# When The Siren Sounds – A Useful Circuit

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In Greek mythology, a
siren was a demonic being (half bird, half woman). Later on this idea
was transformed in art into a mermaid: a combination of a fish and a
woman. Mechanical and electromechanical versions were invented even
later, and electronic models were developed in the last century. Sirens
are characterized by their ability to produce sounds that attract
attention. With the exception of the flesh-and-blood models, they are
thus used to warn people in a particular area of impending danger. The
electronic versions are the most suitable for DIY construction.

#### Siren Circuit Diagram

There are lots of different ways to make an electronic siren. Here
we use a binary counter (IC1) and an analogue multiplexer (IC2, which is
a digitally configurable switch). The counter is a type 4060 IC, which
has an integrated oscillator. The oscillator generates the tone of the
siren. The frequency of this tone depends on the resistance between pin
10 of IC1 and the junction of C1 and R9. The trick here is that the
analogue multiplexer adjusts the clock rate of the counter depending on
the state of the counter.

The frequency of the oscillator decreases as the resistance between
pin 10 of IC1 and the junction of C1 and R9 increases, and the lower the
frequency of the oscillator, the longer the counter remains in its
current state. This means that high frequencies present on pin 9 are
generated for shorter times than low frequencies. The values of
resistors R1–R8 increase in uniform steps of approximately 10kO, with
the result that the output on pin 9 is a series of eight decreasing
frequencies, and this series is constantly repeated in cyclical fashion.
Transistor T1 (BD139) and resistor R10 are included to boost the signal
from pin 9 to a level suitable for driving a loudspeaker.

The sound produced by this circuit may be familiar to some of our
readers (especially if their memories extend back to older types of