CHAPTER 15 ■ COMPARATORS
Repeating the Multiple LED Trick
You’ve just learned that you can add extra LEDs without greater battery consumption. Hmm. The single
yellow (LED7) and single green (LED8) LEDs look a little lonely.
Determining the Number of LEDs that the Battery Voltage Can Support
Yellow and green LEDs usually require between 2.0 V and 2.2 V. We’ll average it to 2.1 V for the formula. Let’s
see how many LEDs we can string in a row for a 9 V battery. Can the circuit support eight LEDs?
2.1 V LED × 8 = 16.8 V
No, obviously a 9 V battery doesn’t have enough voltage for eight LEDs. What about four LEDs?2.1 V LED × 4 = 8.4 V
Looks good. But wait! What happens when the battery runs down to 7 V? Oh well, not enough voltage
for four LEDs when the battery runs down. Let’s try three LEDs.
2.1 V LED × 3 = 6.3 V
Three LEDs are appropriate for a yellow and green LED voltage drop of approximately 2.1 V powered by
a 9 V battery. Recall that only two white LEDs (not three) were used in the headlight circuit. That’s because
the white LED headlights require 3.1 V for each LED, for a total of 6.2 V.
Calculating the Maximum Current Draw
The yellow and green indicator LEDs need to be visible from far away in standard room lighting. Therefore,
the LEDs should be run near their brightest. That calls for a higher current, which is achieved by a lower
resistance.
(9 V battery - (2.1 V LED × 3)) / (30 mA maximum current × 1000) = 90 W current limiting
resistor
The most common resistor value above 90 W is 100 W. But, I’m going to use 150 W in case the battery
voltage starts higher than 9 V. The higher resistor value provides a bit more safety. To double-check that
you didn’t make a math error at some point, plug the numbers into the formula for current presented a few
chapters ago.
(V / W) × 1000 = mA
(9 V battery - (2.1 V LED × 3)) = 2.7 V remaining to be used by the resistor
2.7 V / 150 W current limiting resistor = 18 mA current
That’s below the 30 mA maximum of the LEDs. But, there’s a lurking limitation of the comparator.
According to the datasheet, the typical current it can provide is 16 mA. The minimum current it assures to
provide is a mere 6 mA.
The additional LEDs (much less the motors) can’t be installed until this limitation of the comparator is
addressed in the next chapter.