The circuit shown is from National Semiconductor's Linear Applications Handbook. It makes use of the LM78S40 which is not an easy component to obtain today.
In this circuit found in the December 1972 Radio Electronics magazine. R can be from 1k to 22k,...
This circuit can operate between frequencies from 1 to 15 MHz depending on the coil. For 3.5 MHz...
This metal detector circuit uses an AM radio as an indicator. The radio is placed off-station and tuned...