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My question is on lighting 128 LED's from shift registers. I had originally considered using a string of 16 74HC595's but upon further research, it appears they would not supply enough current to light my high intensity LED's, would it be more practical to just buy some TPIC6C596's (which are rated for higher current) or do some sort of NPN emitter follower's on the 595's? I would have to buy the parts either way but am curious if it would be worth the extra money to just buy the 596's or how difficult would wiring all those NPN's be? I'm not familiar with much transistor circuitry and only know the base equations and uses. I have already coded the needed hardware SPI code to quickly light the registers from my arduino, so now it's just down to hardware selection.

Thanks!

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    \$\begingroup\$ Can you please providea circuit diagram. A good answer is possible with an adequately detailed question. Are you intending to use series current limiting resistors per LED (if not why not?), What system voltage, what desired LED current, What LED part number, what lumen/LEd or candella and cone angle and colour, and ... . More information = better answer. VITAL - are the LEDs on'off or brightness modulated. What LED update rate? Why are yuo not using MM5450 / MM5451 datasheet here :-) but !!!!!!!!!!!!!!! \$\endgroup\$ – Russell McMahon Jan 23 '12 at 7:50
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If you are using the LEDs as a display then I would recommend looking at Charlieplexing them.

3 Pin Charlieplexing

There have even been some chips designed around this idea.

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Since you mention using NPN emitter-followers, it sounds as though you are wanting to switch the high side of the LEDs. Many "general-purpose" logic chips can sink more current than they can source, and would thus be more effectively used to switch the low side than the high side. A 74HC595 should not have any particular problem sinking enough current to light an LED used as an indicator for typical indoor applications. If you want to drive an LED so as to be visible from some distance away even in bright sunlight, you will have to use something more substantial.

If you're driving LEDs, though, I would suggest that you might want to consider using a shift register with controlled-current outputs, rather than one which will switch all it can. Doing so will save you a resistor per LED. Such chips can be had at very reasonable prices (e.g. a STP16CP05PTR, which switches 16 outputs, costs $0.76 in onesies or $0.52 in 5000's). Depending upon the assembled cost of each resistor, using such a chip may be more economical than using a non-current-controlled chip, even if the latter were available for free.

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