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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Single Buss 1V/Octave Keyboard - Need Help!
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PmCimini



Joined: Mar 23, 2008
Posts: 11
Location: Palo Alto, CA

PostPosted: Tue Jan 17, 2012 10:41 am    Post subject: Single Buss 1V/Octave Keyboard - Need Help! Reply with quote  Mark this post and the followings unread

Here is the deal: I am trying to connect a keyboard I salvaged from an old Casio, but it makes contact with a rubber switch. The problem is that the conductive rubber has a rather high resistance, so I don't think this would be effective in this circuit.

Is there anything I can do in order to make this work? I don't really care about component count or anything. I thought about using some CD4066 in order to control the actual circuit, but the datasheet says it has a Ron of about 80 ohms, and I think that is also high, comparing to the 100 ohm resistors...

Please help me, I need to get this pcb sent by friday, but I am actually out of ideas that could work in this circuit.... Thanks

(on the top right of the schematic, there are the 100 ohm resistors and switches)

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Mongo1



Joined: Aug 11, 2011
Posts: 411
Location: Raleigh NC

PostPosted: Tue Jan 17, 2012 1:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

I know you won't want to hear this, but I've seen a number of people install magnetic read switches in keyboards like that, specifically because of the problem you're having.

Other than that I'm not sure what you can do.

Gary
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Mongo1



Joined: Aug 11, 2011
Posts: 411
Location: Raleigh NC

PostPosted: Tue Jan 17, 2012 1:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

One other suggestion -
Take a look at Craigslist and see if you can find an old Organ. I bought one for $20 and am using the Keyboard from it - it works great - nice clean contacts and no hassles.

Gary
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PmCimini



Joined: Mar 23, 2008
Posts: 11
Location: Palo Alto, CA

PostPosted: Tue Jan 17, 2012 2:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

The thing is that I already have the whole layout laid down and it's based on the keyboard theat I have... I could use a reed switch, but I wanted to know if there is a way to do this electronically...
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Mongo1



Joined: Aug 11, 2011
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PostPosted: Tue Jan 17, 2012 3:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Do you have any idea how much resistance we're talking about? The keyboard on this MFOS is basically just a big voltage divider. I guess the keys in your case end up looking like large value resistors.

I'm wondering if you could put a similar value in the feedback loop of U2-A to compensate for it?
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PmCimini



Joined: Mar 23, 2008
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Location: Palo Alto, CA

PostPosted: Tue Jan 17, 2012 3:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

It is about 1kohm... do you think it would work if I put a buffer before U2A?
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Mongo1



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PostPosted: Tue Jan 17, 2012 4:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, U2A and U2B really are buffers, so I don't think that would really fix anything. Have you been able to look at what's really happening with a scope?

Gary
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diablojoy



Joined: Sep 07, 2008
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PostPosted: Tue Jan 17, 2012 11:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

with this cct i am fairly sure the resistance value is not that important
what is vital is the that the resistances have as little variation between them as possible ie they are all closely matched, within 1% is ok better if within 0.1%
with a keyboard such as the one you are using that is impossible
to accomplish as with those type of contacts the resistance varies considerably
take a look at rays matrix scanning keyboard cct, it is a far better choice for the keyboard you have.

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PmCimini



Joined: Mar 23, 2008
Posts: 11
Location: Palo Alto, CA

PostPosted: Wed Jan 18, 2012 12:37 am    Post subject: Reply with quote  Mark this post and the followings unread

I just checked the matrix scanning circuit and it seems very fine! Thank you a lot. However, if I use high-resistance switches (about 1kohm), are there any modifications necessary?
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diablojoy



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PostPosted: Wed Jan 18, 2012 5:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

I wouldnt think you would need to do any mods
as the keys are simply polled as to there state on or off
the correct voltage level to output is decided by the cct independently
of the switches resistance
but ray wilson would be the better person to ask here
try sending him an email

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adhdboy



Joined: Feb 16, 2005
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PostPosted: Sat Feb 04, 2012 9:25 am    Post subject: Resistive switches
Subject description: C5 May need reduced if you use resistive switches.
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C5 is at the end of the chain. The matrix is driven through R5 1K as it is just fine. If it works you're golden if not try reducing C5 to 22pF or 10pF.

Good luck

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adhdboy



Joined: Feb 16, 2005
Posts: 57
Location: denver

PostPosted: Sat Feb 04, 2012 9:29 am    Post subject: Single Buss Keyboard may work with low R switches too
Subject description: 1K to 2K resitance in the switches may actually be alright.
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The design may tolerate up to 1 or 2 K of switch resistance without degrading performance. The voltage from the divider is dropped on 10M so you should be getting very close to the correct voltages regardless of a bit of resistance.
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