This circuit is from a Radio Electronics from March 1970. The crystal depends on the frequency at which the signal is to be captured and the resonant circuit determines the tuned band.
We found this circuit in a 1977 publication for radio amateurs. It serves to test general purpose...
This circuit is from a 1998 Poptronics. The circuit triggers a buzzer after a certain time, waking...
This circuit for electronic voltmeter was found in a publication by Gernsback from 1960. The...