumption: photoresistors consume very little power and are therefore suitable for use in battery-operated devices.
5. Durability: Photoresistors are generally strong and durable and can withstand harsh environmental conditions.

Disadvantages of photoresistors:

1. Non-linear response: The relationship between light intensity and resistance is not linear, making it difficult to accurately measure light levels.
2. Temperature variation: The resistance of the photoresistor can vary with temperature, which can affect the accuracy of light measurement.
3. Limited Dynamic Range: Photoresistors have a limited dynamic range, which means they may not be able to accurately detect very low or very high light levels.
4. Hysteresis: Photoresistors can exhibit hysteresis, meaning that their resistance may not return to its original value after exposure to light.
5. Poor spectral response: Photoresistors can have poor spectral response, which means they may not respond well to lights of different colors or wavelengths. This may limit their use in applications that require accurate color or wavelength detection.