[sdiy] SN76477 project / LF398 sample and hold ?'s

[harry]

Howdi Derek...

Derek Holzer wrote:

> howdy,
>
> just dropped into the list to see what's going on out there. i'm in the
> (long, drawn-out, oft-interrupted) process of building a a little
> synth-in-a-lunchbox based around an SN76477 chip, commonly used for such
> hi-brow applications as video games and pinball machines. too bad i haven't
> got it all the way up yet... anybody else work with this IC that i can ask a
> few details from?

Ask John Blacet. His "Dark Star" chaos noise source uses this chip. He once
posted
an earlier non-VC version of a board for this chip.

>
>
> for the list, i've got a question:
> anyone have a simple schematic for a sample-and-hold circuit using an LF398
> IC? seems like the simplest way is just to have a 0/1 logic input on a
> push-button to trigger it, an output and then the hold capacitor.

The data sheet should do it.  I know that Chris MacDonalds MiniModular uses this

chip as a S/H...

If you really want to use a push button... you could just hang a cap on the
output of
the pushbutton and buffer it with a TL071 (etc).  This would be as simple as it
gets.
If you hold the button down it would follow the input, release and it will hold
the last
value.

>
>
> then i looked at Rene Schmitz's YASH sample-and-hold circuit (
> http://www.uni-bonn.de/~uzs159/ ) and saw a pile of gates and an extra
> transistor leading into the logic input pin. what exactly is all that for if
> all you really need to do is trigger it with a push-button switch? i'm
> curious...
>
> also: i'll be running +5 V into it, so how big should the hold cap be?

wrong answer, dad...  ;^)

The question is... how fast do you need to capture the input waveform (smaller
cap is faster) and how long do you need to hold the sample without the charge
draining
off (bigger cap holds longer).  Its like photography... a very fast film is
usually grainy
and lacks resolution... and the slow one is only good if the subject does not
move...

For a keyboard, the S/H cap must be stable, perhaps for minutes at a time... for
an
effect S/H... that is triggered often... a smaller cap will do fine.

Usually the caps range in the .01uF to 1uF range as a practical starting point.

H^) harry

>
>
> thanks,
> derek
>
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