This modulated siren was found in a 1984 documentation and consists of a variation of the previous circuit (BC15400E). This circuit, when properly adjusted, imitates the siren of the English police, with sounds suddenly changing in tone, in a very interesting effect. As we can see, the two 555 are connected in the configuration of astables, the first being adjusted to a low frequency, producing modulation. Capacitor C1, 10 uF, is responsible for the modulation that is set in potentiometer P1. The value of this potentiometer can vary between 100 k ohms and 220 k ohms without problems. The higher its value, the slower the modulation that can be achieved. The output signal of the first integrated modulates the second via the decoupling pin 5. A resistor, whose value can be changed experimentally in the range of 8k2 to 15 k ohms, determines the frequency modulation of the second oscillator. The frequency of the second oscillator is set in the 220k ohm potentiometer and its average value is given by the 100 nF ceramic capacitor.
