This audio-driven relay was published in the July 1977 issue of Radio Electronics magazine. The circuit uses CMOS components, and the relay must be a sensitive type.
This detector or window comparator was suggested by Harris in documentation from the 90's. The...
This circuit was found in the April 1971 Radio Electronics magazine. It uses common transistors and can...
Found in Radio Electronics from the 90s, this circuit uses a thermistor or NTC. The resistance of the...