Light Sensor

The Light Sensor is a photoelectric device that converts visible or infra-red light energy (photons) into an electrical (electron) signal.

A Light Sensor basically produces an output signal indicating the intensity of light by measuring radian energy in a very narrow frequency range in the Ultraviolet light spectrum, which is called "light" and whose frequencies range from "Infrared" to "Visible".

The light sensor is a passive device that converts this "light energy" in visible or infra-red parts of the spectrum into an electrical signal output.Light sensors are more commonly known as "Photoelectric Devices" or "Photo Sensors" because they convert light energy (photons) into electricity (electrons).

Photoelectric devices can be grouped into two main categories: those that generate electricity when separated, such as Photo-voltaiks or Photo-emitters, and those that somehow alter their electrical properties, such as photo-resistors or Photoconductors. This leads to the following device classification.

  • Photo-emitting Cells – These are photo devices that release free electrons from a light-sensitive material such as cesium when hit by a photon with enough energy.The amount of energy that photons have depends on the frequency of light, and the higher the frequency, the more energy photons have that convert light energy into electrical energy.
  • Photoconducting Cells – These photo devices change their electrical resistance when exposed to light.Photoconductivity is caused by light hitting a semiconductor material that controls the current passing through it.Thus, it increases more light current for a given voltage.The most common photoaleken material is Cadmium Sulfur, which is used in LDR photocells.
  • Photovoltaic Cells – These photoshazes produce an emf in proportion to the radian light energy received and are similar to photoconductivity.The light energy falls on two semiconductor materials compressed together, creating a voltage of about 0.5V.The most common photovoltaic material is Selenium, which is used in solar cells.
  • Photo-connection Devices – These photo-devices are mainly real semiconductor devices, such as photodiodes or phototransistors, that use light to control the flow of electrons and holes along their PN-connections.Photo-connection devices are specially designed for detector application and light penetration with spectral responses adjusted to the wavelength of incoming light.

Photoconducted Cell

A Photoiletken light sensor does not generate electricity, it only changes its physical properties when exposed to light energy.The most common type of photoconductive device is Photodirenç,which changes electrical resistance in response to changes in light intensity.

Photodireners are Semiconductor devices that use light energy to control the flow of electrons and therefore the current flowing through them.Commonly used photoaletken cell is called Light-Based Resistance or LDR.

Use of LDR and PTC in Proteus

LED-Burning Circuit in the Dark with LDR

Light-Based Resistance (LDR)

light sensor

As its name suggests, Light-Bound Resistance (LDR) is made of exposed semiconductor material such as cadmium sulfide, changing electrical resistance from a few thousand Ohms in the dark to only a few hundred Ohms when it falls on the light.

The net effect is an improvement in conductivity with a decrease in resistance for an increase in lighting. In addition, photoresistive cells have a long response time, which requires a lot of time to respond to a change in light intensity.

Materials used as semiconductor substrates include lead sulfur (PbS), lead selenide (PbSe), indium antimonide (InSb), which detects light in the infra-red range, the most commonly used of all photodirenc light sensors, Cadmium Sulfur (Cds).

Cadmium sulfur is used in the manufacture of photoconductor cells, since the spectral response curve is very close to that of the human eye and can be controlled even using a simple flashlight as a light source.Typically, it has a peak sensitivity wavelength (εp) of approximately 560nm to 600nm in the visible spectral range.

Light-Bound Resistance Cell

light sensor

The most commonly used photodirenç light sensor is the ORP12 Cadmium Sulfur photoiletken cell.This light-dependent resistance has a spectral response of about 610 nm in the yellow to orange region of light.When the cell is not illuminated, its resistance (dark resistance) is very high at about 10MΩ, and when fully illuminated it drops to about 100Ω (burning resistance).

The resistant way to increase dark resistance and therefore reduce dark current creates a zigzag pattern along the ceramic substrate.CdS photocell is a very low cost device that is often used for automatic dimming, dark or twilight detection, positioning street lamps in "ON" and "OFF" positions, and for photographic pose meter type applications.

light sensor

There is an important advantage of connecting a light-dependent resistance in series with a standard resistance like this throughout a single DC supply voltage, a different voltage will appear in its connections for different light levels.

The amount of voltage drop along the serial resistance, R2 is determined by the resistance value of RLDR,which is the resistance due to light. The ability to produce different voltages produces a very useful circuit called a "potential divider" or voltage divider network.

As we know, the current passing through a series of circuits is common, and as LDR changes its resistant value due to light intensity, the voltage contained in VOUTwill be determined by the voltage divider formula. The resistance of an LDR, RLDR,can vary from about 100Ω in sunlight to 10MΩ in absolute darkness, and this resistance change is converted into a voltage change in VOUTas shown.

A simple use of Light-Dependent Resistance is like a light-sensitive switch, as shown below.

light sensor

This basic light sensor circuit consists of a switch whose relay output is activated by light. A potential dividing circuit is formed between the photorescist, LDR and resistance R1. When there is no light, that is, in the dark, the resistance of the Ldr is very high in the Megaohm (MΩ) range, so the transistor TR1 is pre-charged with zero base and the relay is uneasy or "off".

As the light level increases, the resistance of the LDR begins to decrease, causing the base pre-refinal voltage in the V1 to increase.At a point determined by the potential dividing network created by R1 resistance, the basic pre-voltage is high enough to position the TR1 transistor in the "ON" position, thereby activating the relay used to control some external circuits.When the light level drops back into darkness, the resistance of the LDR increases and causes the transistor's base voltage to decrease, making the transistor and relay "OFF" at a fixed light level redefined by the potentially divisive network.

By replacing the constant resistance R1 with a pocinciometer VR1, the point at which the relay is "ON" or "OFF" can be preset to a specific light level.This type of simple circuit shown above has a fairly low sensitivity and may not be consistent due to changes in switching point, temperature or feed voltage.By incorporating the LDR into a "Wheatstone Bridge" arrangement and replacing the transistor with a transactional amplifier as shown, the more precise, light-activated circuit can be easily made.

Light Level Detection Circuit

light sensor

In this basic dark sensing circuit, light-dependent resistance LDR1 and the posiometer VR1 form an adjustable arm of a simple network of resistance bridges, commonly known as the Wheatstone bridge, while two constant resistance R1 and R2 form the other arm.Both sides of the bridge form potentially divisive networks throughout the supply voltage, whose V1 and V2 outputs depend on the inverting and inverting voltage inputs of the transactional amplifier, respectively.

The operational amplifier is configured as a Differential Amplifier, also known as a feedback voltage comparator, whose output voltage status is determined by the difference between two input signals or voltages, V1 and V2.Resistance combination R1 and R2 form a constant voltage reference at the V2 inlet, which is adjusted by the ratio of two resistances.LDR – VR1 is proportional to the level of light detected by the V1 photodirenc with a combined variable voltage input.

As in the previous circuit, the output from the transactional amplifier is used to control a relay protected by the free rotating diode D1.When the light level and output voltage detected by the LDR fall below the reference voltage set in V2, the output from the op-amp changes the situation by activating the relay and changing the connected load.

Likewise, as the light level increases, the output will return by positioning the relay in the "OFF" position.The hysteresis of the two switching points is adjusted by the feedback resistance RF to give the amplifier any appropriate voltage gain.

The operation of this type of light sensor circuit can be reversed to position the relay "ON" when the light level exceeds the reference voltage level, and vice versa by changing the positions of the light sensor LDR and the pocinciometer VR1.The posiometer can be used to "pre-adjust" the switching point of the differential amplifier to any specific light level, making it ideal as a simple light sensor project circuit.

Photojonction Devices

Photojonction Devices are mainly light-sensitive detectors made from PN-Junction light sensors or silicon semiconductor PN-connections that can detect both visible light and infra-red light levels.Photo-connection devices are specially made to detect light and this class of photoelectric light sensors includes photodiode and phototransistor.


light sensor

The structure of the photodiode light sensor is similar to that of a traditional PN-join diode, except that the outer housing of the diodes is transparent or has a transparent lens to focus light on the PN junction point for increased precision. The intersection will react to light at longer wavelengths, especially red and infrared, rather than visible light.

This feature can be a problem for transparent or glass beaded diodes such as 1n4148 signal diode. All PN-ports are light-sensitive and the photo conductor, whose PN-junction is always "reverse pre-redeveloped", can be used in a neutral voltage mode, so that only diode leakage or dark current can flow.

The current-voltage characteristic (I/V Curves) of a photodiode with no light (dark mode) in its connection is very similar to a normal signal or rectifier diode.When photodiode is polarized forward, there is an exponential increase in current, as in a normal diode.When a reverse prerequisition is applied, a small inversion current occurs, which causes an increase in the depletion zone, which is the sensitive part of the connection.Photodiode can also be connected in a current mode using a constant pre-voltage along the connection.The current mode is very linear in a wide range.

Photo-diode Structure and Features

light sensor

When used as a light sensor, a photodiode dark current is about 10uA for 0 lux and 1uA for silicon type diodes.When the light falls at the junction point, more holes/electron pairs are formed and the leakage current increases.This leakage current increases as the lighting of the joint increases.

Thus, the photodiode current is directly proportional to the light intensity that falls on the PN-connection.One main advantage of photodiode when used as light sensors is that they react quickly to changes in light levels, but one drawback of this type of photocihas is the relatively small flow of current, even when it is completely lit.

The following circuit shows a photo-current-voltage converter circuit that uses a transactional amplifier as an amplification device.The output voltage (Vout) is given as Vout = I P *Rε and is proportional to the light intensity properties of the photodiode.

This type of circuit also uses the properties of a transactional amplifier with two input terminals at approximately zero voltage to operate the photodiode without pre-resciencing.This zero pre-configuration op-amp configuration gives the photodiota a high impedance loading, which leads to less effect of dark current and a wider photostream linear range than radian light intensity.Capacitor C f is used to prevent oscillation or peak gain and to adjust output bandwidth (1/2πRC).

Photo-diode Amplifier Circuit

light sensor

Photodiodes areversatile light sensors that can make current flow in nanoseconds both "ON" and "OFF", and are widely used in cameras, light meters, CD and DVD-ROM drives, TV remote controls, scanners, fax machines and copiers. and integrated into operational amplifier circuits as infrared spectrum detectors for fiber optic communication, burglar alarm motion detection circuits and numerous imaging, laser scanning and positioning systems, etc.


light sensor

An alternative photo-connection device to photodio is the Phototransistor,which is basically an amplified photodiode.The phototransistor light sensor has a collector-based PN-connection reverse polarity that exposes it to the radiant light source.

Phototransistors work in the same way as photodiode, except that they can gain current and are much more sensitive than photodiode, and the currents are 50 to 100 times higher than that of the standard photodiode and can be easily converted into a phototransistor light sensor by any normal transistor.

Although some phototransistors allow a base connection to control sensitivity and use light photons to produce a base current using light photons, which causes a collector to flow emitter current, it consists mainly of a bipolar NPN Transistor whose large base region is not electrically connected. Most phototransistors are NPN types whose outer casing is either transparent or has a transparent lens to focus light on the base connection for increased precision.

Photo-transistor Structure and Features

light sensor

In the NPN transistor, the collector is positively preposed relative to the emitter, so that the base/collector connection is inverted.Therefore, when there is no light on the port, normal leakage or very small dark current flows.When the light falls to the base, more electron/hole pairs are formed in this region and the current produced by this movement is amplified by the transistor.

Usually the sensitivity of a phototransistor is a function of the DC current gain of the transistor.Therefore, general sensitivity is a function of collector current and can be controlled by connecting a resistance between the base and the emitter, but darlington phototransistors are often used for applications of the very high precision optocuptor type.

light sensor

Photodarlington transistors use a second bipolar NPN transistor to provide additional amplification or when a photodetector needs higher sensitivity due to low light levels or selective sensitivity, but its response is slower than that of an ordinary NPN phototransistor.

Photo darlington devices consist of a normal phototransistor whose emitter output is connected to the base of a larger bipolar NPN transistor.A photodarlington device produces a very precise detector, as a darlington transistor configuration provides an equal current gain to a product of current gains of two separate transistors.

Typical applications of phototransistor light sensors are opto-isolators, corrugated opto switches, light beam sensors, fiber optics and TV type remote controls, etc. When detecting visible light, sometimes infrared filters are required.

Another type of photo-connected semiconductor light sensor worth mentioning is photo-thyristor.This is a light-activated thyristor or Silicon Controlled Rectifier , SCR,which can be used as a light-activated switch in AC applications.However, their sensitivity is usually very low compared to equivalent photodiode or phototransistors.

To help increase their sensitivity to light, photo-thyristors are made thinner around the door connection.The disadvantage of this process is that it limits the amount of anode current that they can change.Next, for higher current AC applications, they are used as alternative devices in opto-couplers to replace larger and more traditional thyristors.

Photovoltaic Cells

The most common type of photovoltaic light sensor is Solar Pili.Solar cells convert light energy directly into DC electrical energy in the form of a voltage or current to a resistant charge such as a light, battery or motor.Photovoltaic cells, then, are similar in many ways to batteries because they provide DC power.

However, unlike other photo devices that we examined above, which use the intensity of light even from a flashlight, photovoltaic solar cells work best using the radian energy of the sun.

light sensor

Photovoltaic cells are made of single crystalline silicon PN connections, the same as photodiode with a very large region sensitive to light, but without reverse pre-redetermination.They have the same properties as a very large photodiode in the dark.

When light energy is illuminated, it causes electrons to flow through the PN connection, and a single solar cell can produce an open circuit voltage of about 0.58v (580mV).Solar cells have "Positive" and "Negative" sides, just like a battery.

Single solar cells can be serially connected to create solar panels that increase output voltage, or can be connected in parallel to increase the current available.Commercially available solar panels are rated in Watts, the product of the output voltage and current (Volt times Amp) when fully lit.

Features of a typical Photovoltaic Solar Cell.

light sensor

The amount of current available from a solar cell depends on the intensity of light, the size of the cell and its efficiency, which is usually very low between 15% and 20%.

Other materials used to make photovoltaic cells include Gallium Arsenide, Copper Indium Diselenide and Cadmium Tellurid.Each of these different materials has a different spectrum band response and therefore can be "adjusted" to produce an output voltage at different light wavelengths.