[sdiy] Keyboard matrix decoder

[harrybissell]

Hi Luis...

unless you are a glutton for punishment... why not use a DAC instead of the
R/2R ladder.

If you do the R/2R ladder you'll probably have to match the resistors to 0.1%

Another good trick is to make the 2R by series of two R resistors (make R by
parallel
of 2R...) this way you use onlt one value...

If you drive with CMOS, you need to make the ladder resistors MUCH larger than
the
internal resistance of the CMOS... often hundreds of ohms... so using 50K -100K
might
be appropriate...

Or... use a DAC ???

H^) harry

Luís Marka wrote:

> Hi,
> I got a broken Yamaha DX27, and I am planning turning the dead thing into a
> CV keyboard. Besides pitch and modulation wheels, I still get two sliders
> that can be turned in CV sources as well! My home brew modular is hungry for
> a CV source!!
> The DX's keyboard matrix is an arrange of 6 rows by 12 columns, and I am
> modding it to 8 by 8, as required in the decoder circuit I am building. An
> inverted Trigger signal will feed the latch IC, 74HC373, which will feed a
> R/2R network. But right now, two things are puzzling me: First, I've seen
> R/2R networks using almost every resistor value, from 1K/2Kohms to
> 100K/200Kohms. I am planning to use matched 10K metal film resistors. Does
> it matter at all?
> And second: Some designs use a 4050 buffer between the latch and the R/2R
> network, others plug the network directly on the 74373 outputs. Is that
> buffer needed?
>
> That's it for now. All my best,
> Luis Marka