Here we have an astable oscillator built around 555, which gives an alarm tone of 1.8 KHz directly driving a speaker. This is basic alarm circuit, which is later used in many other projects in the book. While the circuit shows only 1.8 KHz operation, it can be adapted to different applications by changing frequency and duty cycle after manipulating the timing components as per the formula.

The circuit is shown in Schematic 9. Resistances Rl, R2, and capacitor C determine the frequency. Changing these values change the frequency. You can also manipulate the tone by changing the capacitor at Pin 5. You can modulate the tone also. Try connecting a 10 K resistor from secondary of the transformer, i.e., before the rectifier and connect it to Pin 5, the Control terminal. Output will wobble at the mains frequency of 50Hz. Output at Pin 3 is coupled to the speaker of 8 ohms. The capacitor C2 is essential to block DC. You can also vary the frequency by adding a preset potentiometer in the in the timing components. Try changing the timing capacitor also. You can increase the volume by amplifying the output with a transistor. You will find such applications later.

Construction is straightforward. Use a small Vero board suitable for ICs. Mount the capacitors and resistances close to the board. C2 is an electrolytic capacitor. IC should be fixed finally and if you have not got the skill of soldering well, use of IC base will be helpful. If you wish to vary the tone, solder 100K variable resistance (preset), between power supply and IK (Rl) and change the tone as you like it.

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