This alarm siren circuit has a latch. We found it in an article in the January 1975 Radio Electronics magazine. Note the power given by Rx and the speaker. It can be increased by changing Q1.
This configuration, explored several times on this website, shows how to use a 3-terminal voltage...
This circuit is from a 1967 Triplett manual. The circuit uses components of the time, the common...
Applications for this circuit include hearing aid, stethoscope, etc. Transistors are general...