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One-IC two-tones Siren

Double tone Police sound
Single tone old ambulance sound

Circuit diagram:

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 is suitable to be installed on bicycles, battery powered cars and motorcycles, but also in models and other games. With SW1 positioned as shown in the circuit diagram it reproduces the typical dual tone sound of Police or Fire-brigade cars, by the oscillation of IC1A and IC1B gates. With SW1 in 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 changing the values of C1, C2, C5, C6 and/or associated resistors. There is no power switch: leave SW1 in the low position (old-type siren) and the circuit consumption will be negligible.

author:RED Free Circuit Designs,

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