Wailing alarm

This circuit simulates the sound of an American police siren. IC2 is configured as a low-frequency astable multivibrator, producing a cycling period of approximately 6 seconds. The slowly varying ramp waveform generated on capacitor C1 is fed to the emitter follower transistor Q1, which subsequently frequency modulates the alarm generator IC1 through resistor R6. IC1 operates at a natural center frequency of around 800 Hz. The circuit's operation results in an alarm output signal that begins at a low frequency, increases over 3 seconds to a high frequency, and then decreases back to a low frequency over the next 3 seconds, continuously repeating this cycle.

The circuit utilizes an integrated circuit (IC) configured as an astable multivibrator, specifically IC2, which serves to generate a low-frequency square wave. This square wave is characterized by a period of about 6 seconds, allowing for a gradual modulation of the output frequency. The ramp waveform produced across capacitor C1 is essential for creating a smooth transition in frequency, which is critical for simulating the characteristic sound of a police siren.

Transistor Q1, functioning as an emitter follower, amplifies the ramp signal while maintaining the same voltage level, allowing for effective coupling to the frequency modulation input of alarm generator IC1. The frequency modulation is achieved through resistor R6, which influences the extent of frequency variation based on the input signal from Q1. IC1, which is designed to generate alarm sounds, has a default center frequency of approximately 800 Hz, making it suitable for producing a siren-like tone.

The unique cycling behavior of the output signal is a result of the design of the astable multivibrator, which ensures that the frequency modulation follows a specific pattern. Initially, the output frequency starts low, gradually increases to a peak frequency over a duration of 3 seconds, and then decreases back to the starting frequency over the next 3 seconds. This continuous cycle creates the distinctive wailing sound associated with police sirens, making the circuit effective for alerting purposes in various applications. The design is versatile and can be adapted for different sound frequencies by adjusting component values, particularly those related to the timing of the astable multivibrator and the characteristics of the alarm generator.This circuit simulates the sound of an American police siren. IC2 is wired as a low frequency astable that has a cycling period of about 6 seconds. The slowly varying ramp waveform on Cl is fed to pnp emitter follower Ql, and is then used to frequency modulate alarm generator IC1 via R6. IC1 has a natural center frequency of about 800Hz. Circuit action is such that the alarm output signal starts at a low frequency, rises for 3 seconds to a high frequency, then falls over 3 seconds to a low frequency again, and so on ad infinitum.