Hi,
Could someone please recommend a reasonable way for connecting 4 x 8 channel relay modules to arduino?
Each channel draws ~20mA. If all of them get active I'm looking at a 640mA draw 
hence my problem 
I'm planning to use 4 daisy-chained 74HCT595 for reducing the number of pins needed on the arduino (Mega2560) side.
TIA,
Pedro
You need 20mA to turn on a relay?
Look at TPIC6B595 instead to sink current thru the relay coils to active them.
74HC595 is not a good current sink, TPIC6B595 is made for that.
tpic6b595.pdf (427 KB)
CrossRoads:
You need 20mA to turn on a relay?
Look at TPIC6B595 instead to sink current thru the relay coils to active them.
74HC595 is not a good current sink, TPIC6B595 is made for that.
Most of those Asian relay boards use an input opto-isolator for each channel, so yes 20 ma input current to drive the opto's internal led. Plus the current consumed by the relay coils (if 5 volt and powered from the arduino 5V pin) one can see that these relay boards are pretty current hungry. Note that one ends up with double isolation, once through the opto and once via the relay coil/contact air gap isolation. So paying twice what is needed only once for galvanic isolation, kind of a waste. But the relay boards are pretty cheap so there is that.
Ah - so you need to source 20mA, vs sink 20mA?
Recommend cd74AC164 instead then.
Really appreciate your input guys
Thinking about it a bit further I recalled "old-school" electronics ... 
What about the idea of sourcing both 74HC595s and the relay boards from an external source (by means of, say, a 7805 - properly heat-sinked - these can source as high as 1,5A).
There would be transistors between the 595s outputs and the relay boards (1 per pin). These would source my relays.
Total current sourcing for a 74HC595 is 70mA, tops. So I thought of resistors (1k) betwen the outputs and transistors' bases... I'd have around 5mA drawn from each output, giving a total of 40mA per 595 chip. These 5mA would be enough to turn on transistors with, say Hfe = 30...
As an additional doubt... If I go ahead with this scheme... should I ground this ext PS's with arduino's ?
I promise when I get a grip on Eagle I'll put up a schem! 
(atm working in geda, linux)
Too hair-brained or has this idea some merit?
And btw ... Merry Christmas to all!
TIA,
Pedro
Why muck around with the 74HC595? Just go right to a part that can source 20mA without all the fooling around - cd74ac164
Secondary source for relay current is fine.
Be sure to connect all grounds together.
I would look for relay boards that use 12V to power the relays (otherwise you will need a 5V power supply capable of providing about 3A) and only draw a few mA from the output pins. Then you can use 74HC595s to drive them if you are short of pins.