Wailing Alarm Siren

Authors Avatar

Wailing Alarm Siren

by Tony van Roon

Parts List:

R1,R5 = 4.7K      C1,C4 = 100uF/25V, electrolytic

   R2 = 47K       C2,C3 = 0.01uF (10nF), ceramic

   R3 = 10K          T1 = 2N3702 (NTE159, TUP, etc.)

   R4 = 100K    IC1,IC2 = LM/NE555, MC1455P, etc

   Rx = *Notes       LS = Loudspeaker. *Notes

Notes:
This circuit provides a warbling sound to any alarm circuit. IC2 is wired as a low frequency astable with a cycle period of about 6 seconds. The slowly varying ramp waveform at C1 is fed to pnp emitter follower T1, 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.
*The Loudspeaker LS and the resistor marked "Rx" should be together 75 ohms. If you have a standard 8-ohm speaker then Rx is 67 ohms. The nearest value is 68 ohms. So for a 8 ohm loudspeaker Rx is 68 ohms. For a 4 ohm loudspeaker Rx is 71 ohms, for a 25 ohm loudspeaker Rx is 50 ohms, etc, etc.
BUT, the Rx value is not very critical. It is just there as some sort of volume control. Experiment with it. C2 and C3 are 0.01uF (10nF) and a simple ceramic type will do the job. I tested the circuit at 9, 12, and 15 volt. My choice would be 9volt alkaline for battery operation or 12volt for use with a small powersupply. Output pin 3 of IC2 is NOT connected; just in case you are wondering... :-) In my prototype I used LM555 timers.

Join now!

Police Siren 

This circuit produces a sound similar to the police siren. It makes use of two 555 timer ICs used as astable multivibrators. The frequency is controlled by the pin 5 of the IC. The first IC (left) is wired to work around 1Hz. The 47uF capacitor is charged and discharged periodically and the voltage across it gradually increases and decreases periodically.This varying voltage modulates the frequency of the 2nd IC. This process repeats and what you hear is the sound remarkably similar to the police siren.Two presets VR1 and VR2 are ...

This is a preview of the whole essay