# Police Siren (speaker dual 555)

#### trigg

Joined Mar 6, 2012
2
Hello everyone, I'm quite the beginner when it comes to circuits but I'm working on this project in school of a "Police Siren" sounding circuit where two 555 timers work together to generate a police siren like sound.

I managed to make it work without any issues but what I am having problems with is explaining why and how it works. The project does not require any thorough explanation of how the 555 timers work, only a general idea.

I understand that the left 555 timer runs at low frequency which controls the second 555 timer, which handles the frequency of the sound. But what roles the capacitors have I'm not sure about.

The one connected from the left 555 from output 5 to ground I believe isn't really necessary for the circuit to work but has some other function.

Also I don't really understand the functions of the other capacitors, but they have to do with the oscillation. Am I right?

As for the resistors, the 10K ones connected in series I believe can be swapped to adjust the frequency of the sound. The value of these must somehow be the reason for the right 555 generating a higher frequency (sound) that the left one with the 1k and 68K.

Would a potentiometer plugged in where the 10k resistor is, between 3 and 5 be able to adjust the frequency of the shifting between the two pitches? Or would a potentiometer plugged in where the 68k and 1k are adjust this?

I hope you can help me with understanding this rather basic circuit and hope it is possible to explain it to someone like me.

Regards AW

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#### Wendy

Joined Mar 24, 2008
23,480
This is a dual tone siren used in EU, rather than the wail used in the states. Pin 5 of a 555 oscillator can be used as a VCO (Voltage Controlled Oscillator). If you want a wail let me know, I still have a schematic for a transistorized version kicking around somewhere.