What Is Phototransistor?

What Is Phototransistor?

Are you curious to learn more about phototransistors and what they can do? As one of the most reliable sensing components in use today, phototransistors are a crucial component of modern electronic devices. Consisting of a reflective surface, light source, and transistor elements all combined into one small package, their immense versatility allows them to detect light and convert it into useful electrical signals for a wide range of applications. In this blog post we’ll explore how phototransistors work, examine some common uses for them, discuss how to select the correct type for your needs, as well as go over some important considerations before using phototransistors in projects. Let’s dive right into our discussion!

Definition of a Phototransistor

A phototransistor is a type of transistor that responds to light. It consists of two parts: an emitter and a collector.

The emitter is typically made of semiconductor material, such as silicon, gallium arsenide or germanium. A photon, or particle of light, striking the emitter causes electrons to be emitted from it. These electrons then travel towards the collector, which collects them and forms an electrical current.

Definition of a Phototransistor

The amount of current generated depends on the intensity of the incident light; therefore phototransistors are typically used for light detection and control applications such as optical switches and infrared detectors. Because phototransistors can detect low-level changes in lighting, they are also useful for photoelectric sensors, which convert light signals into electrical signals.

The first phototransistors were made of PN-junction devices that used light energy to create an electrical current. These early devices were bulky and expensive, making them impractical for widespread use. Over the next few decades, technological advances improved on this design, leading to smaller and more reliable versions of the phototransistor.

Modern phototransistors are now much more compact and inexpensive than their predecessors, making them a popular choice for many applications. They are used in everything from consumer electronics to industrial machinery, and their low power consumption makes them ideal for portable devices such as cameras and cell phones.

The phototransistor has also been adapted for use in optical communication systems, where its light-sensitive nature can be used as an effective signal detector. Phototransistors have even been utilized in medical imaging technologies, where they detect the faint signals emitted from nuclei of cells or molecules. [1], [2], [3]

Phototransistor Structure

The typical structure of a phototransistor consists of three layers, including an emitter layer, a collector layer, and an optical sensitive base layer. The emitter layer is usually made up of metal or semiconductor material while the collector is generally formed using doped silicon substrate. This creates a junction between the two layers that allows photons to enter and trigger electrons to move across them. When enough photons enter the base layer, electrons are released and move to the collector layer, which then builds up an electric current in the circuit.

The optical sensitive base layer is composed of either a single junction or multiple junctions based on the phototransistor’s design. For single junction phototransistors, only the light received by one side of the transistor will be detected while for multi-junction types, both sides of the transistor can detect light. This makes multi-junction phototransistors more sensitive than single ones. Furthermore, most modern phototransistors come with built-in lenses that allow them to detect light from wider angles than normal transistors. [1], [2]

Phototransistor Characteristics

Now that we have an understanding of the structure and components of a phototransistor, let’s take a look at some of its key characteristics.

Phototransistor Characteristics

Low-cost visible and near-IR photodetection

The phototransistor is relatively inexpensive and easy to manufacture, making it an attractive option for low-cost visible and near-infrared photodetection. Phototransistors can be used in applications such as automatic lighting control, industrial automation, motion detection systems, and security cameras.

Fast response time

One of the key advantages of a phototransistor is its fast response time. When light hits the transistor’s base layer, electrons are released almost instantaneously and move towards the collector layer. This means that signals can be detected within milliseconds, which makes phototransistors ideal for applications requiring ultra-fast reaction times such as automotive perception systems or robotics vision processing.

Available in a wide range of packages

Phototransistors come in a variety of packages, which allows them to be used in various applications. The most popular types of phototransistor packages are epoxy-coated metal can, plastic-encapsulated package, and surface-mount packages.

The epoxy-coated metal can package is the most common type and typically includes a light shield that blocks out ambient light while allowing only the desired wavelength of light to enter the transistor. Plastic encapsulated packages are ideal for use in rugged environments as they offer superior moisture and chemical resistance compared to other types of phototransistor packaging. Finally, surface-mount packages are becoming increasingly popular due to their small size and versatility in circuit design. [1], [2], [3]

Advantages of Phototransistors

Since now you are aware of what a phototransistor is, it is time to discuss the advantages of using them.

Phototransistors produce a higher current than photodiodes

The first and most important advantage of phototransistors is that they produce a higher output current than photodiodes. This makes them ideal for applications requiring more power such as light-driven motors, relays, and switches. As compared to photodiodes, phototransistors can generate greater current at a given voltage level, allowing them to be used in applications with more demanding power requirements.

They produce voltage

Phototransistors are able to convert light into electrical voltage. This means that they can be used in a variety of applications such as remote controls, medical imaging and even optical communication systems. The amount of current generated by the phototransistor depends on the intensity of the incident light and this makes them ideal for detecting low-level changes in lighting.

This makes them different from photo-resistors which rely on resistance to detect light. In contrast, phototransistors are more reliable and produce more accurate results with less power consumption than photo-resistors.


Phototransistors are highly cost efficient compared to other types of transistors. This is due to their simple structure and the fact that they require fewer components as well as less energy consumption. Additionally, phototransistors require no external power source which further reduces their cost.

They are simple

The other key characteristic of a phototransistor is that they are incredibly simple devices. As compared to other electronic components, the phototransistor does not require complex wiring or circuits and can be easily installed in a variety of applications.

Small size

Another major benefit of using phototransistors is their small size. This makes them ideal for many applications that require a high level of sensitivity, but with minimal space requirement, such as optical switches and transportation systems. Furthermore, because the electronic components used to make phototransistors occupy very little space, it is possible to design them into tiny packages or even into tiny integrated circuits (ICs). This allows for increased functionality at a reduced cost and makes installation simpler than ever before. [1]

Advantages of Phototransistors

Disadvantages of Phototransistor

However nothing is 100% perfect and phototransistors also have their limitations. It’s important to be aware of them before making any decisions when it comes to using phototransistors.

Some phototransistors are not capable of handling voltages over 1,000 Volts

Some phototransistors, especially the older models, are not capable of handling voltages higher than 1,000 Volts. This makes them unsuitable for applications where high voltage is needed such as lightning detectors or high-voltage switches. Silicone phototransistors are one of such phototransistors which cannot handle voltages higher than 1,000 Volts.

They are vulnerable to surges and spikes of electricity

Phototransistors are vulnerable to high voltage surges and spikes of electricity. This is because the base-emitter junction of the transistor can be damaged by an excessive amount of current, resulting in permanent damage to the device. To prevent this, it is important to install surge protectors and limiters before connecting a phototransistor circuit to external power sources.

They do not allow electrons to move freely

The last disadvantage of phototransistors is that the electrons in them do not move as freely as in other transistors. This can limit their current gain, as well as the speed at which they process signals. Additionally, because of this limitation, phototransistors are typically more expensive than regular transistors. [1]

Applications of Phototransistors

As you can see phototransistors are quite versatile components and can be used in a number of applications. In this section we will discuss all the potential uses of phototransistors.

Security systems

Phototransistors can be used in a wide range of security applications. Security systems generally use phototransistors to detect objects moving through the area being monitored, such as people or animals. When an object moves into the field of view of the phototransistor, it increases the amount of light that is detected by the transistor, which triggers a response from the security system. This may include sounding an alarm or activating additional security measures. In some cases, phototransistors are used in combination with infrared sensors to form a more comprehensive detection system that can provide greater accuracy and reliability. Infrared sensors detect changes in temperature, while phototransistors detect changes in light intensity.

Applications of Phototransistors


Phototransistors are also used in encoder systems.

An encoder is an electronic device that converts physical movements into electrical signals.

This can be useful for a wide range of applications, such as robotic arms and machine tools. Phototransistors are often used to detect the rotational movements of objects such as cogs or motors, which allows them to send corresponding electrical signals back to the control system.

Punch-card readers

Punch card readers are devices that use light to read a pattern of holes punched into a card or other substrate. Phototransistors can be used for this purpose due to their ability to detect low levels of light, detecting the presence or absence of holes in the card. In these applications, an array of phototransistors is arranged in an appropriate pattern and situated such that when a punch card is inserted into the reader, each hole passes over one phototransistor. The phototransistor then detects the presence (or absence) of a hole and sends out an electrical signal to indicate its status. By connecting all of the phototransistors in the array together, the information contained on the punch card can be read and processed.In addition to their use in punch-card readers, phototransistors have also found applications in other areas where light needs to be detected such as barcode scanners, optical switches, and automatic door openers.

Counting systems

Phototransistors are often used in counting systems, such as those found in photocopiers, automated teller machines (ATMs), and automated voting systems. In these applications, the phototransistor is used to detect the presence of a document or card which triggers a count. For example, in an ATM system, when a customer inserts their card into the slot, a beam of light shines down onto it that is detected by the phototransistor. This signals for the machine to begin counting the number of cards that have been inserted. The same principle applies to other automated systems such as voting machines; when someone presses a button on the machine, this triggers the phototransistor to register one vote.

Applications of Phototransistors


Phototransistors are utilized to drive relays, which are devices with an electromagnet that act as switches. When a phototransistor is exposed to light the current passing through it will increase, causing the relay to switch on and off. This makes them ideal for automating processes such as turning lights on or off automatically depending on certain conditions.

Lighting control systems

Phototransistors are widely used in lighting control systems due to their unique ability to detect and respond to changes in light levels. Phototransistors can be used in a variety of applications, such as controlling indoor/outdoor lights automatically or manually, dimming lights for energy efficiency, and helping create aesthetically pleasing lighting effects. In many cases, phototransistors are used together with other components such as timers and switches to form a complete lighting control system. For example, when combined with a timer, phototransistors can be programmed to turn on the lights at predetermined times. This is a great way to save energy by avoiding unnecessary usage of electricity during daylight hours.

Computer logic circuitry

Phototransistors are used in computer logic circuitry for a variety of reasons, including providing input to computers and controlling signals. Phototransistors allow electrical signals to be turned on or off in response to changes in light intensity. By using phototransistors as gates, it’s possible to design complex logic circuits that can respond quickly and accurately to changing environmental conditions. For example, motion detectors often use phototransistors as the key control element in the circuit. When movement is detected by an infrared detector, the signal is sent through a phototransistor which turns on a relay that activates an alarm or other action. This type of circuit responds very quickly and provides excellent accuracy when detecting movement. In addition to motion detection, phototransistors can also be used as switches and amplifiers in complex logic circuits. [1], [2]

Applications of Phototransistors


Is a phototransistor passive or active?

A phototransistor is a type of passive electronic device that is used to detect and amplify light.

Made up of a small, usually transparent, semiconductor diode with one or more additional electrodes, phototransistors are designed to be sensitive to only certain frequencies and intensities of light. When the appropriate frequency and intensity of light strikes the phototransistor’s active element, electrons are generated within the device.The generated electrons create an electrical current which is amplified by the transistor’s internal circuitry. This amplified current is then outputted from the device in order for it to be used in other electrical circuits or processes. The amount of amplification provided by these transistors depends on their design; some models may amplify relatively weak currents whereas others can provide stronger currents depending on how much light is detected.

Is a phototransistor a light detector?

Yes, phototransistors are light detectors and are used to detect the intensity of light. Phototransistors work by exciting electrons in a semiconductor material when exposed to light. When illuminated, current can flow through the device which is then used for sensing or switching applications.

How do phototransistors work

Phototransistors, also known as light-sensitive or photosensitive transistors, are devices that convert the energy in visible light into electrical signals.

This process is known as photoconductivity and it occurs when a photon of light interacts with the semiconductor material inside the phototransistor. When this happens, an electron in the semiconductor material is released and then captured by an adjacent metal electrode inside the transistor. This movement of electrons creates an electric current that can be used to power other electronic components such as LEDs and relays. The most common type of phototransistor is the bipolar junction transistor (BJT) because its structure allows it to interface easily with other circuit elements.

What is meant by a phototransistor?

A phototransistor is an electronic component that acts as a switch and is used to detect light. It consists of two parts: the base, which is usually made up of a semiconductor material, and the collector, which collects electrons from the base when exposed to light. The signal produced by the phototransistor can be amplified and used in many different applications such as switching circuits on or off or detecting pulses of infrared radiation.

What is the difference between a photodiode and a phototransistor?

The main difference between a photodiode and a phototransistor is their output signals. A photodiode produces an electrical current that varies with the amount of light that it receives, while a phototransistor produces an amplified voltage signal proportional to the amount of light detected.

In terms of functionality, photodiodes are typically used for sensing incoming light or detecting changes in light levels due to movement or shadows. Phototransistors, on the other hand, have more sophisticated capabilities such as amplifying weak signals and controlling larger amounts of power.

Photodiodes are manufactured in various shapes and sizes and can be used in many applications including optical sensors, solar cells, and imaging devices; whereas phototransistors are more common in optoelectronics such as light source controllers, LED drivers, and proximity sensors.

In terms of cost, photodiodes tend to be less expensive than phototransistors due to their simpler design and smaller size. However, phototransistors offer higher sensitivity and better noise rejection which make them ideal for applications requiring a greater degree of accuracy.

What is the application of a phototransistor?

A phototransistor is a type of transistor that responds to light. It is most commonly used as an optical switch or for signal amplification in integrated circuits and opto-electronic systems. Depending on how it is configured, it can be used as a light-sensitive switch, as part of a voltage regulator circuit, or even as an amplifier.

The primary application areas for phototransistors are robotics, medical equipment, security systems and consumer electronics. In robots, phototransistors are often used to detect obstacles and other objects in their environment; they may also be used in conjunction with sensors to detect motion. Phototransistors are frequently found in medical equipment such as patient monitors and endoscopic cameras; these devices detect changes in the environment and provide feedback to control systems.

In security systems, phototransistors are used as part of alarm circuits that detect intruders. They can also be used to initiate an alarm when a beam of light is broken. Finally, consumer electronics such as digital cameras and other imaging devices often employ phototransistors for light detection, amplification and signal processing tasks.

What is the symbol of a phototransistor?

The symbol for a phototransistor is composed of two arrows pointing towards each other and intersecting at their base. The left arrow represents the light source (e.g., LED) that generates photons, while the right arrow represents the electrons collected by the collector-base junction in response to incident light. The presence of a resistor (R1) connected between these two symbols indicates that this component is an active device requiring a supply voltage to operate correctly.

Useful Video: What is phototransistor ? || Phototransistor Kya Hota Hai?


Phototransistors are an essential component for any electronics project, as their outputs allow for precise control and quick response times. They can be used to detect light in the visible spectrum or infrared light, depending on the type of phototransistor used. Depending on the application, it is important to consider the characteristics of each kind of phototransistor before selecting one for use.No matter what type is chosen, these devices are relatively easy to set up and use; they require only a few external components to make them work correctly. Phototransistors also offer reliable performance in well-designed circuits, and with proper shielding from electromagnetic interference can last indefinitely. We hope you found this guide useful!

In this article, we explored the fundamentals of phototransistors and discussed the different types available. We also looked at some product reviews and shared useful tips on using these devices in your projects.

If you have any further questions about phototransistors, feel free to leave them in the comments section below. Thank you for reading this guide! We hope it was helpful in your journey towards becoming an electronics enthusiast. Be sure to check out the other guides we have available on our website and join us in the discussion forums! Happy tinkering!


  1. https://www.elprocus.com/phototransistor-basics-circuit-diagram-advantages-applications/
  2. https://www.electronics-notes.com/articles/electronic_components/transistor/what-is-a-phototransistor-tutorial.php
  3. https://circuitglobe.com/phototransistor.html