Use a bi-color LED as an ammeter?

No. A LED requires ~1.6V to light.

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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

Perhaps for _your_ motorcycle. The most common color deficiency experienced by humans, which occurs most often in men and boys, is an inability to distinguish green, red and gray.

So you can do this (after adding a suitable transresistance amplifier to address the 1.6V threshold problem), but if you loan your motorcycle to me I won't be able to tell charge from discharge.

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

Not to mention the fact that the current vs. voltage characteristic of 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.

That would be pretty radical, but I have seen techs who put in a 1M resistor where a 10M was supposed to go.

I'm more or less OK with color codes if it's a 5% type. Four-band color codes throw me off completely. Even so I always check (and I have a well-organized set of parts drawers) I can usually get close enough with an ohmmeter, even in circuit -- and if not I have my wife and children trained.

And no, I can't tell the difference between green and red on a bi-color LED -- this absolutely drives me up the wall when some clever SOB designs it in, like my Yaesu FT-23 where red means "squelch" and green means "transmit" (or visa versa).

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.

I don't think you want to have 2 volts of drop across your battery cable. That would be a terribly inefficient battery cable.

After solving the amplifier problem, just add two extra green LEDs. Configure the bi-color LED to produce a horizontal red or green bar and the auxiliary green LEDs to add green squares above and below the bar. Now you have a red '-' sign for discharge and a green '+' sign for charge.

I read in sci.electronics.design that Tim Wescott wrote (in ) about 'Use a bi-color LED as an ammeter?', on Thu, 30 Dec 2004:

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'?

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

I read in sci.electronics.design that JeffM wrote (in ) about 'Use a bi- color LED as an ammeter?', on Thu, 30 Dec 2004:

Green/blue problem is a very unusual defect. 15 ohms is much easier. Crossed eyes. (;-)

Purple/brown can be difficult, especially under incandescent light. But it doesn't normally cause problems with resistors, because 2.1 and 4.1 aren't preferred values. 75 ohms/15 ohms is a possible confusion.

Maybe that's foveal tritanopia - where the very center of vision is weak on blue. The brain usually manages to fudge things other than the brightness of really pure deep blues since the perceived brightness of most blue things is from stimulation of green receptors and maybe red ones. But blue bands of resistors, especially 1/4 watt ones, all too often look like green bands.

I have heard of it. I do not know what causes it or if this is something normal.

- Don Klipstein ( snipped-for-privacy@misty.com)

Here is the relevant context from JeffM's post:

You replied:

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?

The schematic in your original post didn't make any sense. The written description makes sense but won't work for the reason JeffM expressed above.

The bottom line answer is that you can sense whether your battery is being charged or discharged, and light up LED's in response to the direction of charge, but doing so will involve more than just LED's and resistors. Also, it may or may not be practical to use a length of cable as the shunt resistor in this application.

--Mac

OK I've put up a crude drawing at

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

I concur with other posters that the circuit doesn't look plausible whichever way I try to allow for ASCII character shifting.

No that definitely won't work.

Regarding coloured LEDs, about 10% of males are red/green colour blind. Actually, 'blind' seems a bit of a misnomer. One of that 10% said he found it hard with dark greens and reds but bright ones were okay. Everyone fades into monochromatic vision at low intensities, so I would guess that such people just have a brighter threshold for those colours.

In an application such as described, I would have two separate LEDs.

You can get triangular body LEDs, which would be nice for indicating the direction of flow.

Unfortunately for your circuit, they take several volts across them before that 2 ma. will pass through them. If the battery cable drops that much voltage, it will glow in the dark.

Through a wire, yes. Through an LED in series with that wire, no.

Just so I'm clear - You're being sarcastic, right?