This circuit can be used in a signal level indicator with LEDs. The circuit is from an Electronics Now dated January 1996. The power supply must be dual.
The FET of this circuit of a 1971 publication may be the BF245. The circuit serves as a voltage...
This circuit was found in an old European documentation. When R2 is cut off the circuit triggers...
Found in Popular Electronics from the 90s, this circuit can employ other low power audio...