Buffering DOUT Triggers

[creatorlars]

I'm continuing to play with ideas incorporating my Midibox stuff with my analogue drum gear and was wondering -- what is the best way to buffer/protect the DOUT triggers from external voltages?  I am thinking you probably just need an LM324 and a diode at each output?  Would a hex inverter be better?  Do you think the diode alone is enough?

I've been mixing the DOUT triggers and other sources by running each trigger through a diode and then a 10K resistor to ground and this seems to work okay.  But what I'd like is to not mix the signals to only go one way only (into the drum circuit.)  That way the "trigger input jacks" for each drum voice also double as "trigger outputs" for the DOUT triggers.  (So I can layer up several drum sounds from a single trigger by patching.)  I just need to make sure the DOUT is protected for when I use the jacks as inputs for other trigger sources.

I hope this makes sense. :)  Thanks for any help.

[nebula]

Have you considered opto-isolators?

[creatorlars]

No, I haven't -- but is that really necessary?  Seems like it could bulky/expensive -- you'd need one for each output right?  I just need a simple solution to stop external voltage going back into the DOUT board.

[nebula]

No matter what you use, you'll need one for each output.

But ... how much are you out if you blow up a 74HC595 ... about a buck or so?  Is it worth it to add complexity to your circuit?

If you decide to go the opto route, you're fully isolated, and I'm pretty sure I've seen arrays of at least 4 of 'em in a single (SMT) package.  There are definitely numerous ways to go about it, but I think optos are the best means of the type of isolation you're talking about, and aren't really that expensive compared to other ICs you could use.

(Please somebody else stop me if I'm being completely stupid)

[lylehaze]

If you're looking for isolation, optos are hard to beat.

If you want to "logically OR"  more than one trigger source into a single trigger input, you can also try open collector circuits. They allow "mixing" without output overcurrent issues.

If you need a voltage conversion, and optional polarity inversion,you might want to use an op-amp wired as a comparator.. someone else did that here recently to trigger a family organ drum unit.

Have Fun,

LyleHaze

[creatorlars]

Looks like optos are the way to go then.  For some reason, I had it in my head that this would be a lot of trouble, but I guess if I can find a quad package type then that would be fine.  I'll just make a board layout for a DOUT that includes them.  Any tips/schematics or specific part numbers would be very helpful if any come to mind.  Otherwise I guess I'll just look through the Mouser catalog and datasheets. :)

[creatorlars]

Attached is my modified PCB layout for a DOUTx2 board with optoisolator-protected outputs.  I'll be happy to share all the Eagle files and an etching layout after I test it, if anyone's interested.  I could also do an "expander board" to use with a normal DOUT board.

All resistors are labeled "220" right now, but the pull-up resistors to +V at the opto's transistors should be 10K, I think.

[creatorlars]

I'd like to confirm that the above layout has been tested and works quite nicely.  I'll make a wiki post with all the info/files soon.

[monokinetic]

Hey creatorlars,

thanks for coming up with this, I want to trigger some diy analogue drum synth stuff from my MBSeq as well, so this will really help!

Please do document it on the wiki, I'm sure others will find this useful as well :)

David

[nebula]

I'm glad it worked for you.  Good job!

[TheAncientOne]

I'm with you guys on the opto-isolator idea. I plan to connect to an analogue synth: these things have +/- 15 Volt supplies and a bit of bad or careless patching can trash logic chips easily. If you just blow a shift register- no problem, but if the output stage in the chip went short to 5 Volts, then ther is a more serious possibility of other chips getting damaged.

Another thought is that with optos the fairly noisy ground of the MIDIbox gear will not be connected to the  analogue system, helping keep your background noise lower.

I have had a small experiment with some high speed optos, as a way of isolating the MIDIbox data paths, and it seems to work. There are quite a few good design for I2C isolators too, which might open up the possibilities of an isolated and possibly remote CV output stage. I've had no time recently, but I will get back to this as soon as I can get on with my electro-music stuff.

[Antix]

Hi,

Anybody can say me what are tre trigger output on DOUT

I am going to build MB808 drumseq stand alone but i don't know

the pin on DOUT that trigger the drum voice.

can You help me?

regards