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Electronics: LED flasher

Posted on 1998-10-23
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Last Modified: 2012-08-13
I am looking to build an led flasher of sorts. (led hooked to a 74164). Problem is the 164 powers up w/ d0-d7 low and I don't want all leds on on startup-  
|      164     |  
|-------|------|        DON'T want this
        |
        /\ led
        |-------- r1---+5v

Thought your not suppose to drive LED's directly even though HCT devices can drive 20ma.
|      164     |  
|-------|------|        DON'T want this
        |
        \/ led
        |-------- r1---gnd

thought about using:
|      164    |  
|------|------|
       |---r1---+5
       \/ led
       |--------gnd

when 164 powers on +5v goes thru r1 to 164 ground, led powers up off, Questions is what happens when 164 goes to 5v won't it go thru the LED.  what the best (proper!) way to wire this??? wiring diagram + explanation would be great!
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Question by:bculver
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5 Comments
 
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Author Comment

by:bculver
ID: 1127543
Edited text of question
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Expert Comment

by:RoadWarrior
ID: 1127544
*smiling* hard to read your diagram, wish they allowed non-prop text here, i think from how i see it, you need another resistor  in series with the LED, 220 0r 330 ohms should do *smile*

Regards

Road Warrior
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LVL 12

Expert Comment

by:mark2150
ID: 1127545
Stick an inverter (7404) on the output of the counter. Tie the LED to the +5 rail via a resistor. Figure for 10mA or about 330 ohms. TTL logic *sinks* current rather than sources current. Thus trying to use the high state to feed an LED to ground generally won't work. The low state can sink about 20mA, but if you drive *all* to 20mA you'll overheat as you'll exceed total draw.

M

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Author Comment

by:bculver
ID: 1127546
mark2150: Was not looking to add extra active parts (IC's). Passive components ok (resistors etc.). If inverting the output with ic's is the only answer, let me know and i'll give you the points anyways. Thanks.

RoadWarrior: Glad you liked my schematics! A pictures worth a thousand words right?:}
 I believe that adding a resistor in series with the led brings me back to the original problem of driving the led with the IC. thanks though!
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Accepted Solution

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mark2150 earned 150 total points
ID: 1127547
TTL is ACTIVE LOW. You're only going to get the LED's to light in response to a 0 output. You want the LED's to come on in response to a 1, you're going to need inverter. 74x04/06/14 will all work. Check datasheets for drive limits, etc.

M
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