This circuit was obtained in a Popular Electronics of the 90s. The circuit uses integrated from the time, uncommon today. Note that the speaker has a higher impedance than ordinary speakers.
We found this circuit in an old oscilloscope publication. With it, it is possible to observe the B...
This configuration was found in a 1976 documentation. Note the use of pin 4 input for reset. The...
The LM109 can be found with different origins. The circuit shown specifically is from Motorola's Linear...