orange_hand Posted May 25, 2011 Report Share Posted May 25, 2011 Hi Guys, I would need your help regarding a special type of red LED’s that I would like to use in my MB 6582. I have attached the data sheet for the “flat top†LED which is called “Bright Red L-424HDTâ€: I wonder what the dimension of the series resistor (R40 to R55 on the Base PCB) should be for this LED ? I know that there is a formula to calculate the resistance: R = (Ut – Uf) / If but I am not sure if I read the data sheet correctly. Let me calculate an example: Ut = 5 Volt; Uf = 2,1 Volt; If = 10 mA -> this would mean the resistance is R = 290 Ohm -> the LED would have 66% luminous intensity Ut = 5 Volt; Uf = 2,3 Volt; If = 20 mA -> this would mean the resistance is R = 108 Ohm -> the LED would have almost 100% luminous intensity I would appreciate if somebody could confirm that my calculation is correct. As I have ordered the base board parts from SmashTV and there are 220 Ohm resistors forseen, I would assume based on this calculation that they would fit for purpose, wouldn't they ? Sorry I ask this because I don’t have the appropriate knowledge of electronics and just try to get things together from what I could find on various websites. Thanks OrangeFlat Top LED KINGBRIGHT L-424 HDT Datasheet.pdf Quote Link to comment Share on other sites More sharing options...
jojjelito Posted May 25, 2011 Report Share Posted May 25, 2011 Hiya, Your calculation seems valid enough. The problem you will face is that you can't really translate a luminous intensity value of x% into a meaningful way of expressing perceived brightness. That depends on your ambient light settings, LED color, viewing angle and distance to object, plus the number of LEDs that are lit. Hence, people of this forum have told others to eyeball it - i.e watch the box in a darkened setting in the corner of your eye and adjusting the brightness so that it barely begins to annoy you, then take it down a notch from there. So - what to do: Solder a DIL socket on the baseboard for the LED resistors, connect a pot and adjust the value between the center led and either side until you're happy. Then measure VR and select the next larger standard resistor value and go with that. Since the resistors are socketed you can easily swap them if you're unhappy. Hawkeye used something similar in the way of LEDs for his CS tutorial and he stuck with 1kOhm. I suggest you start off from there. Cheers! Quote Link to comment Share on other sites More sharing options...
orange_hand Posted May 25, 2011 Author Report Share Posted May 25, 2011 Hi, thanks for the feedback ! I think I will do it the way you described it. Maybe 1K is too much, but somewhere in between should be ok.... Cheers orange Quote Link to comment Share on other sites More sharing options...
rosch Posted May 25, 2011 Report Share Posted May 25, 2011 you could also do that on a breadboard, if you have one. (or some cheap sockets on veroboard) Quote Link to comment Share on other sites More sharing options...
orange_hand Posted May 26, 2011 Author Report Share Posted May 26, 2011 Thanks, I think I will go for the spider soldering approach on the fly, as I don't have a bread board :-) Cheers orange Quote Link to comment Share on other sites More sharing options...
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