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 2003 circuit serves to digitally show the intensity of a sound captured by a microphone....
The circuit shown in the figure makes it possible to control a stepper motor via a PC's serial port...
This circuit was found in an old American documentation. Its purpose is a proportional scanning signal...