Shift Register w/Driver

No.

Not sure.

Yes.

No.

Another possibility is to use more modern CMOS (like the 74AHCT- series, for one) either for the logic or as buffers--those can drive LEDs directly.

-- HTH, James Arthur

--
No

Standard CMOS driving multiple 20mA LEDs? How can he do that without needing a driver?

-- Cheers, James Arthur

Well.. if he uses superbrights at 1mA each, he can use CD4xxx. Or 74HC at about 5mA each.

More than that and a driver is definitely a plus.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

LEDs in series and a suitable higher supply voltage - classic CMOS is good to ~18v enough for 4x high brightness white LEDs and ~5mA. Might need to be careful about power dissipation if more than one chain can be lit at once. It would be better to buffer them though.

Regards, Martin Brown

at

Sure, but the spec was single outputs driving 1-3 LEDs, @ 20mA per LED. That's 20-60mA per output. I'd use less current per LED, but it's not my spec.

-- Cheers, James Arthur

--
Take a look at the referenced Allegro part; built-in 90 mA drivers.

:

Three in series and 5mA sounds good. 20mA? No, that's too much.

-- Cheers, James Arthur

Gotcha--nice part. I thought you meant no driver was needed using the std CMOS on his parts list.

-- Cheers, James Arthur

Look for TPIC6(a,b,c)595 from TI or ST. The letter indicates the drive strenth. Its similar to an 74'595 chip but with open drain drivers. I use it often.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

Get better LEDs and run them indoors? Modern visible LEDs don't need 20 mA unless you're trying to weld metal. ;)

Running the CMOS off 15V helps the drive amazingly. You could run 3 modern LEDs off a 15-V 4000-series device with no problems, as long as you're indoors (say 2-3 mA per LED).

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

There's a single chip that lights a succession of LEDs if you feed it a ramp voltage, LM3914 or LM3915; it might simplify your design.

The CMOS 4017 has a decade counter and 1-of-10 decoder on the chip; I don't know if they're cascadable ... lessee... nah, you only get the

1. But I'm sure it's doable, maybe with a wide counter and a bunch of

3-to-8 decoders with an enable...

Oh, yeah, the 4017 Ioh is spec'd at -0.2 mA, so you'd need either darlington or logic FET drivers.

Good Luck! Rich

Maybe I need more help than I thought. I was looking at the 'Panasonic: LN21CPH' LED and the specs call for

20mA. Is that wrong, what current is practical for this LED ?

James: Did you mistype '74AHCT' I did not find this, but I did find a lot of 74HCT. Here is what I was able to find: Shift: SN74HC165N - Texas Instruments

-1

Counter: 74HCT393

-1

If you have any better links, please forward.

Schematic: I was just planning on driving the clock of the Shift Register and the Counter with the Timer. Wire up the counter so that when it reached a number like 4, it would reset the counter and set the input of the Shift register to go high.

This would effectively create a Chasing Led with every 4th LED lit, and shifting/chasing to the right.

Let me know if this is un-practical.

Thanks

: 0

I usually run LEDs off much less than 20mA myself, first starting ages ago when I changed a bunch of status indicators in a product to

2-3mA. They ran off +24v IIRC, so that saved a lot of power. I switched the optoisolated inputs from 20mA to 2mA as well.

I recently used an 8 MBPS opto with a 0.5mA threshold (HCPL-0300). Cool, id'nit?

Thing is, here the OP asked for 20mA, so who am I to quibble?, but you guys are right--when a colleague asks me for 20mA I assume he needs it, but our gentle reader may not.

-- Cheers, James Arthur

10001_10001_2...

I really meant 'AHCT', but something like a 74AHCT244 (octal buffer) or, better, 74AC244 / 74ACT, to be used as LED drivers. The 74AC parts drive 24mA.

John Field's Allegro part was really pretty good as far as that goes, shift register and drivers, all in a single package. Nico suggested the TPIC6B595, which is also exactly suited to your task.

What's your LED supply voltage, and what are you trying to do? If it's indoors you won't need nearly 20mA per LED, and if you don't need

20mA you could use ordinary parts.

-- Cheers, James Arthur

So you want a string of 12 LEDs, where three are lit at each count, and when the count reaches 4 it resets to 0. That makes your pattern of LEDs lighting ike this:

0 *000*000*000 1 0*000*000*00 2 00*000*000*0 3 000*000*000*

Where the number on the left is the count, and a * indicates a LED that is lit at that count, while a 0 is an unlit LED.

Is that truly what you want? If so, you don't need the shift register. You can wire the 3 LEDs that correspond to each count in series. Use 4 NPN transistors to drive the LED strings with the 0,1,2 and 3 outputs of the 4017 driving the transistors, like this (one circuit shown, you need 4):

12V ---------------+ | [270R] | [LED] [LED] [LED] | /c 4017 ---[3.3K]---| NPN \e | Gnd ---------------+

The 270 ohm resistor will give you 20 mA based on the 12 volt supply and the nominal 2.2 Vf for your LEDs. Any geeral purpose NPN will do.

Ed

a

I was looking at the 'Panasonic: LN21CPH' LED and the specs call for

20mA. Is that wrong, what current is practical for this LED ? What current would you run it at ?

Thanks

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The 4017 looks like a nice option, the Con is that I am looked into one pattern.

Maybe I should be looking at the Johnson instead of the Decade for the counter.

Here is what I imagined: An array of 16 LEDs w/2 LEDs on each Q separated by several inches.(1 on either side of display)

(Reset the Counter on the 2nd tick) 0 *0*0*0*0*0*0*0*0 1 0*0*0*0*0*0*0*0* 2 *0*0*0*0*0*0*0*0 3 0*0*0*0*0*0*0*0*

(Reset the Counter on the 4th tick) 0 *000*000*000*000 1 0*000*000*000*00 2 00*000*000*000*0 3 000*000*000*000*

(Reset the Counter on the 8th tick) 0 *0000000*0000000 1 0*0000000*000000 2 00*0000000*00000 3 000*0000000*0000