This circuit that generates a special sound was found in a Popular Electronics from the 90s. The base is the 555 and the transistors admit equivalents, and a power stage can be added.
This two-tone decoder has a bandwidth of 24%. The circuit is from a 1994 Electronics Now. The...
Bike speedometer (CB20497E) This circuit can be powered by a solar cell. The circuit is from a...
This is a typical common collector amplifier step configuration. The transistor can be used...