Wailing Siren

We all know that the 555 is a single chip version of a commonly used circuit called a multi-vibrator. The ne555 timer chips is used for basic timing functions through which we can create a tone(sound) of a particular frequency. Here, we are trying to build a circuit using ne555 timer chip. The ne555 timer is operated in astable mode so when the switch is pressed, the speaker produces high pitch sound. If released then its pitch decreases and 30 s after the release of the switch the sound gets switched off.

--> 6V battery clip

--> ne555

--> 8 ohm speaker

--> button

--> bc547, bc557

--> capacitors: 100u, 10n(2)

--> resistors: 220k, 100k, 33k, 22k, 1k, 39, 5.6 ohm

You can see the circuit in the picture in other to guide thoroughly.

We know that the ne555 timer has 8 pins. We ground the pin 1. One end of the 10nF capacitor is grounded and then connect pin 2 and 6 with 33k ohm resistor in parallel. One end of the 10 nF capacitor is grounded where as the other end through the 33k ohm resistor gets connected to the pin 3 and the 1k resistor in a junction.

The other end of the resistor is then connected to the base of the bc547 transistor in series. The emitter of the transistor is then grounded and then the collector is connected to the speaker to the 39 ohm resistor to the 6V power supply all in series.

You then need to ground one end of the 10nF capacitor and the other end to the pin 5.

Ground one end of the 100 uF capacitor and connect switch and 22k ohm resistor in parallel to the capacitor. Connect the 100k ohm resistor in series with 100 uF capacitor which then will connect with the 220k ohm resistor and teh base of the bc557 transistor.

The collector is sourced to 6V power source of the bc557 transistor and the emitter is then connected to the pin 8 and 4 of n555 timer.

1) We connect the 6V battery.

2) we press the switch for about 5 seconds.

- Observes wailing sound with increasing pitch.

3) The capacitor C1 starts charging.

4) When the charging reaches 2/3 rd of the Vcc, capacitor starts discharging and while discharging if it reaches 1/3 rd of Vcc then again the capacitor starts charging which then in result produce the pulses.

5) Transistor is grounded only when it starts conducting so it is connected to the output so then the speaker produces wailing sound.

6) When you release the switch, the sound will stop with decreasing pitch with time. It long last 30 s until the sound is completely switched off.

As we know that the sound does not switch off instantaneously so has limited use but is widely famous where security feature is installed. Can be used in the time of breaking, robbery, or disaster to warn.