Siren Generator

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mark_3094

New Member
I have a schematic for a siren generator that I modified from a book.

I understand that the combination of the two 555 timers (one output connecting to pin 5 of the second) somehow produce a sine wave (AC signal)

What I don't understand is how this happens...

Can anyone explain this to me?

Thanks

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ericgibbs

Well-Known Member
I have a schematic for a siren generator that I modified from a book.

I understand that the combination of the two 555 timers (one output connecting to pin 5 of the second) somehow produce a sine wave (AC signal)

What I don't understand is how this happens...

Can anyone explain this to me?

Thanks

hi,
Pin5 is the control input pin.
As the first 555 astable output is connected to the Control pin of the second astable, the timing of the second astable is modified by the control signal.

So the effect is a siren sound.
Its not a sine wave output, its frequency modulated.

Last edited:

mark_3094

New Member
I thought it would have to be a sine wave for a speaker to work...
What kind of signal does is output?

ericgibbs

Well-Known Member
I thought it would have to be a sine wave for a speaker to work...
What kind of signal does is output?

The signal at the output pin of the second astable would be a squarewave of varying period.

The cap and speaker response will make it 'sound' like a sinewave.

As you dont have any values on the 555 cct, its not possible to say exactly.

mark_3094

New Member
So the wavelength of the square wave changes...
What does the C4 cap do then? I thought it would block the DC signal.
I guess the trough of the wave is not 0v then. What makes the trough a positive voltage? is it the C4 cap?

Thanks

ericgibbs

Well-Known Member
So the wavelength of the square wave changes...
What does the C4 cap do then? I thought it would block the DC signal.
I guess the trough of the wave is not 0v then. What makes the trough a positive voltage? is it the C4 cap?

Thanks

hi Mark,
Cap C1[pin5] on the first 555 is not essential, but preferred.
It decouples the pin to 0V, floating input pins can pick up external electrical 'noise' which can cause the circuit to behave incorrectly.

Calling the output signal 'period' would be more correct than wavelength in this case.
So the period of the square wave changes

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andy257

Member
I thought it would have to be a sine wave for a speaker to work...
What kind of signal does is output?

Not all AC signals are a sine wave. By definition of an AC signal it has to have a negative cycle and a positive cycle. Therefore you can have a square wave, triangle wave that is AC etc etc.

mark_3094

New Member
Thanks. I know about C1 on pin 5, but it's the cap connected to the speaker I'm not sure about

mark_3094

New Member
Does the output in this circuit have a negative cycle?

Thunderchild

New Member
C4 is an ac decoupling capacitor its pretty much standard it prevents DC going through the speaker or to be more precise a constant voltage which would produce no noise anyway. It would make the output appear to be true AC as the output of the 555 is not AC but variable DC wich is the same as AC to the condenser anyhow

Roff

Well-Known Member
Your circuit will not produce a siren sound. It will produce bursts of tone.
If you add a resistor between pin 3 on the 1st 555 and pin 5 on the 2nd one, you will get a two-tone sound, which I suppose sounds similar to some sirens. Add a capacitor from pin 5 to ground and you can get rising and falling tones.

Why did you modify the circuit? What did the original circuit look like?

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audioguru

Well-Known Member
The 555 has a max allowed current of 200mA. If the speaker is 8 ohms then the 555 might be overloaded and blow up.

Its loaded peak to peak output is about 5V with a 9V supply. Then its peak output current into an 8 ohm speaker is 2.5V/8 ohms= 313mA which is too high for a little 555.

The square-wave output will sound like a buzzer, not like a sine-wave.

Hero999

Banned
It depends on the frequency, if it's driven at 1kHz the impedance of the capacitor will be 160R at the fundamental which won't hurt the 555. The DC blocking capacitor also makes the peak voltage half so the load is shared between both output transistors.

Roff

Well-Known Member
It depends on the frequency, if it's driven at 1kHz the impedance of the capacitor will be 160R at the fundamental which won't hurt the 555. The DC blocking capacitor also makes the peak voltage half so the load is shared between both output transistors.
Peak currents are still high because the output is a square wave, not a sine wave.

audioguru

Well-Known Member
Gasp!
The output capacitor is only 1uf?

Hero999

Banned
Peak currents are still high because the output is a square wave, not a sine wave.
There will be peak currents but with 1:mu:F I don't think they'll last for long enough to hurt the 555.

Either way, I agree, bump the capacitor up to 10:mu:F and use a 64Ω speaker

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