Found in a Hands-On Electronics from the 90s, this circuit produces the dial tones of an old telephone. The integrated circuit used is uncommon today.
This circuit was obtained in the manual of the used integrated circuit. Equivalents may be used...
This source whose voltage depends only on the zener diode used, can supply currents up to about 3 A....
This highly sensitive circuit was found in a 1967 General Electric SCR manual. Equivalent...