This LM380-based circuit was found in documentation from the 1980s. The components are current, and the transistor may be a general-purpose equivalent. Note the use of a medium impedance speaker.
This 4-digit counter, using CMOS technology, was found in a 1997 documentation. The circuit is...
This circuit activates a relay that has a lamp as a load. The circuit is from a Radio Electronics from...
This circuit can measure frequencies up to just over 1 MHz and uses CMOS technology. The circuit is...