Phototransistor is a type of BJT transistor that allows current to flow from the emitter to the collector depending on the amount of light exposed to it. It is intenionally exposed to light to control the transistor's conductivity, Just photo diodes which allows current to flow through it in the reverse bias. The difference between a photo diode and a photo transistor is a photodiode allows current to flow in the reverse bias only when it senses light, while a photo transistor allows current to flow from the emitter to the collector only when exposed to light. Photo transistor is more sense sensitive than a photodiode that it's use to determine light signal levels. A photo transistor is just like a darlington transistor with it base intentionally exposed to receive light. It consists of two pins, the emitter and collector for connecting to the external circuit. The base pin is excluded because it don't need to be connected to circuits and it's only activated by sunlight which generates electric current needed to activate the transistor.
Circuit symbol of a photo transistor.
Working principle of phototransistors.
Photo transistors works just like bipolar junction transistors, infact photo transistors can be made by exposing the base surface of an ordinary transistor to bare light. Early phototransistors were made by not covering the plastic encapsulated of the transistor with black paint, making them sensitive to light. Photo transistors are made from silicon or germanium consisting base, emitter and collector. The base is fabricated to have a big surface while the emitter and collector have small surface area.
The base is fabricated to have a big surface while the emitter and collector have small surface area. When semiconductors are exposed to light they undergo optical charge generation otherwise known as photoelectric effect when doped. only doped semiconductors undergoes this effect. That's why most rectifying diodes are encapsuled in black plastic case.
A normal transistor especially a BJT allows current to flow through it from the emitter to the collector only when a little current is applied to the base. As for phototransistors, current is generated at the base by photons impinging it, so it doesn't need to activated by connecting the base to a current source. So photo transistors can only allow current to flow through them only when exposed to light, and it blocks current from flowing through it when it isn't exposed to light or photons. Photo transistors are more sensitive or can sense a little amount of light radiation than photodiodes, because it allows current to flow from the emitter to collector when a little voltage of about 0.7v is produced at the base by photons.
There are different types of phototransistors depending on appllications and mode of construction. Some are use for sensing light while some for detecting Infrared. Those ones for infrared appllications are usually colored in black, while those for sensing light are transparent in color. Photo transistors can be made from TRIACs, BJTs, thyristors and FETs depending on their appllications on either to get switched on or off when light or photons impinges their exposed area.
Circuit applications of photo transistors.
• Encoders.
They're use in robotics for determining the positions and speeds of motors and actuators. It's made by making holes at the egde of a circular piece of material which spins with a shaft attached to the motor or actuator. Light from an LED shines at the one side of the disc and the photo transistor recieves the light when the disc's holes make light pass through it to the photo transistor.
•light activated switch.
The circuit above uses a photo transistor to activate a relay switch which then turn on or off an high voltage circuit. The phototransistor allows current to flow through it when it senses light, and makes current to flow through the base of the transistor "Q2" which then activates the relay circuit.
• Motion activated CCTV camera.
Some CCTV cameras are activated to take vedio record of scences when it senses motion especially of human. It works by systems of BJT and FET phototransistors which is made to activate the camera to take vedio records when it senses light or light is blocked from it.
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Power Electronics