Continuity tester (CB7386E)

This circuit was found in a Hands-On Electronics from the 90's. The FET can be any general purpose.

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Filter with the LM102 (CB12559E)
In the figure we have a circuit suggested by National Semiconductor, using an operational...
Tone control (CB15478E)
Tone controls for audio applications can also be considered filters that must attenuate certain...
1 kHz oscillator (CB12899E)
This circuit was found in an Argentine publication from 1973. It uses a common AM coil (50 + 50...