This speaker overload protection circuit was found in Radio Electronics from the 1980s. The circuit uses a 9 V relay but can be powered by 12 V for a relay of this voltage.
This circuit from an old Argentine publication shows how it is possible to use a tunnel diode in a...
In the figure we have a basic PLL circuit with the LM567 or equivalent. The circuit is from a 1977...
This basic Schmitt trigger was found in an American publication from 1968. The circuit uses...