Footprint creativity/IC-change

[Pax Writer]

Hi Guys

I just submitted a request for a pin-compatible replacement for the 74HC7541 over in the datasheet-forum, but now it occurs to me that really I need a creative piece of advice.
The thing is, that I have eight analogue signals which are supposed to trigger an interrupt on my AVR-processor. They are, however, a bit weak, so on my prototype PCB, I buffered each of them with a 74HC7541 gate.
I think it would be better to use an opamp configured as a comparator, but I have already the 2x10 pin DIP footprint from the '7541. Any good (creative) ideas how to replace that IC with op-amps for the eight signals?
Need info? Ask away and I'll answer.

Analogue in:
Low signal = 0.2V
High signal = (to be) ~1V

Digital(ish) out:
Low signal = 0-0.1V
High signal = 3-5V

Thanks in advance.
/Pax

 

[kchriste]

Why can't you use the 74HC7541? You could sub a 74HC7245 if you tied pin 1 high. You won't find an octal opamp that you could just put in place of the 74HC7541. You'll need to redo your proto board. A couple of 14 pin LM339's would work but would take up a little more PCB room.
BTW, you can get the 74HC7541 at digikey:
http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=DK&keywords=74HC7541&x=17&y=11

 

[Pax Writer]

Hi kchriste

Thanks for your reply. Well, the reason I prefer not to use it is, that my local component-pushers don't carry it, which I was incautious enough not to check before designing the board.
I'll take a look at the 74HC7245, but judging from its name, it'll be a longshot.
As for the redesign, there's no doubt that it will have to be redesigned anyway, but for the first couple of protos, I'll make do with this.
Note: I actually found two 74HC7541s in the back of my component-closet, and they work better than I thought. Too bad they are so hard to come by in Denmark. Maybe I'll just have to accept the shipping costs from digikey...

Thanks again for your detailed and thought-through answer.

 

[Blueteeth]

Digikey can be a bit steep on shipping for non-US customers...in fact, its because of that I have never ordered from them, just get a few friends in the US to tag on my stuff to their order then ship me the components via airmail.

You have some other options though for copmonents:

Farnell (farnell.com). - minimum order (20UKP), but they have almost everything.

RS (rswww.com) - slightly more expensive, but worth it. Pro's.

Ebay. - the 'ol favourite. You can get loads of components from all over the world...and for IC's shipping is genenrally dirt-cheap. Plenty of 'ebay shops' that stock thousands of IC's. - not reliable though if you need a specific part.

The above three are my main distributors, apart from a couple who only deal within the UK. I'm sure farnell will stock it though.

Blueteeth

 

[BeeBop]

Any good (creative) ideas how to replace that IC with op-amps for the eight signals?

Well, why couldn't you build a sub-cct. that plugs into your 2x10 socket?

 

[Pax Writer]

@Blueteeth: Thanks for the tip. Maybe I'll have a look at Ebay. RS and Farnell I know, and I generally think they're good suppliers.

@BeeBop: I have a feeling this will be a semi-stupid question, but what is a sub-cct?

 

[Torben]

@BeeBop: I have a feeling this will be a semi-stupid question, but what is a sub-cct?

'cct' is shorthand for 'circuit', so 'sub-circuit'. What he means is that you don't need a component of the same size/shape as the original; if you needed to, you could build a circuit on another piece of board, give it the same inputs and outputs as the original IC, and plug the new sub-circuit into the socket or holes intended for the original part.

I've seen people do this for various reasons: to replace an op-amp with a different one with a different pinout; to do something like what you're trying to do, etc.


Torben

 

[BeeBop]

Thanks,
Torben, that is exactly what I meant.

 

[Bob Scott]

'cct' is shorthand for 'circuit', so 'sub-circuit'. What he means is that you don't need a component of the same size/shape as the original; if you needed to, you could build a circuit on another piece of board, give it the same inputs and outputs as the original IC, and plug the new sub-circuit into the socket or holes intended for the original part.
Torben

When I worked in San Jose, we called those "daughter board"s. Those would attach to a (example) servo board, and the servo would plug into the "backplane". Odd that nothing was called the motherboard! The backplane was mounted in a Unibody, and the Unibody was mounted in the final product, a huge 10' X10' X 5' wafer inspection machine.

Bob

 

[Torben]

When I worked in San Jose, we called those "daughter board"s. Those would attach to a (example) servo board, and the servo would plug into the "backplane". Odd that nothing was called the motherboard! The backplane was mounted in a Unibody, and the Unibody was mounted in the final product, a huge 10' X10' X 5' wafer inspection machine.

Bob

Cool. If I'd known when entering university the difference between "computer science" and "computer engineering", I would have gone into the CE line of study instead.

Pax, just to clarify: I didn't mean that "sub-circuit" always means what BeeBop and I said before, just that it's a term that you could use to mean that. It can also mean any smaller portion of a larger circuit. I think you know that but I just wanted to clear it up in case.


Cheers,

Torben

 

[Pax Writer]

Hey all of you

Thank you for the input. Incredibly, I didn't consider making a sub-cct (new word in my vocabulary, yay!), but that's an excellent idea for my prototypes.
Have a nice one!
*off to solder*

Best regards.