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 Forum index » DIY Hardware and Software
555 output to synth gate
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Xinod



Joined: Oct 06, 2013
Posts: 3
Location: United Kingdom

PostPosted: Sun Oct 06, 2013 4:48 am    Post subject: 555 output to synth gate Reply with quote  Mark this post and the followings unread

Hi all,
this is my first post, I'm a very beginner with DIY electronics but willing to learn.

I'm trying to use a velleman mk107 kit (schematics here: http://www.niqro.3x.ro/joc2/Velleman_MK107.jpg)
as a basic step sequencer to drive a monotron cv and gate,
following instructions like this video
http://www.youtube.com/watch?v=pJ35ShtB8XI

the cv part of it works alright, I have a few doubts about it but the main problem is the gate:
I'm using the 555 output pin (3) and it seems to be constantly on, ticking
(at sequence speed even when the sequencer is off!)
the tick is a kind of static crack on every step and at the same time there is a constant flow of current which makes the monotron constantly play

Do I need a sort of inverter to get a low signal high only on the step?

I understand that even sorting this issue the gate will be completely dissociated from the cv,
is there a relatively simple way to switch steps off?


Thank you very much for any advice

Dan
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elmegil



Joined: Mar 20, 2012
Posts: 1506
Location: Chicago
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PostPosted: Sun Oct 06, 2013 6:45 am    Post subject: Reply with quote  Mark this post and the followings unread

I can't get the original schematic, so I can't make specific recommendations.

What I'd suggest though is that the 555 probably has a very narrow, downward pulse. This can be adjusted with the selection of resistors and capacitor in the timing section of the 555 circuit.

The "Astable Mode" section of this Tutorial might prove helpful:

http://www.instructables.com/id/555-Timer/?ALLSTEPS

And there is a lot of detail in the datasheet:

http://www.fairchildsemi.com/ds/LM/LM555.pdf
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MusicMan11712



Joined: Aug 08, 2009
Posts: 734
Location: Upstate New York USA
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PostPosted: Sun Oct 06, 2013 10:06 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
I can't get the original schematic, so I can't make specific recommendations.


I couldn't get the link to work either. Try the pdf file here: http://www.allspectrum.com/velleman/MK107/

Steve

PS: Don't thank me; thank PHOBoS--he's the one who gave me the index a few years ago.

PPS: I am not really a builder, though I do like to look at circuits. Maybe someone can correct me if I am wrong, but isn't the 555 out supposed to be sending out clock all the time, with sw1 and sw2 controlling just the sequencer? Or does sw1 also turn off the 555?
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elmegil



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PostPosted: Sun Oct 06, 2013 12:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

So yeah. The low time on that circuit is .693 x the lower resistor (3.3K) x the capacitor (1 uF). That means it's .002 seconds, which is going to be a little tick at best.

To get something more useful, you'll need to change the resistors and/or the capacitor.

As a for instance:

Let's say the fastest rate you would want to clock at is 240BPM. That's 40 Hz (240 / 60) and the length in seconds of one cycle is 1/40 or .025s.

So your cycle has two components. The high time and the low time. The high time is .693 x (R10 + R9) x C and the low time is .693 x R9 x C. The total time for one cycle, once you simplify it a bit, is .693 x C x (R10 + 2 x R9), which we want to be .025s at its fastest. The potentiometer adds to the value of R10, so we're assuming that's at zero resistance for the fastest rate, and as it increases it will slow the entire cycle down (although it won't affect the low time).

Work that through with a desired cycle time of .025 and you get R10 + 2 x R9 = 36K. The easy way to deal with things is to make R9 & R10 the same value, so that would be 12K each.

If we go back and check the low time from there, that means our low time is .008s, which is a bit better than that tick of .002s.



If you want a truly 50% duty cycle at the fastest speed, the duty cycle is R9 / (R10 + 2 x R9) or R9 / 36k = .5, or R9 = 18K. You may not want R10 to be zero (but with the pot there, you could....), but you'd want it to be low, say about 1K. The low time with R9 = 18K is .0125s which is half of that .025 period.




Which comes back around to: I'd suggest trying 12K (or 10K might be easier to find) for both R10 & R9, or 18K for R9 and 1K for R10, and see if you get results more to your liking.
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Xinod



Joined: Oct 06, 2013
Posts: 3
Location: United Kingdom

PostPosted: Mon Oct 07, 2013 6:32 am    Post subject: Reply with quote  Mark this post and the followings unread

thank you very much for the detailed reply Elmegil,
I'll give it a try as soon as possible
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PHOBoS



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PostPosted: Mon Oct 07, 2013 7:50 am    Post subject: Reply with quote  Mark this post and the followings unread

MusicMan11712 wrote:
PPS: I am not really a builder, though I do like to look at circuits. Maybe someone can correct me if I am wrong, but isn't the 555 out supposed to be sending out clock all the time, with sw1 and sw2 controlling just the sequencer? Or does sw1 also turn off the 555?

you're correct.. the 555 provides a constant clock to trigger a shiftregister which can be filled or emptied (reset) with the 2 buttons.
Maybe Cynosure knows a bit more about how it could be used with a monotron.

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Cynosure
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Joined: Dec 11, 2010
Posts: 692
Location: Toronto, Ontario - Canada
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PostPosted: Mon Oct 07, 2013 8:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
Maybe Cynosure knows a bit more about how it could be used with a monotron.

Maybe. I will try.

The MK107 doesn't have a voltage regulator, so you might be outputting too high of a voltage. 5V is the max that you should be sending to the Monotron inputs. Either add a regulator (you may need to adjust some values if you do that) or are a resistor to the + side of the pots for each step so the max output is limited to 5V. Also, add a divider to the gate output.

Also, you need to add a serial resistor to each input on the Monotron. I can't remember the exact value, but I think I used 33k. Maybe start there and increase it a bit at a time if it is acting weird.

As for the gate - I don't see in the video where he has any gate control. Are you just taking the clock output? If that is the case, then every note in the step is always going to be on. You might want to split the outputs of each step, send them through a diode, then a switch and mix them all together. That will give you control to shut some steps off. Have a look at the Baby10 schematic. Another additional mod would be to add a simple PWM control to the gate output so that you can change the note length.
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Xinod



Joined: Oct 06, 2013
Posts: 3
Location: United Kingdom

PostPosted: Wed Oct 09, 2013 12:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you very much for the useful insight guys!
I have dividers in place and a 33k resistor on the monotron cv in,
the voltage can slip out of range but the monotron seems to be able to cope with it.

I had a look before at the baby10, definitely my next attempt on a sequencer.
I didn't breadboard the velleman kit and soldered it straight away (didn't even have a breadboard when I started Very Happy ) so swapping components is a bit awkward right now.

But, if I got it right, Cynosure you're suggesting not to use the clock output to the gate, but to duplicate the steps bus I use to feed the cv and drive the gate? that sounds great!
and makes it very easy to switch single steps off!

Thanks again
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