Phone circuit (CB8709E)

With this circuit, found in a 80s Popular Electronics, it is possible to use two ordinary phones in a basic intercom.

LED Binary Counter (CB4345E)
This circuit came out in an American magazine that no longer exists. It's the Electronic...
Bass and Treble Control (CB918E)
This circuit can be added to any audio amplifier design, consisting of a good tone control with an...
Second order high pass filter (CB7522E)
Using a 741 op amp with symmetrical source, this circuit was found in a 1982 documentation. The...