Re: Opto / relay recommendation

Thanks Jan,

Good explanation, I think I'm starting to understand.

But, would inverting the output states in the controller software
(Master 5) achieve the same end-effect as using an inverter IC ?

Regards,

Rab

>
> Rab: Forgive my intrustion into this question you submit to Steve,

but I

> have used dozens of ULN2003 and ULN2004, and I see a

misunderstanding you

> seem to have, and I THINK I can help:
>
> First question: The ULN2003 is a "driver" for switching loads

heavier than

> "logic gates". Such as an electromechanical relay that requires 12

VDC at

> 100 mA. It is a "current SINK Darlington" (seven of 'em in each 16-

pin DIP),

> so it acts as an INVERTER. That is, the "polarity" of the ULN's

output is

> the OPPOSITE of the PC's port connected to it. Unless you had

added another

> inverter (74HC04, etc.) before the ULN...
>
> Second question: The Gecko drives have opto-isolators as inputs.

That is,

> each input-pin "lights up" a LED in a little device inside that

photo-couples

> that to a "photo-transistor" that then signals the Gecko

circuitry. This

> "hassle" electrically-isolates the inputs from EMI noise from the

nasty ol'

> outside world. It is WONDERFUL in doing that! BUT, for every

advantage,

> there is a DISadvantage: These inputs measure 16 mA load with 5

VDC "common"

> (as specified). That is a bit much for the printer-port. OH, it'd

probably

> "work", but only barely. Better: Use something like the ULN2003

to "buffer"

> the signal to the Gecko inputs. (or, probably better, just

a "dedicated"

> 74HC gate would do fine, so long as that gate would not be expected

to drive

> some other "logic input", as the opto-isolator load "pulls it down"

a bit).

> BUT, the ULN2003 alone INVERTS the logic-sense of the parallel-port

output,

> so you WOULD need an inverting buffer, before the ULN... I have

used

> 74HC540 (EIGHT inverters in ONE package!) when needing

an "inverted"

> printer-port, or the 74HC541 when I just want "isolation", as this

is a

> NON-inverting buffer. It separates the parallel-port a bit from

the outside

> world.
>
> Further to this second question, the ULN will "pull DOWN" only.

That is, it

> will NOT "pull UP", so it is NOT a good candidate for routing

the "step" and

> "direction" outputs to an ordinary logic-gate type of input.

HOWEVER, the

> Gecko inputs, as said, are just LED's, and if you "jumper" those

per the

> instructions for -ve going inputs, and use a +5VDC input-common,

this will

> work. And, though not a "high-speed device", I would think the

ULN2003 could

> be expected to function OK on up into the HUNDREDS of kilohertz.

MORE than

> fast enough for "stepper stuff".
>
> Finally, your third question: Yes, but as said, it is not exactly

ideal for

> this. Do what I said two ¶'s above: Use either the '540 or '541

octal

> buffers, and the ULN2003 after those if you need to switch a relay-

coil or

> light some LED-array, etc.
>
> Hope I have not been TOO confusing! Lemme know! Jan

Rowland

>
>
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