The transistors of this circuit found in a 1965 magazine can be the BC558 and the sensitive 12 V x 50 mA relay. The other components are common.
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...