CHAPTER 11 ■ POWER ON!As the W value increases, the battery life increases but the LED dims (see Table 11-1). As the W value
decreases, the battery life decreases but the LED brightens.
It is important to note that the LED’s brightness depends on the current flowing through it, not the
voltage. Many electronic parts work this way. They are said to be “current driven.”
Selecting Resistors
When I first started experimenting with electronics, I couldn’t figure out how people chose the values for
resistors. Somehow I thought there would be one absolutely correct value for a specific use.
Within a particular range, resistor value selection is a matter of taste. It depends on the designer’s
perception of performance. If the LED is bright enough at 2,200 W, that will save on battery life. If battery life
isn’t an issue but brightness is, perhaps 1,000 W is a better choice. Resistor values are negotiable.
How bright can the LED get? According to the manufacturer’s datasheets, the maximum rated current
for my LED is 30 mA.
You could move the multimeter test probes to immediately before the LED to test the amount of current
going through the LED. You’d reconnect the red alligator clip to the positive terminal of the battery. Then
you’d disconnect the green alligator clip from the LED anode and connect the meter probes in between
(see Figure 11-12).
Figure 11-11. Resistor values for experimenting: from 10,000 W down to 470 W
Table 11-1. Resistance Versus Battery Life and Brightness
Resistance Band Colors Current Battery Life LED Brightness
10,000 W brown, black, orange, gold 0.7 mA 200 hours Very dim
4,700 W yellow, violet, red, gold 1.6 mA 94 hours Dim
2,200 W red, red, red, gold 3.4 mA 44 hours Acceptable
1,000 W brown, black, red, gold 7.2 mA 21 hours Good
680 W blue, gray, brown, gold 10.5 mA 14 hours Bright
470 W yellow, violet, brown, gold 15.1 mA 10 hours Very Bright