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Sparkfun Sequencer with Eurorack, clocking issues
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Sparkfun Sequencer with Eurorack, clocking issues
Sciamano
Hi!

I'm trying to interface a Sparkfun Sequencer with my eurorack modules.

So far, once the kit was completely built, I decided to make it interfaceable with standard eurorack systems.

Here is the PCB:



As one can see, there are many possibilities for connections...

At first, I wired the CV and GATE labels, appearing at the top left of the board, with two jack sockets and....they work!
I'm now able to play a sequence with my modular (GATE -> A-140 ADSR -> A-131 VCA ; Z300 MKII VCO -> A-131 VCA).

But I wanted more....

On the bottom left side of the PBC this sequencer has some extra I/O connections: Clock, Stop, Run and Button!

Here there are the some details regarding those connections:

Signal: As Output -> As Input
CLK: Pulses at tempo generated by clock system -> Causes CD4017 counter to increment
STOP: Indicates sequencer is stopped -> Resets CD4017
Holds clock divider in reset
RUN: Indicates sequence is playing ->Allows 7555 to run
BTN: Indicates button has been pushed -> Pulses cause play/stop to toggle

I picked up the explaination of the whole schematic (https://learn.sparkfun.com/tutorials/sparkpunk-sequencer-theory-and-a pplications-guide), and noticed that those connections can be used to connect two cascading sparkfun sequencer.

Moreover, In the guide it is said that in order to have the two sequencers run togheter and to use those connection properly, one has to cut the copper traces that otherwise will short the input connectors with the output connectors.

So I cut those traces and mounted on my sigle sequencer some male pin headers, in order to be able to choose between the original configuration, and the externally controlled one.



When all the bridges short the male pin headers, the circuit is in the original configuration and all works properly.

When I remove the brigde to allow a clock to drive the sequencer, and feed the CLOCK IN with some clock signal, the sequential part works properly (leds light up at the correct step, rate led is blinking along with my external clock) but no GATE and CV sound are generated !!!!

What could be causing this ?
guest
did you remove all jumpers during testing? if so, try removing only clk_in jumper, and then push the button for start and see what happens. they mix run_in and clk_in to the gate circuit. as for the CV, no idea why that wouldnt be outputing. you should meausre the voltage on the pots and the output mixer stage and see if the signal is getting passed.
glennfin
I want to do exactly what you're doing... use this with eurorack. Keep it up!..please..
Sciamano
guest
Thanks Guest!
So I removed only the clock jumper and pressed the button:
-Leds are blinking when their step is activated, the tempo is set by an external clock.
-CV works, as you said it's the gate output that doesn't work.

Moreover, it has different behaviours when in short or long mode.
In short mode it cannot generate any gate.
In long mode, it does generate gate, but only if there's a non-active step before the active step (by that it means that i can make a 4 step sequence out of 8 steps).

from the guide:
Long gate = running && step switch
Short gate = running && step switch && clock pulse

So the problem may be that the clock pulse isn't big enough, to activate the comparator thet sets the limit for the gate to be outputed, in short mode
Sciamano
I run the clock signal trough a filter (just to overdrive the clock signal a bit, without filtering) and....surprise !!! Short gates are working now.

Yep, but it would be great if it was possible to lower a bit that threshold in the comparator right ??? so no to waste a module just to pump a signal....
guest
try reducing R13 to 22k and see if that helps. there is an issue here with the way they implemented &&, as its level sensitive.
Sciamano
guest
I had at my disposal only a 20K 1% resistor, when I've tried to change R13.

results:
external clock works only in long mode (as before)
long (legato) mode is not "long" anymore, it behaves exactly as the short mode ( I feared this).

maybe a 22K (that extra 2K of resistance )would make it work again with the internal clock and legato mode, but if the external clock is not passing even with a 20K resistor, I doubt it will work with a 22K...

As you said the problem is with the implementation....
Sciamano
Now I have to decide wheter to :

-keep it as it is....when I need it to be externally clockable (like 90% of the times) I will waste a VCA to overdrive that clock a bit...

-adapt the level of the external clock by running it into an op amp...does a simple non-inverting configuration will work ? I have some protoboards, and I could use the 9V battery to feed the op amp....
guest
what voltage are you running the sparkfun board at (is it from a wallwart?)? what voltage is your clock signal going to (0->5, -5->+5,-12->+12,etc)?
Sciamano
- the sequencer is supplied by a 9V battery, but in the end, I will substitute it with a 9V DC wallwart supply (because I'm going to receive a Burton simple quantizer board and I don't want to buy a stock of 9V batteries)
- The external clock i'm using goes from -5V to +5V (it is actually a square LFO)
- The internal CLK_IN signal (that can be found at R13) goes from 0 to about 7.2 V
guest
ok, that helps a lot. you could rework the resitor values to have it function, but then it might not work if you use a different clock source. so the best method would be to buffer the signal, and have it go from 0V to Vcc. you could use an opamp in comparator mode, some transistors, or a digital logic level shifter.

also, if you run it off an external power supply, measure the voltage first. some wallwarts are unregulated, and can be as high as 20V for a 9V supply. since there is no overvoltage protection or regulator onboard, this could damage the sequencer.
Sciamano
Ok, thanks! I do not feel confident enough to rework all resistors' values hihi

I think I'm going to use a op amp in comparator mode, with a diode at the output to take just the positive part of the clock. Then I'll need to scale the signal making it smaller, possibly with the same amplitude as the gate from the sparkfun.

It's the first time I have to design a circuit from scratch so have a few doubts...

I need one op amp as a comparator, and that's fine...I will use a voltage divider at one terminal to set the offset of about 2.5V (sound reasonable enough).
Then I'll pass the signal through a diode and then I'll need to scale it down to 7.5 V....
How to scale it ? can I use a voltage divider ? if so, what kind of single op amp do you suggest ?

Alternaltively I could use a buffer with a non-inverting configuration and set the gain in order to lower the comparator output to 7.5V....

What do you suggest ?
guest
the clock basically goes from 0V to Vcc on the sequencer. so get a rail-to-rail opamp, and connect its powersupply rails to 0V and Vcc, and it should do the right thing. an LM324 or LM358 would probably work if you dont have a rail-to-rail opamp handy.

2.5V sounds like a reasonable threshold. and be sure to put a 10k or so resistor to the input of the opamp from your jack. since its negative rail is 0V, it needs a bit of protection against negative going signals.

or you could use an actual comparator (LM311 or LM339).
Sciamano
OK!
I was in doubt whether to use or not a multi-op amp chip and just using a portion of it (i just need an op amp as a comparator), but from your message I guess it's not an issue.
guest
yeah, its fine to do that. just tie up the pins on the unused opamps. typically i will connect the negative input to the ouput, and then tie the positive input to ground (or midrail).
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