The circuit presented was suggested by Texas Instruments in an opto-electronics manual. The transistor can be the BC548 and the supply voltage depends on the application.
This simple circuit can use any general-purpose transistor. The power supply must be isolated. The circuit...
This is a configuration much explored by assemblers, being found on this site in dozens of similar...
The frequency of this oscillator depends on T, which for the audio band can be a common output...