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Description:
An electronic siren made from
discrete components.
Notes:
The 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. The oscillator action is
more difficult to work out. 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. 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. |
