Light and IR Sensors

LIGHT SENSORS

Photodiode

  • A photodiode is a two-terminal electronic device that allows current to flow when exposed to light.

  • It operates in reverse biased mode.

  • It converts light energy into electrical energy.

  • Symbol : (See original document for symbol)

  • Principle: A photodiode is a reverse-biased p-n junction. When photons of sufficient energy strike the depletion region, electron-hole pairs are created.

  • These electron-hole pairs are separated by the built-in electric field of the depletion region.

  • Holes move toward the anode, and electrons toward the cathode, producing a photocurrent.

Construction

  • Photodiodes are made from single crystal silicon wafers.

  • It consists of two layers: a P-type layer above an N-type layer.

  • A depletion layer is formed at the junction of the P and N regions.

  • It has a small transparent window to allow light to strike the PN junction.

  • The non-active surface is coated with silicon dioxide (SiO_2).

  • The active region is coated with anti-reflection material to maximize light energy conversion into current.

Working

  • When exposed to radiation, photons are absorbed by the diode, creating electron-hole pairs in the depletion region.

  • These pairs are swept apart by the in-built electric field.

Modes of Operation

  1. Photo-Conductive Mode:

    • Operates in reverse biased mode.

    • The current varies linearly with the intensity of incident light.

  2. Photo-Voltaic Mode:

    • Operates without reverse bias (like a solar cell).

I-V Characteristics

  • (See original document for I-V characteristics)

Applications

  • Consumer electronics (compact disc players, smoke detectors, infrared remote controls).

  • Counters and switching circuits.

  • Optical communication systems.

  • Detection of visible and invisible light rays.

  • Safety electronics (fire and smoke detectors).

  • Photo sensors in cameras.

  • Medical applications (pulsed oximeters, instruments for sample analysis).

Solar Cell

  • Principle: Converts light energy into electrical energy using the photovoltaic effect.

Construction

  • Consists of N-type (highly doped) and P-type (lightly doped) semiconductor materials.

  • Conducting electrodes on the top and bottom surfaces collect current.

  • The bottom is fully covered with a conductive layer, while the top layer is partially covered to allow sunlight penetration.

  • Anti-reflective coating is used to reduce reflection.

  • The assembly is enclosed in thin glass for protection against mechanical shock.

Working

  • Based on the photovoltaic effect, generating current or voltage when exposed to light.

  • Sunlight is converted into electrical energy.

  • A depletion layer forms at the N-type and P-type junction.

  • Photons with energy higher than the energy gap produce electron-hole pairs in the depletion region.

  • Electrons move towards the N-type, and holes move towards the P-type, acting as a battery.

  • This movement forms electric current.

Applications

  • Powering space satellites, calculators, and watches.

  • Residential and business power using individual solar PV systems.

  • Large power stations by utility companies.

  • Large electric utility or industrial applications use interconnected solar arrays to form a large utility-scale PV system.

  • Calculators and wrist watches.

  • Storage batteries.

  • Street lights.

  • Portable power supplies.

  • Satellites.

Light Dependent Resistor (LDR)

  • A device whose resistivity is a function of incident electromagnetic radiation.

  • Also called photoresistors or photocells.

  • Made of semiconductor materials with high resistance.

Working Principle

  • Works on the principle of photoconductivity.

  • Photoconductivity: material's conductivity increases when light is absorbed.

  • When light falls on the device, electrons in the valence band are excited to the conduction band.

  • Incident light photons must have energy greater than the semiconductor's band gap.

  • More electrons are excited to the conduction band, creating more charge carriers.

  • More current flows, decreasing the device's resistance.

Construction

  • A light-sensitive material (e.g., cadmium sulfide, cadmium selenide, indium antimonide, cadmium sulphonide) is deposited on an insulating substrate (e.g., ceramic).

  • The material is deposited in a zigzag pattern to achieve the desired resistance and power rating.

  • Ohmic contacts are made on either side of the area.

  • Contact resistances should be minimal to ensure light primarily affects resistance.

Characteristics

  • Resistance decreases when light falls on them and increases in the dark.

  • Dark resistance: resistance in the dark, can be as high as 10^{12} \,\Omega.

  • Resistance decreases drastically when exposed to light.

  • With constant voltage, current increases as light intensity increases.

  • Photocells are non-linear devices.

  • Sensitivity varies with the wavelength of incident light.

  • Different cells have different spectral response curves based on the material used.

Response Time

  • Takes 8 to 12 ms for resistance to change when light is incident.

  • Takes one or more seconds for resistance to return to its initial value after light removal (resistance recovery rate).

  • This property is used in audio compressors.

  • LDRs are less sensitive than photodiodes and phototransistors.

  • A photodiode is a pn junction semiconductor device that converts light to electricity, whereas a photocell is a passive device without a pn junction.

Types of Light Dependent Resistors

  1. Intrinsic photo resistors (Un doped semiconductor):

    • Made of pure semiconductor materials like silicon or germanium.

    • Electrons are excited from the valence band to the conduction band when photons of enough energy fall on it and number charge carriers is increased.

  2. Extrinsic photo resistors:

    • Semiconductor materials doped with impurities (dopants).

    • Dopants create new energy bands above the valence band, reducing the band gap.

    • Less energy is required to excite electrons.

    • Generally used for long wavelengths.

Applications

  • Light sensors.

  • Camera light meters.

  • Street lamps.

  • Alarm clocks.

  • Burglar alarm circuits.

  • Light intensity meters.

  • Counting packages on a conveyor belt.

Phototransistor

  • Tri-terminal (emitter, base, and collector) semiconductor devices with a light-sensitive base region.

  • Have larger collector and base regions compared to ordinary transistors.

  • The principle of operation is similar to a photodiode combined with an amplifying transistor.

  • Light incident on the base induces a small current.

Principle

  • The induced current is amplified by normal transistor action, resulting in a significantly larger current.

  • A phototransistor can provide a current that is 50 to 100 times that of a photodiode.

  • Symbol : (See original document for symbol)

Construction

  • An ordinary bipolar transistor with the base region exposed to illumination.

  • Available in both P-N-P and N-P-N types with different configurations (common emitter, common collector, common base), but common emitter configuration is generally used.

Working

  • The collector terminal is connected to the supply voltage, and the output is obtained at the emitter terminal, while the base terminal is left unconnected.

  • When light falls on the base region, electron-hole pairs are generated, creating a base current (photo-current).

  • This results in the flow of emitter current through the device, leading to amplification.

  • The magnitude of the photo-current is proportional to the luminance and is amplified by the transistor's gain, resulting in a larger collector current.

Applications

  • Object detection.

  • Encoder sensing.

  • Automatic electric control systems (e.g., light detectors).

  • Security systems.

  • Punch-card readers.

  • Relays.

  • Computer logic circuitry.

  • Counting systems.

  • Smoke detectors.

  • Laser-ranging finding devices.

  • Optical remote controls.

  • CD players.

  • Astronomy.

  • Night vision systems.

  • Infrared receivers.

  • Printers and copiers.

  • Cameras as shutter controllers.

  • Level comparators.

HALL SENSORS

Hall Effect Principle

  • If a current-carrying conductor or semiconductor is placed in a transverse magnetic field, a potential difference is developed across the specimen in a direction perpendicular to both the current and the magnetic field.

  • This phenomenon is called the “Hall effect”.

Applications

  • Used in ultra-high-reliability applications such as keyboards.

  • Used to time the speed of wheels and shafts.

  • Used to detect the position of permanent magnets in brushless electric DC motors.

  • Embedded in digital electronic devices along with linear transducers.

  • Sensing the presence of the magnetic field in industrial applications.

  • Used in smartphones to check whether the flip cover accessory is closed.

  • For contactless measurement of DC current in current transformers.

  • Used as a sensor to detect fuel levels in automobiles.

IR SENSORS

  • An electronic device that emits light to sense objects in the surroundings.

  • Measures the heat of an object and detects motion.

  • Objects radiate thermal radiation in the infrared spectrum.

  • Invisible to the human eye but detectable by infrared sensors.

  • The emitter is an IR LED (Light Emitting Diode), and the detector is an IR photodiode.

  • The photodiode is sensitive to IR light of the same wavelength emitted by the IR LED.

  • When IR light falls on the photodiode, resistances and output voltages change in proportion to the magnitude of the received IR light.

  • Five basic elements in a typical infrared detection system: an infrared source, a transmission medium, an optical component, infrared detectors or receivers, and signal processing.

  • Infrared lasers and Infrared LEDs of specific wavelengths are used as infrared sources.

IR Sensor Working Principle

  • An IR sensor consists of an IR LED and an IR Photodiode, together called a PhotoCoupler or OptoCoupler.

IR Transmitter or IR LED

  • A light-emitting diode (LED) that emits infrared radiation called IR LEDs.

  • Emitted radiation is invisible to the human eye.

IR Receiver or Photodiode

  • Detects the radiation from an IR transmitter.

  • Come in the form of photodiodes and phototransistors.

  • Infrared Photodiodes detect only infrared radiation.

Applications

  1. PROXIMITY SENSOR

    • Used in smartphones to find the distance of objects using Reflective Indirect Incidence.

    • Distance is calculated based on the intensity of radiation received after reflection.

  2. ITEM COUNTER

    • Uses the direct incidence method to count items.

    • Constant radiation is maintained between transmitter and receiver.

    • When an object cuts the radiation, the item is detected, and the count is increased, shown on a display system.

  3. BURGLAR ALARM

    • A common sensor application using the direct incidence method.

    • The transmitter and receiver are kept on both sides of a door frame.

    • Constant radiation is maintained, and an alarm starts when an object crosses the path.

  4. RADIATION THERMOMETERS

    • A key application of Infrared sensors.

    • The working depends on temperature and type of object.

    • Faster response and easy pattern measurements.

    • Measurements can be made without direct contact with the object.

  5. HUMAN BODY DETECTION

    • Used in intrusion detection, auto light switches, etc.

    • An intrusion alarm system senses the temperature of the human body.

    • Alarms are set if the temperature is more than the threshold value.

    • Uses an electromagnetic system suitable for the human body to protect it from harmful radiation.

  6. GAS ANALYZERS

    • Used to measure gas density by using the absorption properties of gas in the IR region.

    • Dispersive and Non-Dispersive types are available.

  7. OTHER APPLICATIONS

    • IR sensors are also used in IR imaging devices, optical power meters, sorting devices, missile guidance, remote sensing, flame monitors, moisture analyzers, night vision devices, infrared astronomy, rail safety, etc.