Optical potentiometer (CB22159E)

This voltage-controlled potentiometer using an LDR was found in an English documentation. LDR is common and LED of any type.

Telephne Audio Interface (CB21269E)
This circuit from a Popular Electronics shows how to take the audio signal from a telephone line...
FM Transmitter (CB19344E)
The coil of this transmitter has 4 coils of wire 26 or 28 in the form of 1 cm without core and on it...
Bandpass and Notch Filter I LM3900 (CB15994E)
In the figure we have another configuration of filter that uses the four Norton amplifiers of the...