The purpose of this circuit is only to increase the input resistance of a multimeter, since there is no voltage gain. The batteries are 9 V and operational equivalents can be used.
The presented circuit monitors a line indicating if it is busy if there is an extension. The...
This high-powered megaphone circuit is from a Radio Electronics from December 1964. Note the use...
This linear circuit was found in a 1965 Electronic Design. The transistors are from that time....