Light Sensor

 Light Sensor

The light sensor is a passive devices that convert this “light energy” whether visible or in the infra-red parts of the spectrum into an electrical signal output. Light sensors are more commonly known as “Photoelectric Devices” or “Photo Sensors” because the convert light energy (photons) into electricity (electrons).

Types of light sensor

There are different types of light sensors available; mainly 


Photodiodes, and 


Photoresistors (LDR)

The most common light sensor type that’s used in a light sensor circuit are photoresistors, also known as a light-dependent resistor (LDR). Photoresistors are used to simply detect whether a light is on or off and compare relative light levels throughout a day.

What are photoresistors made of?

A high resistance Semiconductor material called cadmium sulfide cells, highly sensitive to visible and near-infrared light

How photoresistors work?

As its name suggests, photoresistors work similarly to your regular resistors, but instead resistance change is dependent on the amount of light it’s exposed to.

High intensity of light causes a lower resistance between the cadmium sulfide cell

The low intensity of light results in a higher resistance between the cadmium sulfide cells

This working principle can be seen in applications such as street lamps, wherein the day, the higher light intensity results in lower resistance and no light produced.


Photodiodes are another type of light sensor. But instead of using the change in resistance like LDR, it’s more complex to light, easily changing light into a flow of electric currents. Also known as a photodetector, photo sensor

What are photodiodes made of?

Photodiodes are mainly made from silicon and germanium materials and comprise of optical filters, built-in lenses and surface areas

How photodiodes work?

Photodiodes work on the working principle called the inner photoelectric effect. To simply put it, when a beam of light hits, electrons are loosened, causing electron-holes which results in electrical current to flow through.

The brighter the light present, the stronger the electrical current will be

Photodiode light sensor applications

Since current generated by photodiodes are directly proportional to the intensity of light, it makes it favorable for light sensing that requires fast light response changes.

Since photodiodes are responsive to infrared light, it’s applicable for more usages as well.

Here are some of the applications of photodiode:

  • Consumer electronics ranging from compact disc players to smoke detectors and even remote control devices
  • Medical applications such as equipment/instruments used for measuring and analysis purposes
  • Solar energy systems such as solar panels


The last light sensor type we’ll be exploring today is the phototransistor. The phototransistor light sensor can be described as a photodiode + amplifier. With the added amplification, light sensitivity is far better on the phototransistors.

However, it doesn’t fair better in low light level detection as compared to photodiodes.

Since both light sensor types share a similar working principle, do refer to the previous explanation!


Despite the different types of light sensors, it can still be used in a variety of applications as seen below:

  • Consumer electronics
  • Automobiles
  • Agricultural Usages
  • Security applications