This 80's Exar circuit has a carrier detector for frequencies up to 500 kHz. The circuit uses a company 567 which is still a common component.
This circuit consists of a simple continuity test with two transistors forming a relaxation...
We found this circuit in a Popular Electronics of the 90s. The circuit uses a touch sensor formed by...
Found in National Semionductor documentation this circuit combines a tone control with an...