# random frequency generator 555

Discussion in 'General Electronics Chat' started by praondevou, Jan 9, 2012.

1. ### praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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Hi,

maybe someone can help me out here.

I'm trying to generate a pseudo random voltage to modulate a 555.

The idea is to have 4 RC oscillators that run on a few hundred kHz. Since they are not synchronized their frequency will not be the same. (I simulated tolerances by slightly changing the 100pF capacitor values)

Every few seconds then a quad FF is triggered to pass the states of the oscillator outputs to a binary weighted summing amplifier.

From there I want to modulate pin 5 of a 555 which is indicated by the 5k resistors on the right.

The main problem is actually to simulate this, Multisim doesn't work at all how I would like it to.

The OP will use a 555/4017 combination for frequency generation, with the frequency changing every few seconds in a random fashion.

The voltage summing part works quite well even in simulation.
Anybody knows if this will work with the 555? Never modulated one.
This is not a circuit to simulate but one to build and try out I guess. I don't have time for this and I'm tired.

Thanks

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2. ### praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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When only simulating the 555 it seems to work. So it would be only a matter of finding the right values for the offset and gain resistors of the opamp.

Thanks for having a look anyway. Comments, suggestions are welcome.

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3. ### THE_RB AAC Fanatic!

Feb 11, 2008
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In the interest of simplifying it, maybe you could just use 6 RC oscillators made from a hex schmidt inverter IC, then connect the 6 freq outputs through 6 resistors directly to the 555 pin 5 input?

Or maybe even simpler; to use one transistor as a typical noise source, and a second transistor to amplify the noise and provide some low-pass filtering too, then couple the low freq noise output into the 555 pin 5 input?

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4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Have you try to use revers bias PN junction witch generates the noise?
Here is the example such a random frequency generator

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5. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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What are the exclamation points for?

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6. ### praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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I wanted something digital because it's easier to simulate. I'm almost sure that Multisim would have a very hard time to simulate an analogue noise generator (and me too )

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7. ### JCOX Member

Nov 29, 2011
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Is that a program you have on your computer that simulates the circuit?

8. ### praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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Yes. It's called Multisim, also called Electronics Workbench. There are other software packages that can simulate, like Proteus, PSPICE, LTSPICE and probably many others.

9. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Bah, buy a soldering iron and some parts and a 'scope like a real man...

10. ### praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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LOL... I don't need a 3rd soldering iron, and don't have money for a second scope...

11. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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How do you get an Ideal Virtual NPN Transistor using that method?

12. ### JCOX Member

Nov 29, 2011
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I was pricing oscilloscope today and they are very expensive. what would be a good entry level oscilloscope?

Last edited: Jan 12, 2012
13. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Rigol or Owon 60Mhz are around \$400-ish, they are decent DSO's, not lab grade.

For better, look for a Tek, Since you won't be capturing digital or single events, look at some dual or quad channel working and calibrated Tektronix scopes on eBay. They'll mostly be analog in the Under \$500 bracket, but what you can get today for \$200 was a \$18,000 scope in 1988.

14. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Hey, you said; "I wanted something digital because it's easier to simulate. I'm almost sure that Multisim would have a very hard time to simulate an analogue noise generator "

So logically if you are going to build one then you choose a design that will be easy to build and do the job, so the transistor noise generator might be best.

It sounds real funky that you would choose a design (to build) that was chosen because it is "easy to simulate" rather than choose a design that was easier to build and possibly perform better.

I was (obviously) joking in a friendly fashion about being a "real man with a soldering iron" because I like to poke fun at this new breed of "simulator jockeys" that can't actually build anything. Some of the most important design decisions happen with actual testing of the first prototype, and if you just need to build one unit (ie hobby level) then why mess about with a simulator if that is forcing you to choose a more complex multi-chip design?

In 35 years of electronics I have never needed to make anything with an "ideal NPN transistor"... I have only needed to make things with REAL transistors.

Virtual transistors are a bit like virtual sex, people might like messing with it on their PCs but it is never going to PRODUCE anything.

15. ### MrChips Moderator

Oct 2, 2009
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Try running this on your favorite simulator.

16. ### praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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I got the joke, no problem at all. I wanted to propose something to the OP that I could simulate instead of building something that would be rather
difficult to troubleshoot over the internet in case it doesn't work. Especially because the OP doesn't have an oscilloscope...
I also don't really have the time and parts to build circuits for the guys here on AAC.
A simulation is a fast way to confirm if an idea could work or not...

I guess I never used a sim for my own projects.

17. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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All apologies, I forgot the $\reflectbox{?}$ sarcasm indicator.

18. ### THE_RB AAC Fanatic!

Feb 11, 2008
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It looks like the joke's on me.

I saw in the first post that you referenced another thread but missed the point that you were designing something for someone else to build as I did not follow the link to the other thread, so I thought YOU wanted to build one of these.

Due to that mistake my "soldering iron" humour was well off the mark, sorry!

No worries I got that you were joking, something about the little face poking its tongue out tipped me off.