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.
This simple circuit uses a 200 µA galvanometer to measure voltages in three ranges. The circuit...
Today we can call this circuit a baby monitor. It is a microphone amplifier found in a publication by...
This circuit is from Motorola's Linear Interface ICs from 1993. The MC1723 is the equivalent of...