When we see light, it is due to our eyes reacting to the light. Different intensities of light cause the reaction to change, allowing us to notice the difference between a dim and bright light. But, how do you think machines can determine the difference between bright and dim light? Many might think cameras but if you are working on a low budget and you only need to determine the intensity, you can go with a photoresistor.

Photo resistor (c) oomlout
(CC BY-SA 2.0)

But how do photo resistors work?

Photoresistors work by using a semiconductor which changes resistance due to different light intensities. This means that when the light is brighter, the photoresistor will offer less resistance and when the light is brighter. Alternatively, when the intensity of light decreases, then the resistance decreases. This way, you can read the resistance on the photoresistor to determine the intensity of light.

Reading this value is extremely easy, since by connecting it in series to a resistor. Then, you can read the signal from the point of connection between the photoresistor and the actual resistor. This way, you can easily get an analog value for the intensity of light. Since the device itself is quite small and the circuit is extremely simple, many people use photoresistors as opposed to other sensors like cameras.

One common application they can be used is for line sensing, especially in more simple devices. Now, this initially may seem odd since you would initially expect someone to use a colour sensor to determine where light is. But, photoresistors surprisingly work due to a difference in the reflected intensities of different colours. I personally also use photoresistors for this because it is easy to determine differences between colour. Especially since these sensors are extremely simple to implement. Granted, photoresistors may not be that precise but they are still extremely useful, especially considering the cost. This makes photoresistors an extremely important piece of technology which many people, including me, rely on, on a regular basis.

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