Light Sensor
A light sensor is a device that is sensitive to light and can detect light and convert it into an electrical signal.
It can measure the intensity, wavelength, frequency or direction of light.
A light sensor is something that a robot can use to detect the current ambient light level – i.e.
how bright / dark it is.
There are a range of different types of light sensors
Photoresistor
Photodiodes
Phototransistors
A photoresistor is a variable resistor that let current flow easy when it is exposed to light.
It is most used as a sensor to detect changes in brightness.
Streetlights often use a photoresistor to detect when it should turn on.
Photoresistors also known as Light Dependent Resistor (LDR), Cadmium Sulfide cells (CDS cells), photoconductor and sometimes simply photocells are a type of transducer
Which converts energy from one form to another where one of the known forms is electrical energy.
To keep things simple, we will refer to it as “Photoresistor”.
Resistance in a photoresistor inversely varies with the amount of light it is exposed to.
Bright light = Less Resistance and
Low Light = More Resistance.
These sensors are used to make light sensitive devices and are more often found in street lights, cheap toys, outdoor clocks etc.
If you have ever wondered how a street light turns on in the night and switches off in the day,
You will be surprised to find cheap photoresistor circuitry inside it.
Advantages of photoresistor
Cheap and will not make a hole in your pocket if you spoil few.
Commonly found in most robot hobby shops.
Available in different sizes with different specifications.
Easy to design and implement them in circuitry.
Drawbacks of Photoresistor
Highly inaccurate.
Each one behaves differently than other.
If the first one have 150Ω in bright light, the second one can have 500Ω of resistance in the same light.
Very slow for sensitive applications.
They cannot be used to determine precise light levels.
A photodiode is a semiconductor device that converts light into current.
The current is generated when photons are absorbed in the photodiode.
Photodiodes are used to detect light and feature wide, transparent junctions.
Generally, these diodes operate in reverse bias,
where in even small amounts of current flow, resulting from the light, can be detected with ease.
The symbol of photodiode is shown here.
Photodiodes can also be used to generate electricity, used as solar cells and even in photometry.
A photodiode is biased against its easy flow of direction of current, i.e. it is reverse biased so that a very low leakage current flows.
If a photon of sufficient energy is incident on the diode at its junction, an electron is freed and if it is possessing enough energy, it may pass over the energy barrier causing a small leakage current to flow.
The amount of current is proportional to the amount of illumination of the junction.
The phototransistor is a semiconductor light sensor formed from a basic transistor with a transparent cover that provides much better sensitivity than a photodiode.
A phototransistor is a light-sensitive transistor.
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