Fire siren

The LM3909 circuit generates a rapidly ascending wail when the button is pressed, followed by a slower "coasting down" sound upon release. To allow the tone to cease after the button is released, an 18-kΩ resistor can be placed between pins 8 and 6 of the LM3909. The resulting sound resembles that of a motor-driven siren, as detailed in the National Semiconductor Linear Applications Handbook, 1991, page 402.

The LM3909 is a versatile integrated circuit designed for sound generation applications. It utilizes a combination of resistors, capacitors, and the internal timing circuitry to produce distinct audio tones. When the button is actuated, the circuit initiates a rapid oscillation, generating a tone that increases in frequency. This rapid rise in pitch can be attributed to the charging of the capacitor connected to the timing pins of the LM3909.

Upon releasing the button, the circuit transitions to a slower decay phase, where the frequency decreases gradually. This "coasting down" effect is primarily controlled by the discharge rate of the timing capacitor, which is influenced by the values of the external resistor and capacitor connected to the timing pins.

For applications where the sound needs to persist for a short duration after the button is released, the introduction of an 18-kΩ resistor between pins 8 and 6 modifies the discharge characteristics of the timing capacitor. This modification allows for a more extended decay period, enhancing the siren-like sound effect. The circuit can be fine-tuned by adjusting the values of the timing components to achieve the desired pitch and duration of the sound.

Overall, the LM3909 circuit is an effective solution for creating audio signals in various applications, including alarms, toys, and other electronic devices requiring sound alerts. The design simplicity and effectiveness of the LM3909 make it a popular choice among engineers and hobbyists alike.This LM3909 circuit produces a rapidly rising wail upon pressing the button, and a slower "coasting down" sound upon release. If it is desirable to have the tone stop sometime after the button is released, an 18-k © resistor can be placed between pins 8 and 6 of the LM3909.

The sound is then much like that of a motor-driven siren, National Semicon ductor Linear Applications Handbook, 1991 p. 402. 🔗 External reference