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 circuit can operate with signals up to 60 MHz using the indicated operational. We found it in...
This circuit takes advantage of the noise generated at the reverse biased junction of a germanium...
Equivalent transistors can be used on this 80's circuit. Any general purpose PNP transistor can work...