help me understand this

hi in this circuit i can understand that the first 555 timer ic is acting as a astable multivibrator but what is the second 555 ic doing.

can somebody explain me complete working of this project.Thanks

Both 555 are setup as multivibrator.

If you looked at the value of the capacitor at 555 Pin6, you'll noticed that they differ by 1,000 times. The first 555 will produce a very slow oscillation while the second one will produce an audio tone.

The tone of the second 555 is "modulated" by the first to give the low-high-low-high....frequency which resemble a siren.

Edited: the circuit is wrong as the polarity of the 1N4001 from speaker to +12V is reversed and there will be no output to the speaker.

Why would anyone use a 555? You can do the same thing with a simple "inverter" or a "Schmit trigger" Just run the output of one gate into another another gate, then run the output of that gate back into the first gate through an RC parallel network. So easy and way cheaper.

I think you're missing the point Mr Gone

I m Very Thankful To all of u. i m new to electronics project making area. i m not at a level of designing my own circuit thats why i have begun by taking circuits from net.
ofcourse this circuit was of a siren,precisely, a british police car siren.
again a BIG THANKS to all .
if u find anything more to tell me regarding this, that would be just fine i m not going to miss any new thought.

Edited: the circuit is wrong as the polarity of the 1N4001 from speaker to +12V is reversed and there will be no output to the speaker.

Click to expand...

should i change the polarity of the diode.

Either way I think MrFreak needs to do some Googling and complete his coursework himself.

cooltechfreak said:

should i change the polarity of the diode.

Click to expand...

Yes. but only this particular diode. The remaining two are correct.

Sorry for the late reply.