This filter was found in a Texas Instruments manual based on an operational amplifier 741. Other operational can be used and, with the indicated components, the rejection frequency is 2 kHz.
The output power of this wireless intercom for the 27 MHz (11 meters) range is in the order of ½ W,...
This relay drive circuit was found in the July 1958 Radio Electronics magazine. The transistors are of...
I found this circuit in an Electronics Handbook from 1989. The publication no longer exists, but...