led series and transistor

Projects General Guidance
1

i'm decorating a 40' table with little dioramas that get lit up by randomly pulsing LEDs. I just bought a bunch of these but in different colors:

forward voltage around 3.3 for most or 2.2 for red and yellow. 80mA is forward current.

i used this http://led.linear1.org/led.wiz webpage to calculate resistor size if i put them in series of 2 or 3 at a time. the resistors are mostly 1/2W or 1W. usually when i use them with these LEDs they run hot which is a little disconcerting.

i want to put around 6 leds (two or three series of them) into one tip120 which, going through a 1k resistor, goes into a PWM pin in arduino. There will be 24 leds in all and 4 tip120s.

then also i will use a second arduino to pulse 8 lights (4 series) going through 1 tip120, through 1k resistor, to an arduino pin.

can you put so many LEDs on one tip120? would a mosfet be better? will it be okay to do PWM using a transistor? do you think i need a heat sink for the transistor? any idea why the resistors become hot to the touch?

I want to power everything with a 12v, 5amp wall wort. here's a rough idea of what i mean (without actually having the craziness of 32 leds):

2

can you put so many LEDs on one tip120? would a mosfet be better?

Depends on the situation you actually have. From the picture it's not obvious which LEDs are 3.3V and which are 2.2. If you wire it as shown, a TIP120 should be OK. A MOSFET should not be necessary.

any idea why the resistors become hot to the touch?

If the resistor values are the one you've chosen, then some are simply wrong and will probably give too much current on the LEDs but as I mentioned above that depends on which LEDs are connected where. If the two LED set is made of the 2.2V type you get more than 110mA current and almost 1W of power so the resistor will get quite hot after some time (the whole power is converted into heat).

3

did you do ohm's law on the current ?

the simple answer is yes, with the follow up of why choose such an over powered device ?

are you looking at miliWATT and thinking miliAMP ?

the entire circuit draws 80mA for each series array

a simple PN2222 will handle 600mA
the TIP120 does 5,000mA

on your 80mA circuit.

how did you pick the 1k for the base resistor ?

4

I have no idea why you would need two Arduino's to switch some LED's unless I am totally missing something.

LED's and resistors are load, load is controlled by transistor, transistor is switched by PWM into base.

5

bigred1212:
I have no idea why you would need two Arduino's to switch some LED's unless I am totally missing something.

maybe the tutorial shows that pin13 is the one to use when lighting an LED ?

or, one for turning them on and off, the other for pulsing ?

There is no doubt that the OP will find that this is not only a fun project but will learn a lot along the way !