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.
Found in the October 1992 Radio Electronics magazine, this oscillator is controlled by switch S1....
This circuit was obtained in a 1976 amateur radio manual. The circuit can be made with a 2N2222...
The component used in this circuit is not very common today as the circuit is from the 80's. It...