Using CMOS circuits and operational amplifiers of the time, this circuit was found in the September 1977 issue of Radio Electronics magazine. The circuit is powered by a 15 V power source.
Found in a 90s Popular Electronics, this circuit uses two PLL tone recognizers to trigger by touch an...
This Hughes circuit is from 1968. Equivalent modern components can be used. The input impedance is...
This circuit is from 1995 documentation. In the original, the source is symmetrical at 12 V, but...