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Background

I am an electronics novice/hobbyist who is recently using an Arduino connected to a breadboard to prototype a set of navigation lights for a remote controlled hexacopter that I am building.

Similar to nav lights on airplanes, I have four strobing white lights that go on the tips of the back four rotor arms, and this alternates with strobing orange lights (beacon) on the top center and bottom center of the craft. This is all fine.

In addition to the strobing lights, I have two solid green lights to go on the two right rear rotor arm tips, and two solid red LED lights to go on the left rear rotor arm tips. All of this is fine too, but the red LEDs are really dim.

Question

No mater if I run the red LEDs through a battery or the Arduino, with a resistor or without and no matter the brand or MCD rating of the LED, they are always more dim than their counterparts of other colors. How do I brighten the darn red LEDs?

Update

Here are the specs for the LEDs. I got them off of eBay from a Chinese manufacturer. There is not a brand name, but all are from the same source:

Emitted color: red, blue, green, yellow, white 20pcs for one color Size: 5mm Forward voltage: 1.9 - 3.6V DC Luminous intensity: 500-20000 mcd

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    \$\begingroup\$ What kind of LEDs are you using? What kinds of voltages/currents are you applying to these LEDs? Most likely, you just have low-intensity LEDs. \$\endgroup\$ – Shamtam May 12 '12 at 16:51
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    \$\begingroup\$ You're saying no matter what LED or what value resistor you use, the red LEDs are always dimmer than the other LEDs? First, pick a single brand of LED. Then find out the specs of it from the datasheet. You need to know the forward voltage and how much constant current the LED can withstand. 20mA is a usual number, some peopled drive them lower for less power consumption/longer life. You need to eliminate some variables in your question, and also please post a schematic of how you have things connected. \$\endgroup\$ – dext0rb May 12 '12 at 16:57
  • \$\begingroup\$ Thanks dextorb. I will work on a schematic (I have to go teach myself the symbols and all of that first). In the meantime, I posted specs for the LEDs. Also, I am simply running jumpers from Arduino straight to the breadboard. I am pretty sure that Arduino sends out 5v. Currently, I have no resistors or anything, just the LEDs plugged directly into the breadboard. Sorry for lack of detail/dumb comments. I am new to all of this. Thanks! \$\endgroup\$ – Matt Cashatt May 12 '12 at 17:14
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    \$\begingroup\$ IME red leds are simply less efficient that comparable green leds. So you will simply have to dim the green ones. \$\endgroup\$ – Wouter van Ooijen May 13 '12 at 8:30
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    \$\begingroup\$ I got them off of eBay from a Chinese manufacturer. There is not a brand name - This is 90% of your problem. Buy components from a reputable distributor like Digikey or Mouser and a reputable manufacturer like Cree or Kingbright, not ebay. \$\endgroup\$ – Kevin Vermeer May 14 '12 at 18:06
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It is highly likely that your red LEds are of lower luminous effiiency than your white LEDs.

Also, driving an LED incorrectly can destroy it or reduce it's output from then on. Such damage can occur almost instantaneously. Red LEDs have lower operating voltages than White LEDs so are more likely to be damaged by the application of a constant voltage that is too high and not current limited.

The efficiency of LEDs varies widely.
It can be expressed in lumens/Watt = l/W.
The best White and the best red LEDs have similar enough l/W ratings that both should appear about equally bright when driven with the same power input. White LEDs typically have operating voltages = Vf in the 3.0 - 3.6V range and red LEDs have Vf = 2.0 to 2.5 Volts. So a red LED would need about 50% more current to achieve the same power as a White LED as Power = V x I.

You state the LED output range as 500 - 20,000 mCd. That's a 40:1 brightness range.

Note that candela are a measure both of energy out and area of illumination. Less area = more brightness for the same light energy.

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LEDs come in all sorts of brightnesses and viewing angles.

Within some limits, the brightness of the LED is proportional to the current flowing through it, though this relationship is far fron linear.

The amount of current transformed into light is a measure of the LEDs efficiency.

If you want a set of coloured LEDs to match, then examining the manufacturers datasheet is a good start. Also make sure you take into account the viewing angle. Although not strictly true, half the viewing angle results in about four times as bright for the same power and effeciency.

Select LEDs with the same brightness at similar currents and identical viewing angles.

Lastly remember the human eye responds really badly to deep red and deep blue colours (see Wikipedia) so the aparent relative brightness between different coloured lights is somewhat subjective.

I should have included that LEDs are current devices, not voltage. The voltage across (Vf) them is variable between samples and over temperature. (remember LEDs get hot). This means high power ones should be driven by a constant current circuit.

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  • \$\begingroup\$ Your spec says little: Vf: 1.9 - 3.6V DC Luminous intensity: 500-20000 mcd To me that could say that given a random sample, you may get 0.5 or you may get 20 candella. High brightness LEDs take a lot of current, how are you driving them? You should be using a constant current boost circuit such as the National Semiconductor LM3410. \$\endgroup\$ – Jason Morgan May 12 '12 at 17:16
  • \$\begingroup\$ Thanks. I just ordered these high intensity LEDs and they seem like they will be better: ebay.com/itm/… \$\endgroup\$ – Matt Cashatt May 12 '12 at 17:21
  • \$\begingroup\$ You state the LEDs are connected direct to your processor? This is not good. The CPUs pins are probably good for just 5-10mA. You needs much more than that. That is why they are dim. A 20cd LED is so bright you can't look at it. \$\endgroup\$ – Jason Morgan May 12 '12 at 17:23
  • \$\begingroup\$ The datasheet for the parts you just linked is here link \$\endgroup\$ – Jason Morgan May 12 '12 at 17:28
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    \$\begingroup\$ @Matthew Patrick Cashatt You probably killed the LED. When you connect the LED without current limiting to a voltage source, it will start to die. That may be a pop and burnt electronics smell or the color of the LED may change or the brightness may change. I've seen the last two with batteries happen and the LED even when damaged may continue to operate abnormally for some time before it completely stops producing light. I guess that happened to you. \$\endgroup\$ – AndrejaKo May 12 '12 at 17:41
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I had this problem once matching brightness between green and red LEDs. I put the red LEDs on a pwm pin and then used a pot to change its brightness. Use the serial connection so you can see the value of the pot as you tune the brightness. Once you find the level of brightness you want, take the value displayed in the serial connection and hardcode it. Then remove the pot (and its associated code) and you are good to go.

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