The powerful siren that we describe can be powered with a voltage of 12 V, which allows its installation in automobiles (respecting the legal restrictions on its use). Several are the types of sounds that can be obtained from a single adjustment.
Only two transistors are used, but one of them is high power, which allows the production of sound with good intensity on a heavy 4 or 8 ohm speaker. The supply voltage can be between 6 and 12 V. For 6 V we suggest the use of 4 medium or large batteries in a portable assembly.
For the 12 V version, given the current, at least 2 A source or battery must be used. By pressing the pressure switch S1 the capacitor C1 is charged with the production of an increasing frequency sound.
When we release S1 the capacitor is discharged with the continuation of the sound of decreasing form. P1 allows you to adjust the loading time and therefore the leading edge of the sound envelope produced.
ASSEMBLY
In figure 1 we give the complete diagram of the siren.
The printed circuit board is shown in figure 2.
The transistor Q2 must be provided with a heat radiator and P1 can be both a trimpot and a potentiometer of the same value.
The C1 capacitor will have values between 100 uF and 470 uF, depending on whether you want a shorter or faster action to increase the sound. C2 determines the hue and can have values between 22 nF and 100 nF. The smaller value produces a higher pitch.
The resistors are 1/8 or ¼ W and it is recommended to use a heavy speaker for greater audio power. The fuse in series with the power supply is necessary to protect the circuit in case of short.
TEST AND USE
Simply connect the siren to a source or battery and connect S2. Then just press S1 and set P1 to the desired sound type. Pressing and releasing S1 will mimic the siren sound.
We remind you that the use of this type of appliance in ordinary cars is prohibited, with restrictions on use under special conditions.
MATERIAL LIST
Q1 - BC548 - NPN general purpose transistor
Q2 - TIP42 with radiator - PNP power transistor
FTE - Heavy 4 or 8 ohm speaker
P1 - 47 k trimpot
C1 - 100 uF at 470 uF x 16 V - electrolytic capacitor
C2 - 47 nF (473) - ceramic or polyester capacitor
S1 - Pressure switch
S2 – On-Off switch
R1 - 15k - resistor (brown, green, orange)
R2 - 1 k - resistor (brown, black, red)
F1 - 5 A fuse
Miscellaneous: printed circuit board, mounting box, fuse holder, heat radiator for transistor, etc.
