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.