Use a bi-color LED as an ammeter?

Guest
What do you think of this idea to make a simple ammeter for my
motorcycle - Tap into the positive lead in the battery in two spots, so
the battery cable itself is like a shunt resistor. Run wires from
these two spots to a bi-color LED mounted on the dash (and have a
current limiting resistor in-line). The LED glows green when the
battery is charging, and red when it's discharging because I have too
many accessories turned on.

Would this work? Crude ascii below.


|---LED---|
| |
| *<-Resistor
| |
----|+ ---------------------
B |
A |
T |- ---GROUND
----|

Thanks,
-Ryan
 
he battery cable itself is like a shunt resistor
zirco...@yahoo.com
Click to expand...
No.
A LED requires ~1.6V to light.

Use Notepad to draw your ASCII prints
then cut & paste them into your browser.
A monospace font is required (Courier font ).
 
Yes but they are wired into 12v sources all the time with the use of a
resistor. Are you saying the LED would not get enough power to light
up?

-Ryan
 
The resistance of the battery cable is so low that the voltage drop
across it will never be enough for the LED to illuminate.
 
JeffM wrote:

he battery cable itself is like a shunt resistor
zirco...@yahoo.com


No.
A LED requires ~1.6V to light.

Use Notepad to draw your ASCII prints
then cut & paste them into your browser.
A monospace font is required (Courier font ).

Not to mention the fact that the current vs. voltage characteristic of
Click to expand...
any junction diode is exponential, so you want to drive the LED with a
current proportional to the battery current, _not_ a voltage
proportional to battery current.

You _could_ sense the voltage in the manner stated, then amplify it with
a transresistance amplifier that'll drive the LED with a controlled
current that depends on your sense voltage -- but you'll need split
grounds and a good amplifier and all that fun stuff. It's doable, but
see my other post about the suitability of multi-color LED's for
colorblind folk.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
I read in sci.electronics.design that Tim Wescott
<tim@wescottnospamdesign.com> wrote (in <10t8p872h5cps9a@corp.supernews.
com>) about 'Use a bi-color LED as an ammeter?', on Thu, 30 Dec 2004:

if you loan your motorcycle to
me I won't be able to tell charge from discharge.
Click to expand...
I have worked with a number of red-green blind people who had no
problems with resistor colour codes. Can you really not tell a red LED
from a green one? Not even as different shades of what you perceive as
'yellow'?
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
The LED glows green when the battery is charging,
and red when it's discharging
zirco...@yahoo.com

if you loan your motorcycle to me
I won't be able to tell charge from discharge.
Tim Wescott

Can you really not tell a red LED from a green one?
John Woodgate
Click to expand...
That would be pretty radical,
but I have seen techs who put in a 1M resistor
where a 10M was supposed to go.
 
On 30 Dec 2004 13:22:45 -0800, JeffM wrote:

The LED glows green when the battery is charging,
and red when it's discharging
zirco...@yahoo.com

if you loan your motorcycle to me
I won't be able to tell charge from discharge.
Tim Wescott

Can you really not tell a red LED from a green one?
John Woodgate

That would be pretty radical,
but I have seen techs who put in a 1M resistor
where a 10M was supposed to go.
Click to expand...
I went to school with a guy who was red-green colorblind. He bribed me with
a 6 pack to go through his resistor collection and hand him five of each
value. He would go to labs with a piece of styrofoam with rows of carefully
labelled resistors stuck in it. Without that, he was helpless.

Bob
 
OK, Ryan, you are confusing me, and probably others
as well. Can you please draw a schematic of exactly
how you intend to connect these LED's to your
electrical system?
Click to expand...
OK I've put up a crude drawing at
http://myweb.cableone.net/zirconx/LEDammeter.gif

The written description makes sense but won't work for
the reason JeffM expressed above.
Click to expand...
LEDs are available that run off as little as 2ma of current.

If I had two wires to run power from the battery to the bike, current
would run through both of them. This is essentially what I am doing
here, but one wire has much less resistance than the other and the
other has an LED in it. Could I get 2ma would flow through the other
wire?

-Ryan
 
"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:10t8vg9fae925d3@corp.supernews.com...
One of them -- and I think it was the green/blue deficiency -- was very
highly prized for infantrymen during WWII because they tended to see
camouflage easier. Ordinary folk would be so distracted by the color
match that they wouldn't notice the tones being slightly off, the folks
with this deficiency would then be able to see the mismatch in the
pattern.
Click to expand...
My flute teacher (when I was a kid) told me his job was to look at
reconnaisance photos duing the second war - don't remember which type of
colour-blindness he had but they got him to do that for the same reason.
 
On Fri, 31 Dec 2004 06:36:03 +0000, John Woodgate wrote:

I read in sci.electronics.design that Tim Wescott
tim@wescottnospamdesign.com> wrote (in <10t8vbb2dshmk42@corp.supernews.
com>) about 'Use a bi-color LED as an ammeter?', on Thu, 30 Dec 2004:

I'm more or less OK with color codes if it's a 5% type.

Those are usually called '4-band'. Brown, black, brown, gold.

Four-band color
codes throw me off completely.

I think you mean 5-band. They confuse practically everyone. Brown,
brown, black, black, brown? Or is that brown, black, black, brown,
brown?
Click to expand...
I thought that either they'd be bunched up to the left, or the tolerance
band has a wider gap.

Thanks,
Rich
 
On Fri, 31 Dec 2004 05:25:54 -0800, larwe wrote:

Just so I'm clear - You're being sarcastic, right?
Click to expand...
Will you please copy/paste part of what you're responding to?

It's easy to know what the F*** you're talking about when you've got
the post rignt in front of you, typing into google's little input
box, but it sure makes for some lame posts when you're reading the NG
with a real newsreader.

And yes, Rheilly was being sarcastic (about the ammeter), but I'm not.

Thanks,
Rich
 
Blame Google's new service. It's inconsistent and inconvenient.
Sometimes when you click reply, some kind of Javascript-induced text
box opens with nothing quoted in it (that's what I'm typing in now).
Sometimes, it opens the old-style full-page posting window with the
original message quoted and attributed.
 
Besides the fact that there are apparently only two people who care (or
two people who read my posts - take your pick, I'm not fussy) - you
missed the fact that this doesn't make it a quote.
 
On Sun, 02 Jan 2005 15:14:38 -0800, larwe wrote:

Besides the fact that there are apparently only two people who care (or
two people who read my posts - take your pick, I'm not fussy) - you
missed the fact that this doesn't make it a quote.
Click to expand...
Well, you type "QUOTE ->" before it and "<- ENDQUOTE" after it. That will
make it a quote.

Good Luck!
Rich
 
Blame Google's new service. It's inconsistent and inconvenient.
Sometimes when you click reply, some kind of Javascript-induced
text


I hate this too, but the below works for me:
Click to expand...
I just worked out the secret. If you log in to write a reply and check
the "Don't ask for my password for 2 weeks" box, then all subsequent
replies use the non-quoting JavaScript text box. If on the other hand
you leave that box unchecked, then every time you post it will take you
to the old-style full-page posting with automatic quoting.
 
I just worked out the secret.
...[don't] check the "Don't ask for my password for 2 weeks" box,
...[you will get]...automatic quoting.
larwe@larwe.com
Click to expand...
Give that young man a silver wheel mouse.
 
Oh, goodie. Here I was worried it was only me not getting used to this
newfangled stuff. Even peering at them over the rim of my glasses with
a loup doesn't seem to help resolve them.

- YD.

That's why I love my Vishay-Dale axial resistors. Honest to god numbers
Click to expand...
printed on the side instead of color bands. My days of trying to figure
out whether that band is red or orange under awful fluorescent lighting
are over.
 
I love...axial resistors
...numbers printed on the side instead of color bands
Rob Gaddi
Click to expand...
Great--until a sloppy assembler puts them in numbers-down.
 
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