Found in American documentation, this circuit for modulated infrared signals can be tuned to a certain frequency. The photodiode can be of any type.
A narrow range of frequencies around 1 kHz is rejected by the high Q filter shown in the figure,...
This simple converter does not use a transformer and the transistor can be the BD135 for currents up...
This is a circuit from the time when some still called the transistor a crystal triode. It...