Practice code oscillator (CB21696E)

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

Transistor Tester (CB17229E)
We found this circuit in a 1998 publication. It consists of a simple test of transistors using an...
1 000 Second Timer (CB15546E)
The existence of field effect transistors at the input of the integrated circuit (operational...
Null indicator (CB22313E)
This circuit for measuring bridges was found in American documentation from the 60's. Transistors are...