Driving RGB LEDs with an Arduino

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Yes, but with current limiting.

I was thinking a bit more about that...what if the Arduino crashes and switches everything on. Without any resistors it'll burn out, possibly overloading the batteries (I think battery current will be the limiting factor...)

Ok, that makes sense. I'm still getting my mind around this electronics thing... :-)

To drive it directly from the battery you'd probably need four AAs - depending on the voltage needed after the final calculation.

The other option is to put something like this in the box:

Yep. That looks good, thanks!

I don't think there's any way to put the 330 ohm resistors into the Sparkfun board but I assume it just works as-is for switching things on/off.

The schematic for the board is on the product page:

So ... hook one up with a potentiometer, dial "100mA" (carefully, don't fry the pot) and measure the voltage drop across the LED. Got it.

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That's why you use resistors in series with the LEDs to limit current.

Go here

to determine how long your choice of batteries will last at the current loads you're talking about.

I don't think AA's will be adequate for very long.

John

Yes, I understood that. At least you have a constant voltage to work with so you can calculate the current limiting LEDs properly.

No, I understood that. The 330 ohm resistor is to limit the current going from the Arduino output to the transistor.

(I'm unsure how you calculate the 330 ohms but I know why it's there...)

It would probably be bigger though...I need this to fit in a very small space (it has to go inside a musical instrument). The Arduino Pro Mini with one of those boards is *very* small.

If I start off with a high resistance and use my ten-turn wire pot I bet I can do it...it goes down to single Ohms.

I've got a multimeter and a very old (1970s) 'scope

So...MOSFETS have high resistance from gate to source?

A BJT needs the resistor because from base to emitter it's basically a diode. Once you pass the breakdown voltage the resistance from base to emitter is more or less zero and things will get hot without a limiting resistor.

The required collector current divided by the transistor's minimum current gain will dictate the required base current, which determines the maximum value for the base resistor. Above this value, you may not be able to obtain the required collector current.

The output voltage and maximum output current from the uC's logic pins will dictate the minimum value for the base resistor.

If the calculated minimum value ends up greater than the maximum, you need to use a transistor with more current gain or a FET.

t

I've been trying to educate myself on this today. This is the bit I was missing: A transistor needs a minimum amount of current to flow between base/emitter for it to work. I thought it was just voltage...

Best links:

MOSFETs seem to work like my previous idea of a transitor - only the voltage counts.

I didn't find a good "MOSFETS for dummies" page yet... they all launch off into substrate types and formulas before the end of the second sentence.

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Essentially infinite once you charge up the gate capacitance.

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Oops...

Hit the send key too soon, please disregard.

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OK.

Generally, in a common emitter switching circuit, one forces a beta of
ten in order to swamp out the vagaries between transistors.

That means that for every 100mA of collector-to-emitter current drawn
from the supply there will be 10mA through the base-to-emitter
junction.

Then, for 150mA of collector current there should be 15 mA of base
current.

With a 150mA load on the collector of the LED driver, and assuming the
Arduino outputs can get pretty close to the 5V rail when they go hot
means that with 15mA into the base of the driver, and with about a
0.5V drop across the b-e junction the base current limiting resistor
should be:

         5V - 0.5V
    R = ----------- = 300 ohms
           0.015A

Yes. I've been reading up some more and the thing people seem to worry about with MOSFETS is gate capacitance (the gate acts like a capacitor).

Seems that to get it to switch quickly you have to fill/drain the gate as fast as possible.

An Arduino can source/sink 40mA on any pin.

In theory I don't even need the perfboard, the Mini Arduino has holes in the PCB for soldering wires to it.

The transistors and resistors could go directly onto to the Arduino.

One other thing I was wondering: Is the actual transistor inside a TO-220 bigger than a (eg.) TO-92 or is it all just extra packaging to absorb heat?

Lifetime...if it can go for an hour that's probably enough. Colors will mostly be saturated (not white) so that's a maximum of two LEDs lit up, call it 250-300mA power draw for the whole thing. A 2500mA/h battery should be able to power it for quite a few hours.

OTOH the LEDs will only be at full power for short pulses. Most of the time they'll be under PWM control so that could easily double/triple the battery life (assuming the circuit is reasonably efficient).

It's 5K, 2W, one of these:

I would have thought it could survive 100mA with no problems (hey it's MIL-spec!). Maybe I don't understand how the "2W" is calculated.

I looked in my box and I've got a few 2N2222s left over from something else I did.

I think the thing I understand _least_ about transistors is that there's so many different parts.

Seems to me that half a dozen well chosen variations should cover everything.

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Any I/O pin, I think, but why use more than necessary?

So the 2W is end-to end, not what can come out of the wiper thingy?

Yeah, but:

• One which is really good at switching on/off

• One which is quite linear

• One which is more exponential

• ...what else?

After that it's just maximum ratings and changing the external resistors.

I'll give it a go...

D l

ent

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If you like electronics, you should buy a copy of "The Art of Electronics" by Horowitz and Hill.

George H.

- Hide quoted text -

In general you don't want to push much current through the wiper of a pot. Though when used as a variable resistor you don't have a lot of choice.

George H.

The results are in, all components seem to have survived...

I measured the voltage drop on each color at three different currents, 100mA, 125mA and 150mA because I'm not sure what the final current will be yet.

\ 100mA 125mA 150mA

R 2.10V 2.16V 2.21V

G 3.04V 3.13V 3.28V

B 2.81V 3.23V 3.45V

The results fit the LED specs quite well. The rest of the math is easy now I have these numbers.

Thanks for the help!

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My pleasure! :-)