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Wailing Alarm Siren
by Tony van Roon

Wailing Alarm Siren, Schematic

Parts List:
R1,R5 = 4K7       C1,C4 = 100uF/25v      IC1,IC2 = LM/NE555, MC1455P, etc
   R2 = 47K       C2,C3 = 0.01uF              LS = Loudspeaker. *Notes
   R3 = 10K          T1 = 2N3702
   R4 = 100K              (NTE290A) 
   Rx = *Notes            (TUP) etc.
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.

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Copyright © Tony van Roon, CET