This circuit was found in a 1967 General Electric documentation. The components are from that time. The tunnel diode is difficult to obtain today.
This circuit is from a Popular Electronics from the 90s. The circuit has a tone frequency...
This solar powered circuit was found in a 1991 documentation. The transistor is a general purpose...
This circuit is based on a common cathode 7-segment display. The circuit is based on CMOS...