Converting bipolar 12v to single pulse.

[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.

 

[PRHunt]

I wonder if the little 555 timer might do the job. It takes a -ve going trigger and produces a positive pulse output. The duration of the output pulse is set by a RC combination. Just google for datasheets and circuits/tutorials. Here's one to get going with.

You would be looking at monostable (one shot) operation.

To stop the trigger going below 0v, you could try running the trigger voltage through a diode (maybe 1n1418?). The trigger needs to be briefer in duration than the output pulse. So, if the voltage stays at -12v while pressed, you'll need to configure the trigger as an edge trigger. This doc might help. Adjust R1 and/or C1 to get a time constant smaller than the 10ms output trigger.

No idea if this will actually work, but hopefully it will give you some ideas.

Paul

 

[Steenamaroo]

Thanks PR. I've read a little about that suggestion. It totally reminds me of when they tried to make you read a french novel at the age of 14.......lol.
Needless to say I didn't do to well at french.

I can't remember if the problem was retrig on key release, or if it was multiple triggers while a key was held down, but either way some lad was having troubles with a 555.

A chap on prodigy pro has posted a simple little diagram with a cap and resistor functioning as a high pass filter?
I don't understand it, but I'm about to look into it.

 

[Steenamaroo]

Whaddya know, PR. I got the trigger sorted.


Here's what I currently have.

Every time a key is pressed, c25 sees 1.6v (from a battery) for a short period, resetting it to peak.
I guess -1.6v would reset to trough, so here's my next mission.

I have +/-12v available to me. What would be the best way to scale that down to +/-1.6v, without interrupting the -/+12v supplies.


Full schematic http://www.steenaudio.com/mg-1.pdf