Extracting short single pulse from keyboard trigger voltage.

[steenamaroo]

Hi all.
I have a little concertmate mg1 synth and I'm trying to use the internal trigger to reset LFO cycle.

I think I know how to reset the LFO cycle by having a transistor short a cap to ground.
The problem is, I need a very short (<10ms) positive trigger to do this.

The keyboard trigger is bipolar 12v. -12v when pressed, 12v when released.
Ultimately, I need the presence of that -12 to very briefly open a transistor.

I was hoping someone could point me in the right direction.

Many thanks in advance.

 

[PRR]

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[steenamaroo]

PRR, thanks for taking the time to draw that out.

The problems isn't how to reset the lfo though,
It's how to turn -12v into a 10ms one shot pulse.

 

[njm]

That'l do the job. The cap and resistor make a high pass filter that just lets through the rise of the waveform

 

[steenamaroo]

Oh wow. I'm sorry. Thank you both.

That seems to have done the job.

Rather than shorting the cap to ground, I've used this to feed the cap 1.6v from a battery.
That resets the waveform to peak.

So far so good; I just have to take +/-12v from the synth down to +/-1.6 and I'll be able to reset to peak or trough.

 

[PRR]

> having a transistor short a cap to ground.
> The problem is, I need a very short (<10ms) positive trigger to do this.

> The problems isn't how to reset the lfo though,
> It's how to turn -12v into a 10ms one shot pulse.

I'm not sure what I mis-understood.

If instead of "short a cap to ground" you now need, short a cap to +1.6V", that will work nearly the same.

> just lets through the rise of the waveform

I might say "the fall of the wave" (the 'drop' from +12V to -12V), but up/down is how you see it.