Types of Photodetectors

Photo sensors are used in many projects involving sensing of light or shadow.  Whenever, we plan to do a project which involves sensing of light, we have a choice of choosing a wide variety of light sensing devices. Light Dependent Resistor (LDR), Photo Diode, Photo Transistor and Photo Darlington Pair are some of the commonly used photo sensors. How do we select a suitable photo sensor out of all these alternatives? Although there are many types of photo sensors, each one has its own set of properties different from the others. Although it is possible to use one type of photo sensor instead of another, it is necessary to select the best sensor analyzing its properties so as to get better results.

Light Dependent Resistor (LDR)

LDR is one of the widely used photo sensors. They are cheap and rugged in nature. They are basically resistors whose resistance depends on the intensity of light. Their resistance variation is continuous and inversely proportional to the intensity of light. By considering the properties of an LDR, it is suitable for applications where many different levels of light intensity need to be measured. For example, if you are using a photo sensor with an ADC, then LDR suits you the best. Also if your application needs to operate in rough external environments, then also LDR will be a good option as it is rugged in nature. Its response time is slower compared to other semiconductor based photo sensors. The dark resistance of an LDR is quite less compared with the reverse bias resistance offered by photo diodes because of which it may consume comparatively more power than its semiconductor counterparts.


  • Moderate response time
  • Low cost
  • Rugged in nature
  • Resistance varies continually (Analog)
  • Bidirectional

Phototransistor – Features & Circuit Diagram

A Phototransistor is an electronic switching and current amplification component which relies on exposure to light to operate. When light falls on the junction, reverse current flows which is proportional to the luminance. Phototransistors are used extensively to detect light pulses and convert them into digital electrical signals. These are operated by light rather than electric current. Providing large amount of gain, low cost and these phototransistors might be used in numerous applications.

It is capable of converting light energy into electric energy. Phototransistors work in a similar way to photo resistors commonly known as LDR (light dependant resistor) but are able to produce both current and voltage while photo resistors are only capable of producing current due to change in resistance. Phototransistors are transistors with the base terminal exposed. Instead of sending current into the base, the photons from striking light activate the transistor. This is because a phototransistor is made of a bipolar semiconductor and focuses the energy that is passed through it. These are activated by light particles and are used in virtually all electronic devices that depend on light in some way. All silicon photo sensors (phototransistors) respond to the entire visible radiation range as well as to infrared. In fact, all diodes, transistors, Darlington’s, triacs, etc. have the same basic radiation frequency response.

The structure of the phototransistor is specifically optimized for photo applications. Compared to a normal transistor, a photo transistor has a larger base and collector width and is made using diffusion or ion implantation.


  • Low-cost visible and near-IR photo detection.
  • Available with gains from 100 to over 1500.
  • Moderately fast response times.
  • Available in a wide range of packages including epoxy-coated, transfer-molded and surface mounting technology.
  • Electrical characteristics similar to that of signal transistors.

A photo transistor is nothing but an ordinary bi-poplar transistor in which the base region is exposed to the illumination. It is available in both the P-N-P and N-P-N types having different configurations like common emitter, common collector and common base. Common emitter configuration is generally used. It can also work while base is made open. Compared to the conventional transistor it has more base and collector areas. Ancient photo transistors used single semiconductor materials like silicon and germanium but now a day’s modern components uses materials like gallium and arsenide for high efficiency levels. The base is the lead responsible for activating the transistor. It is the gate controller device for the larger electrical supply. The collector is the positive lead and the larger electrical supply. The emitter is the negative lead and the outlet for the larger electrical supply.

Photo transisor

With no light falling on the device there will be a small current flow due to thermally generated hole-electron pairs and the output voltage from the circuit will be slightly less than the supply value due to the voltage drop across the load resistor R. With light falling on the collector-base junction the current flow increases. With the base connection open circuit, the collector-base current must flow in the base-emitter circuit and hence the current flowing is amplified by normal transistor action. Collector base junction is very sensitive to light .Its working condition depends upon intensity of light. The base current from the incident photons is amplified by the gain of the transistor, resulting in current gains that range from hundreds to several thousands. A phototransistor is 50 to 100 times more sensitive than a photodiode with a lower level of noise.

A phototransistor works just like a normal transistor, where the base current is multiplied to give the collector current, except that in a phototransistor, the base current is controlled by the amount of visible or infrared light where the device only needs 2 pins.

Phototransistor Circuit

In the simple circuit, assuming that nothing is connected to Vout, the base current controlled by the amount of light will determine the collector current, which is the current going through the resistor. Therefore, the voltage at Vout will move high and low based on the amount of light. We can connect this to an op-amp to boost the signal or directly to an input of a microcontroller. The output of a phototransistor is dependent upon the wavelength of incident light. These devices respond to light over a broad range of wavelengths from the near UV, through the visible and into the near IR part of the spectrum. For a given light source illumination level, the output of a phototransistor is defined by the area of the exposed collector-base junction and the dc current gain of the transistor

Photo transistors available different configurations like opto isolator, optical switch, retro sensor. Opto isolator is similar to a transformer in that the output is electrically isolated from the input. An object is detected when it enters the gap of the optical switch and blocks the light path between the emitter and detector. The retro sensor detects the presence of an object by generating light and then looking for its reflectance off of the object to be sensed.

Advantages of Photo transistors:

Phototransistors have several important advantages that separate them from other optical sensor some of them are mentioned below

  • Phototransistors produce a higher current than photo diodes.
  • Phototransistors are relatively inexpensive, simple, and small enough to fit several of them onto a single integrated computer chip.
  • Phototransistors are very fast and are capable of providing nearly instantaneous output.
  • Phototransistors produce a voltage, that photo-resistors cannot do so.

Disadvantages of Photo transistors:

  • Phototransistors that are made of silicon are not capable of handling voltages over 1,000 Volts.
  • Phototransistors are also more vulnerable to surges and spikes of electricity as well as electromagnetic energy.
  • Phototransistors also do not allow electrons to move as freely as other devices do, such as electron tubes.

Areas of application for the Phototransistor include:

  • Punch-card readers.
  • Security systems
  • Encoders – measure speed and direction
  • IR detectors photo
  • electric controls
  • Computer logic circuitry.
  • Relays
  • Lighting control (highways etc)
  • Level indication
  • Counting systems

Photo Diode

Photodiodes are basically diodes which are used in reverse bias and they are turned ON when a light intensity above the threshold level is incident on it. It has only two possible levels of outputs, either ON or OFF because of which it can only differentiate between two different intensities of light. It is suitable for applications where detecting a single light threshold is necessary. For example, if you are using this to make a Shadow Counter type of circuit, then this sensor will be most suitable.

Photo DiodeProperties:

Quicker response time

  • Low cost
  • Temperature sensitive
  • Digital in nature
  • Unidirectional

Photo Darling-ton Pair

Photo Darling-ton Pair shares most of the properties of a Photo Transistor but it has more amplification factor meaning that it is capable of amplifying light signals more than a simple Photo Transistor. But this high amplification factor comes at the cost of longer response time. So, it is preferred when receiving very weak light signals for which the amplification provided by the photo transistor is not sufficient.

Photo darling-tonProperties:

  • High Amplification Possible
  • Slower than Photo Transistor
  • Costlier than Photo Transistor
  • Biasing and stabilization is possible

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