555 output to synth gate

[Xinod]

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

[elmegil]

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

[MusicMan11712]

[elmegil]

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.

[Xinod]

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

[PHOBoS]

[Cynosure]

[Xinod]

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 ) 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