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 Forum index » DIY Hardware and Software » Circuit Bending
Sync pulse from LED?
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kudamm99



Joined: Mar 08, 2014
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Location: Dayton, Ohio

PostPosted: Sat Feb 03, 2018 3:49 pm    Post subject: Sync pulse from LED?
Subject description: Can I hack a tap tempo button to provide sync in/out?
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I'm curious to know if there's a way to get Korg-style sync I/O via the tap tempo button and LED on my Akai MPK Mini MkII. That way the arpeggiator could potentially transmit sync or slave to external gear. Thanks.
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PHOBoS



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PostPosted: Sat Feb 03, 2018 4:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

It might be possible but I can't tell without a schematic of the thing. There is a chance the LED is multiplexed which means it
is actually flashing very fast when on which could make it a bit tricky. An optocoupler in series with the LED might be a safe
way to tap into the signal without any chance of damaging the MPK. A vactrol might be even better because of the slow responce
of the LDR which could solve problems if the LED is indeed multiplexed. Of course you'd need some extra circuitry to turn it
into a useful signal.

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Last edited by PHOBoS on Sat Feb 03, 2018 5:45 pm; edited 2 times in total
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kudamm99



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PostPosted: Sat Feb 03, 2018 5:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cool, thanks. I just got a bunch of LDRs to make that $3 low pass gate. I'll try working something up.
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kudamm99



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PostPosted: Mon Feb 05, 2018 3:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here's what I was thinking. It's based on Synth DIY Guy's low pass gate from this video https://www.youtube.com/watch?v=8BOnvSQpoGY. Can I simply tap the MPK's LED as shown? TIA!


IMG_1731.gif
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MPK sync out schematic
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IMG_1731.gif


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PHOBoS



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PostPosted: Mon Feb 05, 2018 5:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'd probably put the LED in series but this is easier to do and might even work better. It depends on the MK circuit that's driving the LED.
It could also be that the LED has a higher voltage drop than the one you are adding in which case it might not light up anymore.

For the circuit I would make a voltage divider with the LDR and a resistor (trimpot might be very convenient) and feed that into a
schmitt trigger. You could make that with some transistors but I would use a CD4093 or CD40106. This will guarantee a very clean
(square) output. I'd also put a resistor (1K) in series with the output as a protection against possible shorts though with the
circuit you've drawn that's probably not necessary. (depending on LDR resistance). Just try it and see how it works for you.

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PostPosted: Tue Feb 06, 2018 6:27 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
For the circuit I would make a voltage divider with the LDR and a resistor (trimpot might be very convenient) and feed that into a
schmitt trigger.


As is, the circuit will not work as there is nothing to discharge the 10 µF capacitor (except maybe for an externally connected resistor on the output, but that would give ill defined behaviour), so once it is fully loaded nothing further will happen.

So at the very least it will need some load. Apart from that 10 µF seems a bit large with the resistance range an LDR can have.

But anyway, I'd follow PHOBoS' suggestion.

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kudamm99



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PostPosted: Sun Feb 11, 2018 4:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Is this along the lines of what was described above?


LDR plus schmitt-trigger CD4093.jpg
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LDR plus schmitt-trigger CD4093.jpg


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PHOBoS



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PostPosted: Sun Feb 11, 2018 4:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

kudamm99 wrote:
Is this along the lines of what was described above?

yes, but swap the positions of the LDR and R1+R2 otherwise the output pulse will be inverted compared to the LED.

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kudamm99



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PostPosted: Sun Feb 11, 2018 5:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Great, thanks so much for your help!
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