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.
Circuit diagram:
R1 = 470K - 1/4W Resistors
R2 = 680K - 1/4W Resistor
R3 = 470K - 1/4W Resistors
R4 = 82K - 1/4W Resistor
R5 = 330K - 1/4W Resistor
R6 = 10K - 1/4W Resistor
R7 = 33K - 1/4W Resistor
R8 = 3.3M - 1/4W Resistor
C1 = 10µF - 25V Electrolytic Capacitors
C2 = 10nF - 63V Polyester Capacitors
C3 = 100nF - 63V Polyester Capacitor
C4 = 100µF - 25V Electrolytic Capacitor
C5 = 10µF - 25V Electrolytic Capacitors
C6 = 10nF - 63V Polyester Capacitors
D1 = 1N4148 - 75V 150mA Diodes
D2 = 1N4148 - 75V 150mA Diodes
D3 = 1N4148 - 75V 150mA Diodes
Q1 = BC337 - 45V 800mA NPN Transistor
P1 = SPST Pushbutton
B1 = 6V Battery (4 AA 1.5V Cells in series)
IC1 = 4093 - Quad 2 input Schmitt NAND Gate IC
SW1 = DPDT Switch
SPKR= 8 Ohm Loudspeaker
Source: Extreme Circuits
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