need help with LED circuit for school project

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Bugster

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Im building a model for my architecture class and i want to incorporate some lighting to this model. Since i have a lot of 3mm white LEDs laying around, i think i will use this.

What i want to do:
I want to run 4x3mm white LED lights off of a 9V battery. I would like a switch on this as well. I know how to wire things up and solder, but what resistor value resistor do i use? Could someone advise? THanks!
 
White LEDs need about 3.5 volts to operate and you want to limit current to 15-20 mA. If you wire up two LEDs in series with a resistor, times two in parallel... you need two resistors:
9 - (2x3.5) / .015 = 133 ohms
You could use 100, 110, 120, 130.

If you wire up one LED with a resistor, times four in parallel... you need four resistors:
9 - 3.5 / .015 = 366 ohms
You could use 300, 330, 360.
 
sweet...i had a design change lastnight as i was going to sleep....im gonna do green LED's instead. Would this be the same?


Also....lead do i solder the resister onto on the LED?
 
Green LEDs require about 2.2 volts so the resistors will be different. Using the formulas above...

If you wire up two LEDs in series with a resistor, times two in parallel... you need two resistors:
9 - (2x2.2) / .015 = 306 ohms
You could use 270 or 300.
+9V___300___+LED-___+LED-___Gnd
+9V___300___+LED-___+LED-___Gnd


If you wire up one LED with a resistor, times four in parallel... you need four resistors:
9 - 2.2 / .015 = 453 ohms
You could use 390 or 430.
+9V___430___+LED-___Gnd
+9V___430___+LED-___Gnd
+9V___430___+LED-___Gnd
+9V___430___+LED-___Gnd

With green LEDs you could even get away with wiring all LEDs in series and one resistor.
9 - (4x2.2) / .015 = 13.3 ohms
+9V___13___+LED-___+LED-___+LED-___+LED___Gnd
 
Frank4d said:
With green LEDs you could even get away with wiring all LEDs in series and one resistor.
9 - (4x2.2) / .015 = 13.3 ohms
+9V___13___+LED-___+LED-___+LED-___+LED___Gnd
Click to expand...
With a resistor that small, you'll still get thermal runaway. Better to keep with the two strings, each with two LEDs. That means (9-2*2.2)/.015~=300Ohms. Here's an ASCII schematic:
Code: 
9v---^^^^---|>|---|>|---GND
9v---^^^^---|>|---|>|---GND
Good luck!

 
what a coincidence, i had the same question for a school project. theres no use starting a new thread about the same thing, so sorry bugster :p but does this work with maybe a hundred led's too?
 
damn...is there any power source that:

a) doesnt need a wall hookup
b) cheap
c) fairly small
d) able to power about a hundred leds?

should i just start a new thread?
 
is there any power source that:
Click to expand...
A 12V sealed-lead-acid battery might be what you're looking for. They weigh a lot for their size (lead isn't light), but they have decent energy capacity, decent current capacity, and they're portable. You can get one at RadioShack if you don't want to order online. Charger's aren't expensive either. If you go the 12V SLA route, you'll want to do the same thing that Frank4d or I suggested, except with 3 LEDs in each string, and a different resistor (which depends on what kind of LED you're using). For ~100 LEDs, that means 33+ strings of LEDs @ 20mA apiece ~=670-700mA. That's well within the capacity of a decent sized (say 7Ah) SLA battery.

 
by wall hookup do you mean no wallwarts/bricks, or do you want to use batteries?

oOo sam lee oOo said:
damn...is there any power source that:

a) doesnt need a wall hookup
b) cheap
c) fairly small
d) able to power about a hundred leds?

should i just start a new thread?
Click to expand...
 
for wall hookup, i meant you dont have to plug it into the wall socket

edit: i just realized that this thread started just two days ago, so now i feel bad, so im gonna start another one (dont wanna jack bugster's thread)
 
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