This circuit can detect static charges or extremely low voltages. The circuit was found in ancient American documentation and makes use of a JFET. A BF245 which is a more modern transistor can be used.
This circuit works both as a grid-dip (transistorized) and as an oscillator. The circuit is from a...
This circuit generates a control tone that serves to adjust amplifiers, check audio cables. The...
Found in a 1980s publication, this circuit uses an operational amplifier from that time. The...