Phone circuit (CB8709E)

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

Door buzzer (CB7560E)
This buzzer with the 555 was suggested by a Popular Electronics from the 90s to be used as a...
Receiver for Heart Rate Monitor (CB25407E)
This is the receiver for the previous circuit found in the November 1991 Radio Electronics...
Series regulator protection (CB8364S)
In the figure we have the configuration of series regulator protector suggested by Motorola in 1968...