This unusual circuit from a Popular Electronics of the 90s uses LED indicators, operating as a bridge. The components used are common. It is based on the tuning of the signal by a PLL.
This circuit can be powered by 2 or 3 small batteries. The resistor can have values between 220...
These circuits from a May 1994 Electronics Now take advantage of the noise from the junctions of...
This network can be used in a sound project to control the tone. The circuit is from an...