Above circuit is an fire alarm circuit which build using a fire alarm sensor component and sound generator component. With simple and cheap component, you can build this circuit for low cost.

The termistor is a fire detector sensor and the NE555 is an audio frequency generator which will resulting the sound of alarm from the speaker.

This anti theft circuit uses timer IC 555 to produce the sound. The alarm will keep “silent” when the thin wire is connected the pin 4 of IC to the ground. When the thin wire is broken, the alarm will set to “crying”…

This is a police alarm circuit.  The two 555 timer ICs used as astable multivibrators. The frequency is controlled by the pin 5 of the IC. Actually, the sound is genereted by combination of sound from both IC 555. One timer generated low frequency and the another generated high frequency. By varying VR1 you can set how fast the siren changes from high freq. to low freq. VR2 sets the siren frequency.