One-IC two-tones Siren

Double-tone Police sound
Single-tone old ambulance sound

Circuit diagram:

One-IC two-tones Siren

Parts:

R1,R3

470K   1/4W Resistors

R2

680K   1/4W Resistor

R4

82K   1/4W Resistor

R5

330K   1/4W Resistor

R6

10K   1/4W Resistor

R7

33K   1/4W Resistor

R8

3M3 1/4W Resistor

   

C1,C5

10µF 25V Electrolytic Capacitors

C2,C6

10nF 63V Polyester Capacitors

C3

100nF 63V Polyester Capacitor

C4

100µF 25V Electrolytic Capacitor

   

D1-D3

1N4148 75V 150mA Diodes

   

IC1

4093   Quad 2 input Schmitt NAND Gate IC

   

Q1

BC337   45V 800mA NPN Transistor

   

P1

SPST Pushbutton

   

SW1

DPDT Switch

   

SPKR

8 Ohm Loudspeaker

   

B1

6V Battery (4 AA 1.5V Cells in series)

Circuit operation:

This circuit is intended for children fun, and can be installed on bicycles, battery powered cars and motorcycles, but also on models and various games and toys. With SW1 positioned as shown in the circuit diagram, the typical dual-tone sound of Police or Fire-brigade cars is generated, by the oscillation of IC1A and IC1B gates. With SW1 set to the other position, the old siren sound increasing in frequency and then slowly decreasing is reproduced, by pushing on P1 that starts oscillation in IC1C and IC1D.
The loudspeaker, driven by Q1, should be of reasonable dimensions and well encased, in order to obtain a more realistic and louder output. Tone and period of the sound oscillations can be varied by changing the values of C1, C2, C5, C6 and/or associated resistors. No power switch is required: leave SW1 in the low position (old-type siren) and the circuit consumption will be negligible.

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