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strange 556 behavior - trying to flash LEDs

Given two LEDs, I wanted to build a circuit that would alternate in turning those LEDs on and off, with only one LED on at any given time. LED 1 on, LED 2 off; LED 1 off, LED 2 on.

I have a few 555s and one 556, so I thought to use an astable multivibrator.

I saw and built this circuit:

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from
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s

but was a little bit irked that a small amount of current still leaked through the first LED to activate the second transistor, and I could tell the first LED didn't quite turn off all the way.

So I built my own circuit, using the second part of a 556 as a NOT gate:

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It worked just fine for about a day or two, but then I noticed that Q2 stayed on ALL the time, while Q1 blinked on and off.

At first I discovered a loose ground, and thought that was it, but then the problem repeated.

Then I thought my 2N3904 transistor blew out, so I replaced it - but still the same situation.

Did I blow out my NOT gate somehow? I don't have another 556, so I can't test that theory yet, but that's where my suspicion lies.

I'm using 4 1.5V AA batteries as the power supply.

Any ideas?

Thanks,

Michael

-- Consider: A million and a half new Americans are murdered every year by abortion. No other issue involves numbers that high. Nothing short of a full-scale nuclear or biological war between well-armed nation states would kill that many people, and we aren=92t in imminent danger of having one of those. Jobs? The economy? Taxes? Education? The environment? Immigration? Forget it. We do not have nine million people dying in a typical president=92s term of office due to bad job programs, bad economic policies, bad taxes, bad education, bad environmental law, bad immigration rules=97or even all of these combined. All of them together cannot provide a reason proportionate to the need to end abortion. Make no mistake: Abortion is the preeminent moral issue of our time. It is the black hole that out- masses every other issue. Presenting any other issues as if they were proportionate to it is nothing but smoke and mirrors.

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Reply to
mrdarrett
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A crude fix for that circuit is to just shunt the first LED with a resistor of about 1K. It will reduce the brightness a bit, but probably not noticably.

Another approach (ASSUMING!! it's actually a CMOS 555) would be to build two of the left transistor circuit, but complimentary (one as shown, and one "upside down" with a PNP transistor 2N3906 emitter to

+9, base through resistor to pin 3, LED / 270R from collector to ground).

You're driving those LEDs pretty hard with a 9V supply.

Oh, not so hard then.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

joke circuits...the JTFC member morons would be challenged to do worse...

I really hate that "any ideas?" crap- like you think you can stump someone... so lez see... duh...the NOT gate staying high means the Q1 base drive has no problem pullin' low, not much of load, but pullin' high to 2/3*Vbatt with a 1.4V double Vbe drop from Vbatt, means Vbatt-1.4V>= 2/3*Vbatt or Vbatt >=3*1.4=4.2V or 4.2/4=1.1V per battery makes this thing marginal. That would explain the "It worked just fine for about a day or two" part. You take it from there...the fix is left as an exercise for the student...and why do ya need two current limiting resistors for the LEDs. Didn't you say you were in management?

Reply to
Fred Bloggs

Great! Thanks. Should have thought of that... the batteries are dead.

I received my Bachelor's in Chemical Engineering in 2001 from the University of California at Davis, and passed the 8-hour PE exam, oh,

3 or so years ago. So truly, electronics are not my area of expertise.

And you? What are your qualifications?

Michael

Reply to
mrdarrett

Paging Jim Thompson, my LED won't blink!!!

--
The Force is dark on one side, light on the other and holds the world
together.

Hmmm, just like Gaffer Tape then.
Reply to
Hot Jock

AFAIR, Fred has a PhD in heckling from UC Davis, and a Masters in applied sarcasm from Roanoke. Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Ah, Fred's from my neck of the woods?

M
Reply to
mrdarrett

Apropos of nothing, I thought I'd pass this along

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- made me think of good 'ole S.E.D.

Bob

Reply to
Bob

Thanks! Toxic Granny is now part of my desktop wallpaper...

Michael

Reply to
mrdarrett

Nothing basically wrong with the circuit. Probably poor handling/soldering killed something. If one is blinking the 555 is not at fault.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

Thanks. This is my first NOT gate circuit (and my first 556 circuit actually) and I thought I blew out something. I blew out mosfets before... (now I can appreciate low R_ds_on MOSFETs.)

I used a breadboard so no soldering.

How come you replied? I thought you killfiled me long ago...

Thanks,

Michael

Reply to
mrdarrett

Put the LEDs in the emitter legs of the transistors...

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Experience: What you get when you don\'t get what you want
Reply to
DaveM

message

I'd have, Vcc-resistor-LED-TAP-LED-resistor-GND. 555 Output connected to TAP.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

Is there some reason you can't do this using a bipolar 555?

+Vcc ------+ | [R1] | [LED1] | 555 pin3 ---+ | [R2] | [LED2] | Gnd --------+
Reply to
ehsjr

Posted two days ago...

Newsgroups: sci.electronics.design Subject: Re: strange 556 behavior - trying to flash LEDs Date: Fri, 06 Jun 2008 16:07:17 -0700 Message-ID:

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

Yeah, the original lunar.org link had that circuit:

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but I didn't build it for two reasons:

  1. I didn't understand HOW it worked
  2. I wanted to eventually replace the 2 LEDs with something that consumes much more current (more than the 555 can drive directly).

Cheers,

Michael

Reply to
mrdarrett 

--
Or this, ;)

.+Vcc --[LED1>]-+
.               |
.               |
.               |
.               |
.               |
.555 pin3--[R]--+
.               |
.               |
.               |
.               | 
.               |
.Gnd --[
Reply to
John Fields

Nasty suggestion.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Now John ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson 

--
Yup.

Haste makes waste. 

Thanks. :-)

JF
Reply to
John Fields

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