This circuit for light modulated in frequency (FM) operating at 50 kHz was found in a 90’s Electronics Now. The circuit has an active filter. Modern equivalent transistors can be used.
Two operational amplifiers are used in this source obtained in an American documentation...
This circuit is suggested by the Linear Applications Handbook of National Semiconductor, being used...
This simple circuit for headphones or small speakers was found in documentation from the 90s. The...