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  #1  
Old 03-25-2011, 03:19 PM
tonewill tonewill is offline
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Default MD24: Analog drum circuit trigger

Hello,
I just received, built and tested my MD24. All seems to be fine, very nice.

I'm using it to trigger percussion sounds on a 'clone' of an old analog drum machine I built on stripboard. The sounds trigger fine but there is something I didn't anticipate and wonder if there is a solution for. I'll try and explain:

The two cymbal sounds have connections in the circuit that when +6v is applied alters the attack of the cymbal giving them a quick initial decay so they sound like they're being struck. I thought I'd use a couple of outputs from the MD24 (only +5V I know) for this so that a MIDI note can be used as a control to switch this on/off.

The problem is that when you release the MIDI note, it doesn't just remove the +5V, but drops to 0V (I think) which 'chokes' the sound completely.

Is there any firmware tinkering that could solve this or is it not possible? I suppose you could use the output from the MD24 to trigger a transistor switch, but I'm not very up on all that and it would be a bit of a pain to add this in as there's very little room inside the box I built it in.

Thanks for any help,

Barry.
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  #2  
Old 03-28-2011, 05:29 PM
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Barry,

There is a "beta" firmware version that has a fixed-length output pulse. (This feature will return with updated MD24 firmware & hardware, coming soon.)

More info here:

http://forum.highlyliquid.com/showthread.php?t=312

Let me know if this would be useful--we can make arrangements to get you a replacement IC with the functionality.

Otherwise, you'd have to solve the problem either in sequencing (ie, adding extra note commands to control that "attack" signal) or by adding some kind of delay circuit to the MD24 output. That could be something like a 555 timer in "monostable" mode, for example.

Hope this helps...
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Old 03-28-2011, 07:17 PM
tonewill tonewill is offline
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Hello John, thanks for the reply.

It's not that I need the note event to be a short pulse, in fact, the 'control' note would probably stay on for a whole song.

On the original drum machine, selecting a particular rhythm would add the +6v signal to alter the attack of the sound for that rhythm, while another rhythm would remove it. In other words, a continuous voltage is required to alter the sound, and the sound itself is triggered from another note. I hope that makes sense.

The problem I have that I didn't anticipate is that the MD24 doesn't just 'remove' the +5V signal when you release the note, but drops it to 0v. This is as it should be of course, but doesn't work in this case as it chokes the sound. I suppose I'll have to do it some other way, although other than a physical switch on the front of the machine, I'm not sure how at the moment.

If you or anyone has any ideas that might solve this, I'd love to hear it.
Thanks again,
Barry.
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Old 03-28-2011, 09:50 PM
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Barry,

Ok. I think I understand better. The MD24 also has a "note toggle" output mode, that could be used so that:

One particular note would switch the signal "on"...and the output would remain "on" until the same note was received again. (See mode 03h in the firmware 1.0/1.1 manual.)

Hope that helps. If you can use CC messages or program change messages, the MD24 output could also be toggled using these...
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Old 03-28-2011, 10:11 PM
tonewill tonewill is offline
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Thanks John, I appreciate your help, but that's not quite it.

The problem is that when toggled off, however it's done, 0v is going to the circuit rather than 'nothing' which is what it needs. I suppose you could say it's grounding the connection rather than just cutting it altogether (like a toggle switch) which is what is needed.

The 'on' mode works fine, but the 'off' mode doesn't unless you physically remove the MD24 output from the connector on the circuit. I'm finding it difficult to explain it well, I'm sorry about that.

If you think of anything else, please let me know though.

Thanks again,
Barry.
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Old 03-30-2011, 08:46 PM
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Hi Barry,

You can try using a diode in series with the MD24 output. For example:

(MD24 output) --|>|-- (trigger connection)

With this configuration, when the MD24 output is "on" (5V), it will send the trigger pulse. But when the MD24 output is "off" (0V), the trigger connection will effectively "float", which I think is what you're looking for.

You can also do something similar with a PNP transistor, if this doesn't quite work.

Let me know if we're getting closer....
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Old 03-30-2011, 09:38 PM
tonewill tonewill is offline
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Hello John,
You've understood exactly what the problem is now, and that sounds like it might just work! I'll give it a go tomorrow and I'll post the result.

Many thanks,
Barry.
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Old 04-01-2011, 01:58 PM
tonewill tonewill is offline
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Hello John,
I tried using a diode between the MD24 and the circuit voltage input but it didn't work. It didn't do anything at all either with or without the control note being triggered.

I think I'm going to have to use a transistor as you suggest. All the transistor switch info I've seen has the load between the positive voltage and the collector. I'm not sure how to do this in my circuit however because I just have a single connector on the circuit that should be either +6V or floating. There's no 'return' as such if you see what I mean.

I know it's not really an MD24 problem, but any help would be appreciated if you get time as I'd like to get this working.

Many thanks,
Barry.
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Old 04-04-2011, 02:44 PM
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Barry, please check out this page:

http://www.kpsec.freeuk.com/trancirc.htm

Note the difference in circuits where there is an NPN transistor vs a PNP transistor.
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Old 04-04-2011, 08:32 PM
tonewill tonewill is offline
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Thanks John, that's a good site I've already been there to try and figure this out. I think I'll have to find a general electronics forum as it's not really related to the MD24 directly.

Just for the record though, imagine a wire coming from the +6v of the power supply into a SPST switch and onto a connector on my PCB. That switch is what I need to replace with a transistor.

All the examples I've seen would require:
A wire coming from the +6v rail of the power supply going to 'the PCB' then returning from the PCB to the collector. Not the same.

Thanks for all your help anyway,
Barry.
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Old 04-11-2011, 05:14 PM
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Barry,

This is the PNP switch diagram from the freeuk page:



In this diagram, "+Vs" would be your 6V. "Chip output" can be the output from the MD24 (assuming MD24 ground is connected to drum circuit ground). The load is the location on the circuit board that requires connection to 6V to be triggered...

Note the difference from the NPN diagram:



Not sure if this helps, but please keep us posted with your progress.

Last edited by John; 04-11-2011 at 05:17 PM.
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  #12  
Old 04-12-2011, 08:42 PM
tonewill tonewill is offline
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Thanks a lot John, much appreciate your help. I don't have any PNP transistors at the moment so I'll have to get a few when I next order some parts which will probably be soon. I think it's just a case of getting the breadboard out and trying it to see what happens. I will post back once I've tried it or anything else that may work.

Thanks again,
Barry.
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