An electronic siren made from discrete components. A push button switch
is used to create the sound, which rises in pitch when the switch is pressed,
and falls in pitch when the switch is released.
The Electronic Siren Circuits sound produced imitates the rise
and fall of an American police siren. When first switched on the 10u capacitors
is discharged and both transistors are off. When the push button switch is
pressed to 10u capacitor will charge via the 22k resistor. This voltage is
applied to the base of the BC108B which will turn on slowly. When the switch is
released the capacitor will discharge via the 100k and 47k base resistors and
the transistor will slowly turn off. The change in voltage alters the frequency
of the siren.
Electronic Siren Circuits Oscillators action is as follows. As
the BC108B transistor switches on its collector voltage falls and so the 2N3702
transistor is switched on. This happens very quickly ( less than 1us). The 22n
capacitor will charge very quickly as well. As this capacitor is connected
between the collector of the 2N3702 and the base of the BC108B, it soon reaches
almost full supply voltage. The charging current for the capacitor is then much
reduced and the collector emitter voltage of the 2N3072 is therefore increased;
the collector potential will fall. This change in voltage is passed through the
22n capacitor to the base of the BC108B causing it to come out of saturation
slightly. Electronic Siren Circuits as this happens its collector voltage will rise and turn off the
2N3072 transistor more. This continues until both transistors are off. The 22n
capacitor will then discharge via the 100k, 22k resistor, the closed push
button switch, 9V battery, the speaker and 56 ohm resistor. The discharge time
takes around 5-6msec. As soon as the 22n capacitor is discharged, the BC108B
transistor will switch on again and the cycle repeats. The difference in
voltage at the collector of the BC108B (caused by the charging 10u capacitor)
causes the tone of the siren to change. As the 10u capacitor is charged, the
tone of the siren will rise, and as it is discharged, it will fall. A 64 ohm
loudspeaker may be used in place of the 8 ohm and 56 resistor, and the values
of components may be altered to produce different sound effects.
Electronic Siren Circuits current drain is fairly high in this
circuit so a suitable power supply is required. The duration the tone takes to
rise and fall is determined by the 10u and 22k resistor. These values may be
varied for different effects of Electronic Siren Circuits.
Electronic Siren Circuits
Electronic Siren Circuits Figure