Found on an 80's Elektor, this circuit uses common components, being the diode a general purpose germanium. The coils select the frequency range of the indicator that reaches 60 MHz.
This circuit monitors the voltage of a 12 V battery. The indicated levels let you know if the...
This circuit turns on an old cassette recorder when the telephone rings, thus enabling the...
This circuit is from an old Popular Electronics, but it can be easily implemented nowadays, even...