View Full Version : Powering LEDs

06-16-2008, 01:38 PM
The inner-kid in me wants to build a lego city with real working stoplights and streetlights and such. I figured I would use a 595 controller and hook it up to vixen to control the stoplights and such. My question is what do I have to do to hook them up? Each channel should only ever have 4 LEDs on them (and they are the small LEDs). Do I need to build some sort of relay for them or can the 595 controller power them directly?

06-16-2008, 05:44 PM
The ULN2803 chip should easily handle 4 LEDs per channel. Four would not draw more than 12 milli-amps. No additional interface is needed. Let your inner-kid run wild.

06-26-2008, 05:53 PM
Just use the same resistor-LED configuration used on the 595.

good Luck


10-11-2008, 11:47 AM
I have a 595 that I plan on using to run LEDs direct of the chips no SSR. I was wondering if I could run several; say 4 in series per channel? From the specs it looks like they are 100mA so that would be 400mA per channel. I have spoken to one person that ran them without resistors, so do I need them if I do 4?

The LED I am looking at are:

http://cgi.ebay.com/50-PCS-10mm-40-5-Chips-White-LED-100mA-280Kmcd-NEW_W0QQitemZ310042808324QQihZ021QQcategoryZ66954Q QtcZphotoQQcmdZViewItemQQ_trksidZp1742.m153.l1262

P. Short
10-11-2008, 12:37 PM
If they are in series the same current will be flowing through all of the LEDs, so they don't add up. If I were you I would try not to run the chips at their max rating, instead limit the current to maybe 75 mA or so.

However, the voltages will add, so you will need to find a power supply good for at least 16v (20 v might be better), and configure your 595 board to use split voltages (5V for the 595 chips, 20V for the ULN 2803 chips). And I would strongly encourage you to use current limiting resistors, regardless of what you may have heard elsewhere.

You could also connect the LEDs in parallel. This would require four times as many current limit resistors, though. In that situation you should be able to get by with just 5v if the LEDs end up in the 'typical' portion of their range. You would end up with perhaps 1V across the current limit resistors, so you would use perhaps 12 Ohm current limit resistors, for about 83 mA current through the LEDs. You might see some variation in the LED brightness, depending on the power supply and resistor tolerances and on how consistent the LED Vf characteristics are.

10-11-2008, 01:13 PM
Thanks for the reply.

I found a cool calculator for current limitting resitors for LEDs