This circuit takes advantage of the hysteresis of a relay to provide a latching effect. The circuit acts as a trigger and the components must be according to the used relay.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...