This simple sensitive circuit was suggested in a July 1992 Radio Electronics issue. The relay is a double-contact changeover relay. Sensitivity adjustment is made at R2.
Found in 1965 General Electric documentation this circuit applies to a LASCR used as an optical...
This micro FM transmitter uses a tunnel diode as an oscillator. I found it in an English...
Found in a 90s Popular Electronics, this circuit uses two PLL tone recognizers to trigger by touch an...