Given that you have two ways to adjust the brightness of an LED, resistor and voltage, which should you use? That is, should you increase voltage (by adding batteries) or decrease resistance, to get a brighter LED? The answer is in how power is used:

The battery (or power supply) generates power, the LED and resistor both use power, but they do so in different ways. The LED uses the power to make light (more power, more light). The resistor does not make light, it makes heat (more power, more heat). And as you know from the last quiz, any voltage left over from the LED is used by the resistor.

That voltage & current in the resistor is lost forever as heat and doesn’t do anything useful in our circuit. Since it’s inefficient to just pump all our battery power into the air as heat, we should make the power used by the resistor as small as possible, and the best way to do that is to keep the voltage low.

The upshot? If you need to make an LED brighter, adding batteries is wasteful: you’re better off using a smaller resistor! If you are making up a power supply, by adding up AA’s in a pack, try to have about half or one volt minimum ‘headroom’ above the highest forward voltage, so that you can have a small resistor, around 100 or 200 ohms.

Going lower than that isn’t suggested because the forward voltage can vary, and resistors can vary, and the battery can vary and all these little variances of 0.2 Volts or so add up and you won’t get the brightness you want.

Controlling the brightness of LEDs (light emitting diodes) lets you get more out of your LED design. If you’re a fashion designer, you can create more subtle effects in your LED studded clothes when you make colored LEDs brighter. For around the house, making your LEDs shine brighter will make it easier to read books and newspapers.

It is not difficult to make an LED brighter, however trying to make your LED shine too bright or shine too bright for too long, could result in a shorter LED life or a LED that doesn’t light at all.

Preliminary Calculations
Construct a table with two columns and four rows. Label one left column, “Current (mA)” for current in milliamperes. Label the right column “Brightness (mcd)” for luminous intensity in millicandelas.

Examine the current/brightness curve given in the manufacturer’s LED specification sheet. Mark four points on the curve with your pencil and label them “1” through “4.” Write the current level for the first point on the curve in the row one, column one of the table. Write the brightness level for the first point on the curve in row one, column 2 of the table. Continue in this fashion for the remaining three points you marked on the curve.

Subtract from the battery voltage (you are going to use to power your LED) the LED “forward voltage” from the manufacturer’s specification sheet. Label this result as “voltage drop across the brightness resistor or Vbr.” Divide Vbr by the highest current value in the table to determine the value of resistor you will need to make the LED the brightest. Label this result as “Resistor for highest brightness or Rb.”

Divide “Vbr” by the lowest current value in the table to obtain the resistor value for the lowest brightness level. Label the result as “Rlb” for “resistor value with lowest brightness level.”

Connect the positive terminal of the battery to the left lead of resistor Rbl. Connect the right lead of resistor Rbl to the positive terminal of the LED (anode). Connect the negative terminal of the LED (cathode) to the negative terminal of the battery. Observe the brightness level of the LED.

I am about to begin to work on an electronics project that involves the output of red light. After playing around with a bunch of LED’s, I realize that red and yellow are the weakest as far as “luminous” goes, no matter how much current you drive through the LED.

How could I make red and yellow more radiant? Like, amplifying the output? I searched “light amplification” and articles about lasers came up, but I’m not sure if that’s what I’m really going for.

If you are correcting the LEDs directly to the button cells, then you are depending upon the resistance of the cells to limit the current through the LEDs which is poor practice. You need to use a higher voltage and a resistor in series with the LEDs to control the current.

The current should be limited to the maximum rating of the LEDs. For example, if the LED rating was 20mA, then the required resistor value would be R = (Vbat – Vled)/20ma. If the three LEDs are in series then Vled is 3 times the voltage of one.

The wavelength number is the frequency (color) of the LED. It’s tells you nothing about brightness.

High brightness type LEDs will give you the most light for a given current.