Need help please :(

[HanisPF]

Can anybody explain me how does this circuit works ?

[pebe]

Both 555s are wired to run as oscillators. There is a connection with no number against it – it is pin2.

With the maximum value of R shown the first 555 runs at 1cycle every 3secs. T=1.4 x .047 x 47 (in megohms and microfarads). The output is a 5Vpp near-triangular waveform at pin6 which varies (with a 15V supply) from 5V to 10V.

That waveform is fed to pin 5 of the second 555 and is used to vary its frequency. With pin5 at 10V the frequency is the normal one calculated as in the first one. F = 1/1.4 x 0.1 x 0.1 = 70Hz. As the voltage on pin5 falls the frequency increases.

So the output from pin3 will be like siren where the frequency goes from 70Hz up to a higher frequency and back down again every 3 secs.

I hope that explains well enough. If not just shout.