Overcoming 8.5mA current limitation

[PaulG.]

I'm neck deep in designing interfaces between low-cost USB digital output modules and various hardware. I seem to be having a hard time relay modules that will work with 5V at 8.5mA. Since I'm looking at up to 24 outputs I don't want to solder discrete components to a protoboard. I really like the Omega relay boards that I have used in the past but they are very expensive. Any suggestions for a less expensive alternative? Thanks in advance.

[SnowMule]

Hardware drivers?  Buffers?  Have a board printed... makes things much easier.
I like the ULN2003 series.  Cheap and easy to interface to.  Run it through an inverter if you don't want the line to "turn on" when the DAQ device initializes (digital lines init as inputs, internal pullup drags all the lines high).

[crossrulz]

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@SnowMule wrote:

(digital lines init as inputs, internal pullup drags all the lines high).

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Depends on the board.  I have a DAQ board whose DIO lines are pulled low.  So you have to read the spec.

But I full-heartily agree with having a small PWB built with the buffer lines.  You can get boards really cheap from Advanced Circuits.  And they give you pop-corn with your boards!

[SnowMule]

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@crossrulz wrote:

Depends on the board.  I have a DAQ board whose DIO lines are pulled low.  So you have to read the spec.

But I full-heartily agree with having a small PWB built with the buffer lines.  You can get boards really cheap from Advanced Circuits.  And they give you pop-corn with your boards!

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I saw "Low-Cost USB" and my mind went to 650x/600x.  But yes, the 634x's and a few others have pull-downs.

AC doesn't do the popcorn thing anymore (at least not the last few orders I've put in; maybe the receiving guys are taking 'em out though ). 

Lately I've been going through ExpressPCB... they do good work for cheap, and their software's real easy to use.  >pic related

[crossrulz]

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SnowMule wrote:

AC doesn't do the popcorn thing anymore (at least not the last few orders I've put in; maybe the receiving guys are taking 'em out though ).

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Maybe you just didn't order enough.  Just last week I got a bag of popcorn as I watched my buddy unpack his brand new boards.  But let's blaim receiving.

[PaulG.]

Thanks for all the input, guys. There's so much I/O out there in the Arduino world that I thought it would be easier to find something off the shelf.

Making my own board with ExpressPCB might be a viable option. Using the ULN2003 (only 7 outputs) would make me a little OCD but I can stuff a 2803 (8 outputs) into a board just as easily. 

[SnowMule]

My issue with off-the-shelf stuff, especially for system-level parts, is that very few of them actually do what I need/want it to. To fit design requirements and space contraints on manufacturing benches, I usually opt to do my own designs. Lets me incorporate everything for a particular piece of equipment on one board, rather than requiring a bunch of external subassy's and hacking them together.