I'm new to all this and I have two LEDs (super bright white) from Radio Shack connected in series. When I connect the circuit to the Arduino the lights turn on but very dim. Is it because I am drawing too much power then the 5v from the arduino? Basically my students want to create headlights for their robot. I figured two LEDs in series would do the trick and not take up an extra pin on the board. Is there a better way to do this?
The specs for the LED are below
Reverse voltage: 5V
Continuous forward current: 30mA
Peak forward current: 100mA
Power dissipation: 120mW
Operating temperature range: -25°C to 85°C
Storage temperature range: -25°C to 100°C
Lead temperature: 260°C
Forward voltage: 3.6V
Reverse current: 60 μA
Luminous intensity: 16,000 mcd
Peak emission wave length: CIE coordinates typically X: 0.3, Y: 0.31
Viewing angle: 22°
The only things that matter to this discussion are:
* Continuous forward current: 30mA
* Forward voltage: 3.6V
Two in series requires 7.2 volts, so you need to power each one individually because 5 volts isn't enough.
Also, 30 mA is too much for an Arduino port to supply, so you will need a driver transistor to turn the LED on and off.
Since the forward voltage is 3.6 and you want to use 5.0 volts, you need a resistor R=(5-3.6)/0.03 = 46.6 = 47 ohms.
So, you would need TWO 47 ohm resistors (one for each LED) and two driver transistors if you want to control each LED separately, or one if you want to control both at once.
Lastly, paralleling the two LED's is not a good idea because their forward voltage drop is not exactly equal and one will surely draw more than the other, giving an unbalanced amount of light.
To be sure... the Arduino will drive the switching transistors only, the transistors will control the LED's. You cannot connect the LED's directly to the Arduino ports (they draw too much current and will destroy the chip).
Of course you know that you need an appropriate resistor between the Arduino port and the base of the transistor as well......
Hope this helps.