[sdiy] Generating +5V gate signals from +3.3V logic

[Andre Majorel]

On 2012-03-05 12:19 +0000, Steve Lenham wrote:
> >>Ok, so what do you do for gate outputs then, Andre?
> >
> >What Tony Allgood does in some of his modules : 75 ohm resistor
> >inside the feedback loop. The part I haven't figured out is how
> >to generate a +5.00 V (or, if possible +5.000 V) voltage without
> >using too many components. I suspect a voltage reference is
> >needed plus one comparator and one op-amp per gate.
> 
> Why not just use a CMOS analogue switch to gate your chosen
> reference voltage to the output buffer opamp?
>
> If you are using a non-inverting opamp, the input impedance can be
> made v.high, so switch on-resistance is not an issue and cheap
> 405x/74HC405x devices can be used.

Interesting. Don't you mean 4016/4066 ?

> You get three or four switches in a single package

That seems on a par with quad comparators like the LM2901 and
LM339 : 4 channels per DIP14.

> plus, if you choose HCT parts they will accept Tom's 3.3V
> logic without further translation.

My gate outputs usually see a 30 VPP swing at the input. Would
a 74HCT handle that ?

I see an advantage to comparators : they lend themselves to
positive feedback. Useful for generating gates from slow moving
signals. Are there advantages to analogue switches besides
compatibility with 3.3 V logic ?

-- 
André Majorel http://www.teaser.fr/~amajorel/