The frequency of this circuit depends on C1 and, for the indicated values, it is 1 kHz. Equivalent operational amplifiers can be used, and the power supply must be symmetrical.
Found in a Motorola documentation from the 70s, this circuit makes use of transistors of the time....
This is one of many Morse code audio oscillators that can be found in this section. The circuit...
Here we have the way to activate two common cathode displays with 4026 integrated circuits. The...