Resistors before LEDs

[Screaming_Rabbit]

Hello all

I'll use the MEC switches for my MidiBox, with built in LEDs. http://www.mec.dk/files/LED%20tecinfo.pdf

My question to the electronic pros:

Are the 220 Ohms resistors on the DOUT Board high enough or do I need further resistors before the LEDs?

Greets and thanks

Roger

[illogik]

i'm no pro so i'm not sure but i think the 220R resistors will do just fine; it's just to lower the output voltage of the shift registers so it wont kill your leds. you could compare the data (Vmax) to the Vmax of a "regular" led

cheers marcel

[raphael]

Hello Roger,

In short: 220R is just fine.

I've found a interesting link for all the midibox users who are asking this question from time to time:

http://www.theledlight.com/resistancecalculator.html

Just enter the 'supply voltage' --> 5V

The LED forward voltage V_f (voltage drop) --> 2.1 V in your case (take the typical value)

The LED forward current I_f --> 20 mA in your (and the most) cases

You will get 145R. So 220R is Ok (try a resistor with 200R or 180R for brighter LEDs but beware of overloading the outputs of the DOUT! 25mA is the maximum!).

Easiest way: Do it with your calculator. The formula is:

(V_batt - V_f)/I_f = R

So: (5V-2.1V)/0,02A = 145 R

Hope this helps,

Raphael

[Screaming_Rabbit]

... it does help  :D

Thanks a lot Raphael!

Greets, Roger