This keypad telephone encoder circuit was found in ancient American documentation. The components used are common and are not difficult to obtain.
March 2017 - Microchip's new 8-bit PIC16F15354 microcontrollers can now be obtained from Mouser...
This signal generator found in Japanese documentation of 1973 uses an unusual integrated circuit...
This circuit that converts very low currents into voltage was obtained in the 80's used component...