Optocouler using transistor...

[Kedar264]

Transistor turns on when mentioned base current is provided ,but the process behind it is to generate small amount of carriers in base region

We can do this by shining light (of suitable wavelength) on to the semiconductor of the transistor

So, I've opened 2n2222a transistor and spotted under the laser and here's the result,   it worked!!!

Maybe this can be used as an optical isolator or optocoupler

See the video
 https://youtu.be/hdIEQJqHyvI

[xrunner]

Yea it's interesting and fun but they already make them - they are called phototransistors -

A phototransistor is a light-sensitive transistor. A common type of phototransistor, called a photobipolar transistor, is in essence a bipolar transistor encased in a transparent case so that light can reach the base–collector junction. It was invented by Dr. John N. Shive (more famous for his wave machine) at Bell Labs in 1948,[5]:205 but it wasn't announced until 1950.[6] The electrons that are generated by photons in the base–collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain ? (or hfe). If the emitter is left unconnected, the phototransistor becomes a photodiode. While phototransistors have a higher responsivity for light they are not able to detect low levels of light any better than photodiodes.[citation needed] Phototransistors also have significantly longer response times. Field-effect phototransistors, also known as photoFETs, are light-sensitive field-effect transistors. Unlike photobipolar transistors, photoFETs control drain-source current by creating a gate voltage.

http://en.wikipedia.org/wiki/Photodiode

[JohnnyBerg]

I remember scraping of the paint of a OC71 end 60's.
Yes, it works, even after 45 years

[T3sl4co1l]

Yup!

Likewise, you can use a photosensor as a transistor, if the semiconductor is accessible in some way (electrical connection or electric field).  FET example: http://sparkbangbuzz.com/cds-fet/cds-fet.htm

Tim