MLA 8 N, David. Name required. Email required. Please note: comment moderation is enabled and may delay your comment. There is no need to resubmit your comment. Notify me of followup comments via e-mail. Written by : david. User assumes all risk of use, damage, or injury. You agree that we have no liability for any damages.
Summary LDR and photo diode are two very commonly used photo sensors LDR refers to the Light Dependent Resistor LDR LDR is the most commonly used photo sensor Photodiode is used in applications that can only do with two values: either on or off LDR varies the light depending on certain factors Photodiode has a quicker response time as opposed to LDR which is analog LDR is a bidirectional resistor whereas photo diode is a unidirectional resistor Both LDR and photo resistor have low costs LDR is best suited for rugged environments Examples of LDR in use is street lights Examples of photodiodes in use is precision equipment like laboratory appliances e.
Author Recent Posts. Photodiodes, on the other hand, have an instant response. Although an LDR is tuned to the visible spectrum of light, photodiodes are sensitive to both visible and infra-red lights. As nouns the difference between photodiode and photodetector is that photodiode is a semiconductor two-terminal component whose electrical characteristics are light-sensitive while photodetector is any device used to detect electromagnetic radiation.
The key difference between the diode and photodiode is that diode is the semiconductor device which conducts when forward biased applied to it exceeds the barrier potential while the photodiode is the devices which conduct when the light is incident on it. A Light Dependent Resistor also known as a photoresistor or LDR is a device whose resistivity is a function of the incident electromagnetic radiation. Hence, they are light-sensitive devices. The sensitivity of the LDR switch is fairly low.
To increase the sensitivity of light sensing, few modifications can be applied. The fixed resistor R1 is replaced with a potentiometer VR1. The transistor is replaced with an Operational Amplifier and the light dependent resistor is incorporated into a Wheatstone bridge. The new and more sensitive light sensing circuit using a light dependent resistor is shown below. These voltages are connected to non-inverting and inverting inputs of the operational amplifier respectively.
The operational amplifier is operated as a differential amplifier whose output is a function of the difference between the two input voltages V1 and V2. This is also known as a voltage comparator with feedback.
The feedback resistor Rf is used to provide required voltage gain. The output of the operational amplifier is connected to a relay which can control an external circuit. When the voltage V1 due to light sensing by LDR falls below the voltage V2, which acts as a reference voltage, the output of the amplifier changes its state.
This causes the relay to activate and the load is switched on. As the intensity of the light increases, the output switches back and the relay is turned off. Here the relay is turned on when the intensity of light is less.
The operation can be reversed by reversing the positions of the light detecting resistor and potentiometer. Now the relay is turned on when the light level increases and exceeds the level set by the reference voltage. Photodiode is in the class of photo junction device which is basically a PN junction light sensor. They are generally made from semiconductor PN junctions and are sensitive to visible light and infrared light.
When light is incident on a Photodiode, the electrons and holes are separated and will allow the junction to conduct. Photodiodes are constructed like any other conventional junction diodes.
A typical photo diode is shown below. The opaque coating used in signal and rectifier diodes is absent in photo diodes.
This makes the diode transparent enough to permit light and to affect the conductivity of the junction. A photo diode is biased against its easy flow of direction of current, i.
If a photon of sufficient energy is incident on the diode at its junction, an electron is freed and if it possesses 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. In the absence of light, the current — voltage characteristic of a photo diode is similar to that of a normal diode.
Similar to a normal diode, there is an exponential increase in current when a photo diode is forward biased. When it is reverse biased, a small leakage current called reverse saturation current will appear and causes an increase in the depletion region.
Dark current is the current when the intensity of the light is 0 Lux. LDRs or photoresistors have a long response time. They may take several seconds to change conductivity after exposure to light. Photodiodes, on the other hand, have an instant response.
Although an LDR is tuned to the visible spectrum of light, photodiodes are sensitive to both visible and infra-red lights. The biggest disadvantage of photodiodes is that their reverse leakage current is still in the micro-ampere range — even when subject to light. Therefore, they require an operational amplifier circuit for light detection.
Photodiodes have a response time in nanoseconds. These are used in sophisticated applications including cameras, imaging and scanning devices, CD and DVD readers, optical fiber communication, motion detection, and positioning sensors.
Phototransistors Phototransistors are similar to photodiodes except they provide amplification to the current. These are generally designed using normal NPN transistors with their collector-base PN junction exposed to light via a transparent case or a clear lens. Due to current amplification, their output current is 50 to times greater than photodiodes.
The base region is electrically isolated or has control for sensitivity. As phototransistor already provides current amplification, unlike a photodiode, and requires no external amplifier for its operation. A phototransistor is simply a typical transistor with a base-collector exposed to light.
The NPN phototransistors are connected in a circuit with their base-collector in a reverse-bias configuration. When exposed to light, the base current increases and is amplified by the transistor. The sensitivity of a phototransistor depends on the DC gain of the transistor. The output current can be controlled by the resistance between the base and the emitter of the phototransistor. For higher sensitivity applications, such as optocouplers, Darlington phototransistors are used.
The output current amplification is the product of the current amplification of the two phototransistors.
0コメント