I have successful completed this project but it does not work the way I expect. can anybody help me here. this alarm start well low, raising until it reach the high level, I want when it reach the high level then it start descend slowly until it stop.

 

Wht does it do now?

 

Wht does it do now?

When I switch it on, it gradual raise until it reach high level, then it keep alarming at that level till I release the press switch

 

When I switch it on, it gradual raise until it reach high level, then it keep alarming at that level till I release the press switch

Where is the switch? It's not shown in your schematic.

 

Where is the switch? It's not shown in your schematic.

The switch is between battery and +3-9v terminal shown in the schematic. my real schematic is as shown below

 

Releasing the switch removes the power, so the circuit no longer works. In that circuit, when power is applied the two 2N551s should start oscillating. First one transistor turns on then the other, then the first and so on. Is that happening?

 

yes, the whole circuit is working well. but what it does not is, it does not lower the alarm as in the real air raid, it just continuing unless you switch it off

 

... not completely sure, but try replacing C4 with a 10 uF electrolytic capacitor, positioning the + terminal towards the speaker.

 

... not completely sure, but try replacing C4 with a 10 uF electrolytic capacitor, positioning the + terminal towards the speaker.

Ok I will try that, thanks

 

Ok I will try that, thanks

No, there is something missing in this circuit. I tried replacing cap 0.1 with 10uf but there was no sound, speaker started distorting

 

... ok
The thing to do is create a LT Spice simulation.
That way you can obtain an understanding of the circuit operation.
.... A free download;
... A better link.

 

It appears that the circuit is doing what it was designed to do. When it is switched on, the multivibrator starts running. The 47 uF capacitor capacitor charges up through the 22 Kohm resistor and gradually biases the output transistors on. It will then continue at full volume until it is turned off.
Regards,
Keith

 

I agree with @KeithWalker above - the circuit seems to be working as designed. You would probably need to add a second oscillator that continuously changes the volume between the output of the astable main oscillator (comprised of VT1 and VT2 on the original schematics) and the output amplifier to get the continuous fade in / fade out effect. This secondary oscillator runs at a very low frequency.

 

yes, the whole circuit is working well. but what it does not is, it does not lower the alarm as in the real air raid, it just continuing unless you switch it off

The way it is described as working is exactly the way the sirens in this part of the state work. They duplicate the old mechanical siren with a tone that starts low and rises and then stays loud at the higher pitch until the power is switched off. THAT is exactly how they are supposed to work.
Tjhe sirens that wail up and down in frequency have a second oscillator that varies the frquency of the first one.

 

It appears that the circuit is doing what it was designed to do. When it is switched on, the multivibrator starts running. The 47 uF capacitor capacitor charges up through the 22 Kohm resistor and gradually biases the output transistors on. It will then continue at full volume until it is turned off.
Regards,
Keith

I disagree. What would be the point of the transistor mutlivibrator? Check this link to the operation of the oscillator/output stage.
siren output
Now think of the right transistor of the multivibrator. It will act as a periodic switch opening and closing, charging and discharging the 47uF capacitor, raising and lowering the pitch of the output section as the multivbrator changes states.

 

OK, if the multivibrator is not working, check the collector voltages and the base voltages. The collector voltages should be going up and down .

 

Here is an LTSpice simulation, modified with a resistor in parallel with C3.
It yields a variation in the output waveform, but not one of uniform or regular appearance
... click on Vout.

 

Here is an LTSpice simulation, modified with a resistor in parallel with C3.
It yields a variation in the output waveform, but not one of uniform or regular appearance
... click on Vout.

I see no reason to put a resistor across C3. And since I do not have the simulator I can not open the ".ASC" file.
In the simulation look at the base voltages and the collector voltages of VT1 and VT2, they should be ramping up and down. If not, the multivibrator is not working.

 

Here is an LTSpice simulation, modified with a resistor in parallel with C3.
It yields a variation in the output waveform, but not one of uniform or regular appearance
... click on Vout.

please change the format of attacment to jpg

 

please change the format of attacment to jpg

What does it sound lie? The sound is what matters here.