Types of Light Sensor and its Application.

Light Sensor

light sensors are semiconductors device and this is operation is based on the change in the resistance and current flow in the circuit when the light falls on them. 

there are three type of light sensor:-

1. Photodiode
2. Phototransistor
3. Photoresistor

 photodiode is semiconductor junction diodes which are connected into a circuit in reverse bias, so giving a very high resistance. With no incident light, the reverse current is almost negligible and is termed the dark current.when the light fall on the junction, extra hole electrons pair are produced and there is an increase in the reverse current and the diode resistance drops.

The photodiode is two terminal electronic devices which when exposed to light the current start flowing in the diode. It is operated in reverse bias. Basically, it converts light energy into electrical energy.

Working

The junction of the diode is. Illuminated by the light source, the photons strike the junction surface. The photons impart their energy in the form of light to the junction. Due to which electrons from valence band get the energy to jump into the conduction band and contribute to the current. The reverse current is very nearly proportional to the intensity of the light. For example, the current in the absence of light with a reverse bias of 3 volts might be 25uA and when illuminated by 25000 lumens /m^2 the current rises to 375 uA . The resistance of the device with no light is  120k ohm and with light is 8k ohm. A photodiode can thus be used as a variable resistance device controlled by the light incident on it. Photodiode has the very high response to the light.

Application

• photodiodes are condensed as one of the significant optoelectronics devices which is extensively used in the optical fibre communication system.
• it is used in the character recognition technique and IR remote control circuits.

Phototransistor

The phototransistors have light-sensitive collector base p-n junction. When there is no incident light there is a very small collector to emitter current. When light is incident, a base current is produced that is directly proportional to the light intensity. this leads to the production of a collector current which is then a measure of the light intensity.

Working

The two arrow point towards phototransistor indicate that the phototransistors are triggered by the light rays incident on it .the output of the phototransistors is taken from the emitter terminal and the light rays are allowed to enter the base region. the magnitude of the photocurrent generated by the phototransistors depends on the light intensity of the light falling on the transistor.

Application

• Counting system
• Encoder sensing
• Object detection
• Printer and optical control remote
• Light detectors.

Photoresistor

A photoresistor has a resistance which depends on the intensity of the light which is falling on it.decreases linearly as the intensity increases. The cadmium sulphide photoresistor is the most responsive to light having the wavelength shorter than about 515 nm and the cadmium selenide photoresistor for wavelength less than about 700nm.

Working

When the light falls on the photoresistor, some of the valence electron, which absorbs energy from the light and breaks the bonding with the atoms. the valence electron which breaks the binding with the atom is called the free electron. Because when the light energy falls on it, material get heated up and the electrons movement gradually increases, again if the energy increase then the electrons comes out from its threshold boundaries.

Application

• Streetlight
• Alarm device
• Street solar lamp
• Clock radios.