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Trigger and shape white noise
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samsam



Joined: Mar 01, 2017
Posts: 22
Location: France

PostPosted: Fri May 05, 2017 9:41 am    Post subject: Trigger and shape white noise Reply with quote  Mark this post and the followings unread

Hi everyone.

I have made a very simple circuit to create white noise, so far it works fine.
Here is the circuit i have found.
I have used bc549 for the transistors

Posted Image, might have been reduced in size. Click Image to view fullscreen.

On the other side i have a 555 timer that create a pulse.

Would there be simple a way to trigger the white noise with the 555 timer (pin 3) and add a decay control to create a percussive tone ?

I have found a schematic for a simple hi hat but it doesn't work at all on my breadboard. Sad

Here is the schematic of the hi hat i have found.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Thanks you
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electrotech



Joined: Apr 24, 2013
Posts: 38
Location: Ayrshire Scotland

PostPosted: Fri May 05, 2017 1:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi samsam,

Well, the second circuit you've posted is certainly one way of doing what you want... however, it relies on the energy of the input trigger going through a voltage-doubler circuit which then powers the output transistor (forming a simple VCA) and the indicator LED. In fact, the LED takes nearly all the available current, so make sure that you use a low-current, high intensity LED (preferably red in color) and I would also increase it's current limiting resistor from 1k to 2.2k.
The trigger input voltage should be around 12V and an ordinary bipolar 555 (not a CMOS type) should be used. Directly connect to pin 3 on the 555.
Also, if the input pulse is too short it might not work.
If you get the LED flashing you should be able to get some audio output.
Lastly, I would add a 220R-470R resistor in series with the 1M decay potentiometer so that the minimum resistance can't be turned all the way down to zero as this is effectively shorting the output from the voltage-doubler to ground and without any current limiting resistance might damage the input drive circuit.

Hope this helps.

Andy
EDIT : I've realised since posting this that the two 10uF capacitors plus diodes are NOT a voltage-doubler circuit here at all - they're actually part of the envelope generator...

Last edited by electrotech on Fri May 05, 2017 3:52 pm; edited 1 time in total
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samsam



Joined: Mar 01, 2017
Posts: 22
Location: France

PostPosted: Fri May 05, 2017 2:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks electrotech for the reply,

On the breadboard i didn't even get the led flashing. It was silence and no light Sad even at 12 volt.
The transistors i used were bc549. Those transistors works on the first circuit i made and they have worked to create white noise. I guess they should be allright to make the sound i want.

I will try again tomorrow. My timer are ne555. Are they bipolar ?

Do you suggest any other way to do this ?
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electrotech



Joined: Apr 24, 2013
Posts: 38
Location: Ayrshire Scotland

PostPosted: Fri May 05, 2017 3:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi samsam,

I was wrong with my circuit description, sorry Embarassed
The input part (trigger) is not a voltage doubler at all in this circuit because the input pulse doesn't go negative, only positive.
It's actually an envelope generator with a fast attack and an exponential decay.
If you didn't get the LED flashing then the envelope waveform voltage must be too low therefore no signal output either. The drive signal needs to be quite powerful (low impedance) for this to work and yes, an NE555 is a bipolar type and should be OK.
There will be other ways to do this but not as simply as this one and I can't think of another way at the moment.
You should try to get this one to work - aim to get the LED flashing first because if that part doesn't work then the rest won't work either.

Andy
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samsam



Joined: Mar 01, 2017
Posts: 22
Location: France

PostPosted: Sat May 06, 2017 5:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the help Andy.

I have done the circuit again this morning and it works. I have probably wired something wrong yesterday, I checked a 100 times.

Sometimes it s better to redo the all thing i guess.

Thanks for your help.
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electrotech



Joined: Apr 24, 2013
Posts: 38
Location: Ayrshire Scotland

PostPosted: Sat May 06, 2017 9:37 am    Post subject: Reply with quote  Mark this post and the followings unread

That's good that you've got it working thumleft
Yes, it's very easy to plug components into the wrong holes on breadboards, I'm sure we've all done it at some time - I know I have.

Andy
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richardc64



Joined: Jun 01, 2006
Posts: 679
Location: NYC
Audio files: 26

PostPosted: Mon May 08, 2017 7:43 am    Post subject: Reply with quote  Mark this post and the followings unread

electrotech wrote:
I was wrong with my circuit description, sorry Embarassed
The input part (trigger) is not a voltage doubler at all in this circuit because the input pulse doesn't go negative, only positive.


No, you were right. When the trigger goes low it's less positive than it was an instant earlier, i.e., negative. Doesn't matter if it's a continuous stream of square waves, or a.c., or spaced out, one-at-time trigger pulses. It doubles the amplitude of the trigger pulse -- in theory. These circuits always fall short of putting out twice what's put in.

In other news: At 6volts I needed 27k to make an ultra-bright LED merely bright.

That noise source samsam found will work @ 9V. The one-transistor VCA is known to work at 5V.

I'm always happy to see success with simple circuits, so Congrats to samsam.

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