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March 7, 2010
Tutorial - LED Laden Bike Helmet
about 5 years ago
On a separate note, I have heard that it is not preferred to limit several LEDs with the same resistor, as the failure of one LED will cause the others to overload. Is there a way of avoiding this without using an individual resistor on every LED?
Interesting little project. Planning something similar myself. A couple of comments that could improve the tutorial.
1. You state “power will be flowing … to the white LEDs first then to the red ones”, but your diagram shows the opposite.
2. In the first part of the tutorial you are calculating the value of the LIMITING RESISTOR. In the second part, you are calculating the resistance of the groups of PARALLEL LEDs. This is a bit confusing as for how it relates to the selecting the correct resistor values.
Each resistor and LED combination (red or white) will see the same 3.7V from the power supply. If the resistance of the LED array is LOWER, a BIGGER resistor (or combination of resistors) is required to limit the current. For the white it will be R1. For the red it will be R1+R2. This is why the white LEDs need to be “1st”. R1 is actually used by both.
I calculate R1 = (3.7-3.4)/.12 = 2.5 Ohm and R2 = (3.4-2.4)/.06 = 16.7 Ohm (approximately)
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