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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
CGS10 mini keyboard expand to 32 keys
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Werebear



Joined: Nov 23, 2009
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PostPosted: Fri Jan 12, 2018 10:04 am    Post subject: CGS10 mini keyboard expand to 32 keys
Subject description: 74c922 datasheet expansion to 32 keys possible with Kens design?
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Hi

I've been looking at CGS10 mini pedalboard/keyboard schematics using (MM)74c922 that gives a sixteen key keyboard with CV and gate outputs. According to the data sheet the 74c922 can be expanded to 32 key input using a four input NAND gate, diodes and pull up resistors. However, I can't see how the second keypad switches or more precisely what it switches between.

There is probably a simple answer that I am unaware of but there seems to be nothing to switch between on the second keypad shown in the datasheet unless they connect to X1 to X4, which is not shown. It may be that that connection is implied but I don't know enough to be certain.

I have ten of these chips winging their way from China (£7.odd including P&P) so will be able to experiment soon and hopefully be able to produce 32 key keyboards/control surfaces for future projects.

Sorry to post the entire datasheet as I couldn't see how to extract the desired schemo.

Thanks

Jim


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Werebear



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PostPosted: Fri Jan 12, 2018 10:18 am    Post subject: Image from 74c922
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This is the schematic from the datasheet that I want to use but there appear to be connections missing.


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PHOBoS



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PostPosted: Fri Jan 12, 2018 11:11 am    Post subject: Re: CGS10 mini keyboard expand to 32 keys
Subject description: 74c922 datasheet expansion to 32 keys possible with Kens design?
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Werebear wrote:
..unless they connect to X1 to X4, which is not shown. It may be that that connection is implied but I don't know enough to be certain.

without really looking into it that would be my guess. I think I have some of those chips too, maybe I can do some experiments.

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Werebear



Joined: Nov 23, 2009
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PostPosted: Sat Jan 13, 2018 4:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks PHOBoS. Hopefully mine will arrive before too long and I can play with them to see how this works.

I think part of the problem is not understanding the conventions of schematic drawing, particularly digital stuff where some connections are not shown. I've studied electronics at college and university but that was a LONG time ago and now with synth DIY I'm teaching myself (mostly) analogue electronics a bit at a time through a senior haze. Such fun!

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gasboss775



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PostPosted: Sat Jan 13, 2018 10:32 am    Post subject: Reply with quote  Mark this post and the followings unread

The X outputs are definitely meant to be extended down to the bottom grid of 16 switches. It is very annoying when things like this are assumed ( by the writer ) to be implicit. Causes a lot of grief during the earlier stages of the learning process and sometimes even to those of us longer in the tooth with our hobby.

Werebear thanks for posting this, I reckon it will come in useful at some point as I have one of these 74c922 chips laying around somewhere.
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Werebear



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PostPosted: Sun Jan 14, 2018 4:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks GASBOSS for pointing that out. I was kinda sure that's what was going on.

When my chips arrive I'll have a play and post the results.

And still learning all the time!

Thanks.

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gasboss775



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PostPosted: Sun Jan 14, 2018 7:23 am    Post subject: Reply with quote  Mark this post and the followings unread

Werebear wrote:
Thanks GASBOSS for pointing that out. I was kinda sure that's what was going on.

When my chips arrive I'll have a play and post the results.

And still learning all the time!

Thanks.


You're welcome.

Are these still being sold? If yes can you tell me where?
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Werebear



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PostPosted: Sun Jan 14, 2018 10:47 am    Post subject: Reply with quote  Mark this post and the followings unread

After a few searches I found them here:

https://www.ecrater.co.uk/p/25678197/10pcs-ic-mm74c922n-mm74c922-fsc

But they may be on Ebay. I had no luck with RS, Rapid, CPC, Mouser et al and googling for replacement chips didn't find anything suitable or I would know how to use!

I've just noticed that actually I paid £5 for delivery so total is more like £12.odd but that's for ten IC's. Suppose it will be different if you're not in the UK.

By the way have just checked the page and they claim to have only one available.

Jim

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gasboss775



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PostPosted: Sun Jan 14, 2018 11:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Werebear wrote:
After a few searches I found them here:

https://www.ecrater.co.uk/p/25678197/10pcs-ic-mm74c922n-mm74c922-fsc

But they may be on Ebay. I had no luck with RS, Rapid, CPC, Mouser et al and googling for replacement chips didn't find anything suitable or I would know how to use!

I've just noticed that actually I paid £5 for delivery so total is more like £12.odd but that's for ten IC's. Suppose it will be different if you're not in the UK.

By the way have just checked the page and they claim to have only one available.

Jim


I reckon there's a good chance they could be fakes particularly at that price for 10. This doesn't necessarily mean they won't work, but they might not be quite up to spec. I got a new old stock RS part of eBay a couple of years ago.

The brand mark is for Fairchild and according to their website they don't make these chips anymore. I did find a Fairchild datasheet from 1987 for this chip on one of those sites that specialise in old datasheets.
The Chinese make copies of many obsolete chips, sometimes successfully sometimes not.

It's a good bet that if the manufacturer's website no longer lists the part that it could be a fake. I wish you good luck. Let us know how you get on with them, Ive had some good luck with fakes and a lot of the Chinese ebay sellers have free postage included in the deal so it can be worth the gamble!
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Werebear



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PostPosted: Sun Jan 14, 2018 11:52 am    Post subject: Reply with quote  Mark this post and the followings unread

To be honest those are actually my thoughts but my experience is that I haven't been burnt yet! I've had CA3080s from China that friends told me wouldn't work work very well in T.H.s VCO 1 - the first VCO I've been able to build with a repeatable scale that I could actually play a tune on - and if the specs aren't quite there then the sounds coming from my speakers are far too exciting for me to care right now.
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Werebear



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PostPosted: Fri Jan 26, 2018 6:54 am    Post subject: Reply with quote  Mark this post and the followings unread

OK> The chips arrived all ten of them. I've munged together a switch matrix of 27 switches on a board. I've used a CD4012 for my four input NAND gate for the extended schematic.

I beadboarded the CGS 10 as shown in Ken's schematic with just 16 switches/keys and it worked beautifully even apart from I don't have a multi turn 100k trim pot just a single turn and I made the R2R from 5% 100k resistors soldered together to give the 200ks. I have 1% 100k's but didn't want to waste them prototyping. I also had to solder two 1uF together to give me something close to 2.2uF for the keybounce cap. I'm very pleased with the way this just worked first time on the breadboard. I can already see several uses for these chips for me in future, one being single octave keyboards to control the note of drone oscillators in a tanpura/shruti type instrument and of course Kens suggestion of having it control the root note of a sequencer. Plus you're not dependant on a cap in a sample and hold which will eventually drain - I like my notes looooooooong!

One thing to be aware of is that you cannot play legato(?). Press a second switch and the tone doesn't change until you release the first key. And I haven't tried the data available pin yet for gate.

Moving on to the extended version - I can't get it to work. The first sixteen notes are there and scalable with the trim but once you go beyond the first sixteen it behaves oddly. You press a switch and you hear a note but release the switch and the tone drops to the value of one of the first sixteen notes. I'll put up some video and the schem of my switch matrix to see if anyone can spot an obvious fault but it may be as GASBOSS says these chips from China might not be up to spec or possibly this wasn't going to work anyway. I don't know enough to know at the moment.

I had hoped this would be an easy way to build keyboards. I have a couple of 2 octave keybeds I've been hoarding to use with something like this. But the search continues.

I hope to put up video and specs of what I've done today.

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Werebear



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PostPosted: Fri Jan 26, 2018 10:09 am    Post subject: Reply with quote  Mark this post and the followings unread

First some video, with my dulcet tones or at least dull tones.

The first is just the CGS10 with a sixteen switch matrix as shown in the original CGS schematic. If you wince at what I describe as a scale remember the R2R is made from 5% 100K resistors exclusively. It will probably sound sweeter when I break out the 1% and invest in some 1% 200K and get a multi turn trim for the span control.

The second demonstrates the behaviour with the expansion suggested in the MM74C922 data sheet. I'll post schems of what I did to realise this but this video is to demonstrate the behaviour. Basically keys pressed in rows Y1 to Y4 generate voltage which stays at that level after key release. Keys played in the rows Y1+ to Y3+ - which are connected to Y1 to Y3 by diodes and to Vs by 100Ks - put out voltage but that voltage drops after key release. If I play key 16 which is column X1 switching to Y1+ after release the voltage/tone drops to that put out by key 0 which is X1 switched to Y1. In my head this appears significant behaviour but don't know signifying what!

In the extended version scaling doesn't appear to be an option, even keeping the keys pressed down and twirling that trim to destruction. Oh well.

Schems in next post - yeah I know, not helpful since I keep referring to them in this post


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