Light Sensor

Light Sensor

A Light Sensor generates an output signal indicating the intensity of light by measuring the radiant energy that exists in a very narrow range of frequencies basically called "light", and which ranges in frequency from "Infrared" to "Visible" up to "Ultraviolet" light spectrum. The light sensor is a passive devices that convert this "light energy" whether visible or in the infrared 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).

Photoelectric devices can be grouped into two main categories, those which generate electricity when illuminated, such as Photo-voltaics or Photo-emissives etc, and those which change their electrical properties in some way such as Photo-resistors or Photo-conductors. This leads to the following classification of devices.

  Photo-emissive Cells - These are photodevices which release free electrons from a light sensitive material such as caesium when struck by a photon of sufficient energy. The amount of energy the photons have depends on the frequency of the light and the higher the frequency, the more energy the photons have converting light energy into electrical energy.

   

  • Photo-conductive Cells - These photodevices vary their electrical resistance when subjected to light. Photoconductivity results from light hitting a semiconductor material which controls the current flow through it. Thus, more light increase the current for a given applied voltage. The most common photoconductive material is Cadmium Sulphide used in LDR photocells.

 

  • Photo-voltaic Cells - These photodevices generate an emf in proportion to the radiant light energy received and is similar in effect to photoconductivity. Light energy falls on to two semiconductor materials sandwiched together creating a voltage of approximately 0.5V. The most common photovoltaic material is Selenium used in solar cells.

  • Photo-junction Devices - These photodevices are mainly true semiconductor devices such as the photodiode or phototransistor which use light to control the flow of electrons and holes across their PN-junction. Photojunction devices are specifically designed for detector application and light penetration with their spectral response tuned to the wavelength of incident light.

The light sensor kit consists of a light sensor

Functions

The light sensor enables a robot to detect light. Robots can be programmed to have a specific reaction if a certain amount of light is detected. The light sensor uses a cadmium sulfosolenide (CdS) photoconductive photocell. The CdS photocell is a photo resistor, meaning that its resistance value changes based on the amount of incident light.

Benefits

The light sensor can give the robot vision and make it easier for it to operate autonomously.

Applications

The light sensor allows the robot to follow a light source, such as a beam of light from a flash light or similar. The light sensor is placed on the top section of the robot or on a lower section that is not covered. Figure 2 shows the sensor placement. This sensor can be used to help the robot avoid getting stuck under furniture. The light sensor can enable your robot to detect and follow a beam of light, or avoid getting stuck underneath furniture by avoiding shadows. Figure 3 shows Squarebot following a light source placed above it. In conjunction with clever programming, it can be used to conserve battery power by shutting down the robot in the absence of light. In conjunction with filters, the robot can be given color vision to detect various colors.

light/ dark senso

Opposite is a simple light/ dark sensor. This can be connected as an input or switch to another circuit. The sensors has three green wires (1, 2 and 3). Wire 2 should always be connected to one of the inputs. If wire 1 is also connected then the sensor acts as a dark sensor. If wires 2 and 3 are connected to the inputs then sensor operates as a light sensor.

The preset resistor allows the person using the circuit to alter its sensitivity to light/dark.