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74hc595 Shift Register and current per pin... — Parallax Forums

74hc595 Shift Register and current per pin...

BamseBamse Posts: 561
edited 2006-03-20 19:34 in General Discussion
I'm controlling 16 LEDs with two 74hc595 shift register and I'm trying to figure out the safe current per LED...
Reading datasheets in general is new to me so I wonder if I got this correct...

My datasheet (http://www.st.com/stonline/books/pdf/docs/1989.pdf) says Io=35mA (Output current per pin I guess).
But Icc=70mA (Since Vcc is input voltage, I guess Icc is max current) which give me 70mA/8 = 8.75mA per pin if all pins go high...

I don't have any specifications on my LEDs but they are red and I think I saw that the voltage drop is 2V somewhere...
So if I'm using 5V, the voltage over my resistor in series with the LED would be 3 V and therefore it should be 3/0.00875 = 340 ohm...

Did I get this correct ???

/Bamse

Comments

  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-03-15 23:04
    · The 70mA total current is an absolute maximum, one you shouldn't go near.· The 74HC595 isn't an LED Driver, afterall,·it's all about Data signals.· By dead-reckoning, your 8mA / pin gets you close (8 x 8mA = 64mA.)

    · So, if you want to honk current, then you should drive your LEDs through a transistor buffer.· Or you can live on the edge.· It's only when they're all on that you're pushing the envelope.· All Things Must Pass.

    · That's how I see it.
  • NewzedNewzed Posts: 2,503
    edited 2006-03-15 23:15
    Why don"t you play it safe and put a ULN2803 between the register and the LEDs.· Tie all the LED anodes to 5 Volts, and tie each cathode to the proper pin on the 2803 via·a 270 ohms resistor.

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    Sid Weaver
    Do you have a Stamp Tester yet?
    http://hometown.aol.com/newzed/index.html

    ·
  • BamseBamse Posts: 561
    edited 2006-03-16 01:39
    Looks like I'm going to live on the edge and go for the 8mA per pin...tongue.gif

    The whole idea with this project is a "Button masher" game for two players. Each player have 15 LEDs to light up.·First to 15 LEDs winns, you work against each other·so the idea is to get 15 pushes more than your opponent...

    The 5:th, 10:th and 15:th LED will be replaced with a lamp through a ULN2803 and the 16:th LED will be a fancy tower light and bell like on a slot machine. I probably·need a relay for that one...

    So there will be two units of 16 LEDs so to say...

    Thanx for your help...

    /Bamse
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-03-16 09:47
    Bamse,
    There are other 'high power' shift registers out there. Much higher. Google is your friend.

    With all the LED devices around today, the manufacturers have combined functions to reduce chip count.

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • BamseBamse Posts: 561
    edited 2006-03-16 15:36
    I decided to go with small 12V lamps in stead, the LEDs are nice but you pretty much have to look straight at them to see them...
    A lamp would be viewable from a bigger angle and it just looks better... scool.gif

    So the plan is now to keep the shift registers and buffer them with ULN2803 or similar...
    The other benefits are that I can connect a 12V car battery through ULN2803 and if I need more than 500mA, I could use two of the arrays to get a full amp... smilewinkgrin.gif

    /Bamse
  • Paul BakerPaul Baker Posts: 6,351
    edited 2006-03-16 17:02
    Bamse said...
    I decided to go with small 12V lamps in stead, the LEDs are nice but you pretty much have to look straight at them to see them...
    A lamp would be viewable from a bigger angle and it just looks better... scool.gif
    How about diffused lens LED's? as long as you have a line of sight to the LED you will see it illuminated. You can convert clear lens LEDs to diffused yourself by using a high grit sandpaper and gently sanding the finish to a frosted glass appearence.

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    ·1+1=10
  • BamseBamse Posts: 561
    edited 2006-03-17 20:14
    I think I'm going to give the sandpaper a try just to see how it looks.

    I got some of the lamps (12V, 60mA) and it looks and works great, the ULN2803 does get warm after 5 minutes but not really hot.
    However I already have a bunch of LED's and if I can get them to look as good as the lamps that would save me a few bucks...

    Thanks everybody...

    /Bamse
  • nick bernardnick bernard Posts: 329
    edited 2006-03-17 21:21
    after thought
    the tpic series shift registers from TI are goooood. i use em alot for driving relays and LEDS
    TPIC6C595N et al

    roxon

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    engineer, fireman, bowler, father, WoW addict [noparse];)[/noparse]
  • BamseBamse Posts: 561
    edited 2006-03-20 19:34
    I roughed up the surface with some 100 grit sand paper and the LED is visable from a much greater angle... tongue.gif
    I'm still going with the Lights though, they just looks cooler...

    The TPIC6C595N looks awesome, I saw that Digikey have them for a buck each, I'll definitly order some later this week...

    Thanks for the tips...

    /Bamse
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