Phone circuit (CB8709E)

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

IR tone remote control (CB20972E)
This circuit is from a Popular Electronics from the 90s. The circuit has a tone frequency...
Door buzzer or alarm (CB20492E)
This solar powered circuit was found in a 1991 documentation. The transistor is a general purpose...
Electronic die (CB21712E)
This circuit is based on a common cathode 7-segment display. The circuit is based on CMOS...