Using the 555 and 4020, this timer was found in the July 1994 Radio Electronics magazine. The relay is voltage-dependent, and the transistor is general purpose.
This limit alarm was found in a 1980s US documentation. Comparators can be equivalent and the...
Radio Electronics published this circuit in a 90's edition. The circuit makes use of an uncommon...
This circuit serves to indicate when a transmitter is operating by detecting its signal. The...