This traditional configuration found in a Radio Shack document can use 2N2646. More keys can be added, and the power can be supplied with voltages from 9 to 15 V.
This 1967 Electronics World circuit is suitable for frequencies up to 1 MHz. The setting applies...
Here is a very useful circuit suggested by ICOTRON (Siemens) capable of eliminating the...
A frequency divider for digital signals can be constructed based on a single 4018 integrated...