The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
The circuit shown is only for analog telephone lines. I found it in Popular Electronics (1991). It...
These circuits use loads of 50 ohms to allow the use of a voltmeter in evaluating the output of a...
This circuit can be used to add a delay to a logic signal as shown by the input and output...