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.

Below is stereo audio amplifier circuit based n single chip STK465. The Power IC will deliver 30W power output on each channel.

Schematic diagram:

This  power inverter uses only 9 electronic components. This inverter circuit will turn 10 to 16VDC into 115V / 60Hz square wave AC power. It was able to operate AC equipment up to 25W.

The first section of the 555 timer is wires as an astable oscillator with R2 and C1 setting the frequency. The output is available at pin 5. The second section is wired as a phase inverter. That output is available at pin 9. Resistor R3 and R4 keep output transistor Q1 and Q2 from loading down the oscillator.

Read more about 25W Inverter circuit

This in a really simple electronic buzzer circuit based on timer IC NE555. The IC 555 act as astable multivibrator which will generate audio signal frequency. The frequency value is depended the value of 56K resistor and 10nF (0.01 uf) capacitor… you may replace the 56K resistor with a variable resistor to adjust the frequency.

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”…