Practice code oscillator (CB21696E)

In the figure we have an astable multivibrator driving a speaker to produce audio signals for Morse code practice.

Low Level Activayed Simple Shield (CB15481E)
The circuit shown in the figure is the version activated at the low level, of the previous circuit...
NiCad Charger (CB4979E)
This circuit is from an old American documentation using LEDs in the rectification. The circuit...
High Input Impedance Amplifier (CB3945E)
This circuit consists of an emitter follower, presenting unit voltage gain implemented with an...