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From the Q&A

With TJ Byers


Screaming Sirens

Question:

I would like to add a circuit to my motion detector, ultrasonic and other types, that would trigger at least two sirens (preferably three or four) in a sequence of one at a time at half-minute intervals; i.e., first one siren, then two sirens, three sirens, and so on. The whole circuit should then reset itself after two minutes if no further motion is detected. The trigger times between the sirens and the reset time should be adjustable.

Melvin Fisher
Lebanon, PA


Answer:

Here’s just what the doctor ordered.

IC1, a one-shot multivibrator, is the master controller in this circuit. I’d use a pair of relay contacts driven by the motion detector to trigger IC1. When pin 2 goes low, the output, pin 3, goes high and stays high for up to two minutes. This activates RL1, the first siren, and resets the siren timers (pin 4), which now start timing out according to their R1/C1 values. As each timer expires, its corresponding relay pulls in, sounding another siren. When IC1 times out, its output goes low, which removes power to RL1 and the sequencer timers. Notice that each timer is independent of the others; each stands on its own, and will perform even if the other timers fail (except for IC1). The time delay for each timer is adjusted by R1. While this is only a three-stage sequencer, you can add as many timers as you wish by simply piling on more 555 circuits off IC1.


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