The Mouse and Rat repellent circuit using astable multivibrator which use sound waves or ultrasonic so it escaped away. Use transistor and piezoelectric.
This AC 220V siren circuit is a simple siren that use 220V AC as power supply. It is small size and use a piezo speaker cause use low power and saving.
This is Blown Fuse indicator circuit by LED Display. A Fuse be the equipment protects that use often most. Because of cheapness can use protect electronics
I like to build simple audio alarm circuit using transistors. Although it is old circuit, it still useful. It uses just two transistors, other components.
This is police siren circuit using 555 IC, with PCB layout, using 2 IC-555 as frequency oscillators and transistor to drive sound to the speaker [...]
This is Simple 555 Alarm sound circuit, for beginner as buzzer sound. IC1 is pulse generator, to transistor PNP for boost current to speakers.
Often drivers forgot to turn off lights when parking. They cannot start the machine, ran out battery. This Headlight Warning buzzer circuit can help.