One-IC two-tones Siren

Double-tone Police sound

Single-tone old ambulance sound

Circuit diagram:

Siren 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 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.