The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
This dual-LED circuit indicates three signal levels. It was found in the January 1996 issue of...
This circuit was obtained in a 90s Electronic Design. The circuit uses components from that time....
We found this circuit in Linear Technology Applications 1993. The maximum output current is 5 A...