This simple circuit needs a high gain stage to compensate for losses. It consists of a passive tone control obtained from Electronics Today International of the 90s.
An OR gate can be improvised using NAND gates of a 4001 as shown here. The diodes are...
Based on an LM317T, this circuit found in Popular Electronics works with signals of frequencies up to...
This circuit causes an instrument needle to oscillate to signal an event. The circuit is powered by...