This circuit produces a loud tone that changes pitch rapidly ideal for a warning device.

It is advised to use a paper or mylar cone speaker of at least 1W. Do not use high quality speakers for home entertainment systems.

Circuit Voltage: 6 to 12 volts

ASTABLE:
Set the Range 2 - (31Hz-850Hz) and use VR1 to change frequency.
Range 3 - (700Hz-18KHz) can also produce some very piercing sounds)

TRIANGLE WAVE ASTABLE:
Adjust the warble rate using VR1. Start with VR1 set at minimum, fully anti-clockwise.

The output is fed to the Frequency Modulation input of the Astable which causes the output frequency to vary in proportion with the voltage. The more voltage, the more deviation from the original frequency creating a rapidly rising and falling tone.

The output from the Astable is connected to the Darlington Driver which boosts current for the speaker.

By turning VR1 on the Triangle Wave Astable gradully clockwise, the speed of the warbling will increase to a point where it matches the frequency of the second astable. This can produce some very unusual sounds. For a standard siren, turn VR1 fully anti-clockwise

• Frequency Modulation and associated terminology
• Principle of radio transmission

• Fire Alarm
• Elderly Panic Alarm
• Attack Alarm