Solved

# Electronics: LED flasher

Posted on 1998-10-23
Medium Priority
574 Views
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

|      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!
0
Question by:bculver
[X]
###### Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

• Help others & share knowledge
• Earn cash & points
• 2
• 2

LVL 1

Author Comment

ID: 1127543
Edited text of question
0

LVL 11

Expert Comment

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

0

LVL 12

Expert Comment

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

0

LVL 1

Author Comment

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!
0

LVL 12

Accepted Solution

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
0

## Featured Post

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Monitor input from a computer is usually nothing special.  In this instance it prevented anyone from using the computer.  This was a preconfiguration that didn't work.
Lithium-ion batteries area cornerstone of today's portable electronic devices, and even though they are relied upon heavily, their chemistry and origin are not of common knowledge. This article is about a device on which every smartphone, laptop, an…
This video teaches viewers how to process images for a time-lapse video. Programs required: Adobe Lightroom, Adobe After Effects, Video Editing Program. In Adobe Lightroom: Import sequence image files into Adobe Lightroom: Develop settings of an I…
This course is ideal for IT System Administrators working with VMware vSphere and its associated products in their company infrastructure. This course teaches you how to install and maintain this virtualization technology to store data, prevent vuln…
###### Suggested Courses
Course of the Month10 days, 8 hours left to enroll