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RGB LED simple project. need help I only have one day

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SpeedyRex

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I am doing a project about LEDs for my physics class. My idea is to have 3 LEDs (1 red, 1 green, 1 blue).

What I want to do is have one battery (or battery pack) power all 3 LEDs. I want to hook up a potentiometer to each LED so I can control its output to create the different colors in the rainbow.

I have to have this done by Monday morning (4/27)

Is this possible to do? if so can anyone provide a link or info about what supplies I will need and a basic schematic?
 

Sceadwian

Banned
POT's aren't particularly practical for dimming LED's. About 90% of the travel of the POT will be useless and the LED will go from full brightness to complete darkness in that 10% range in the middle, but it will work. Use a 1K pot for each LED. One end of the POT gets connected to the + of the battery, the other gets connected to the - end, The lead in the middle gets connected to the LED's anode, and the cathode of the LED gets connected to the - lead of the battery. Also use a resistor in series with each LED to limit the current when the POT is at one extreme end of it's travel. That resistor value and the wattage rating of the POT's you need to use depends on the voltage you're going to be powering the LED's from. I would recommend using a 10 turn POT to give you at least some chance to be able to set a specific brightness to the LEDs.
 

tcmtech

Banned
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Sceadwian got it close but use a 1 meg with a 1k in series with each LED. You will have a far better chance of getting a noticeable dimming effect. But ultimately you may still be getting a very dim LED at full resistance.
A better way is to use a PWM system and vary the on Vs off time of each LED.
 

Sceadwian

Banned
1 meg with 1k in series? That'll be at most 4ma through the Blue and green LED's...
 
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tcmtech

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I dont know how you came up with 4 ma. The OP did not put a battery pack voltage on his post. you didn't reference a voltage source either!
Are you assuming he is using a 4001 volt power source? :p:D

Or are were we just messing with him and I just ruined it?
OOPS! my bad. :eek:
 

Sceadwian

Banned
I'm asuming voltage just above that required to actually drive the LED's.
 

tcmtech

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Same here. I was assuming a 4.5 - 6 volt power source with the 1 meg 1k and the LED all in series.
given a 3 volt average drop through 1.001 meg the LED would only be getting .000002997 MA or 2.997 nA. LED's tend to be rather dim at that current!

Although assuming its a 6 volt power source I would drop the 1 k and us a 150 ohm.

Or do you have some insider info in this thread too? :D :)
 

Sceadwian

Banned
1k to 150 that works =)
 

SpeedyRex

New Member
So far I have it set up with all LEDs connected to the same 3v power supply (2 AA batteries) with a potentiometer in series with each LED. Sceadwian was right, the pots only affected the LEDs in that 10% middle portion :(
 

tcmtech

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what resistance is the pot you are using. I may not actualy be high enough.
If you have high power LED's you may actualy have to get way high on the pot resistance. Say 5 - 10 meg.
The other option is to use a PWM circuit to control each LED.
 

Sceadwian

Banned
tcmtech, I'm not sure I follow this high meg pot for controlling an LED brightness. You could never adjust the pot low enough to allow the LED to brighten. Even 1/10th of a percent into it's rotation the resistance on the center tap is going to be 5k, how's is an LED going to light through that?
 
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tcmtech

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OPPs! Did I forget to mention its in series with the LED not parallel. It gives you the ability to vary the actual running current from nanoamps to 10's of milliamps linearly while still keeping the proper voltage across the LED.
 

Sceadwian

Banned
TCM, yer starting to scare me here.... If you use a 5meg pot in series with an LED as I said 1/10th of 1% of the travel range will cause the LED to go completely dim, that's even worse than using a 1k pot with the LED fed from the center tab that I suggested.... What you're describing makes absolutely no sense. There's a way to get something close to linear using a small POT and an extra series resistor but I haven't sat down to work it out yet.
 
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tcmtech

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I just redid the running test and well very big oops on my part! I miss read my test pot. its a 10k not a 1 meg. :eek::eek::eek:
With a standard LED it does in fact give you a full range of dimability. I did need to use a 20 k on a High brightness LED though.

Very sorry about that. I did not read the pots numbers closely enough.
I just assumed it was 1 meg because it came out of the one meg tray in my pot drawer.

To do the test I just grabbed a 1 meg and hooked it up to the 5 volt source on my bread board and started turning it to see how well it dimmed. Well first try the assumed 1 meg worked full range with a smooth even dimming effect! To bad it was really a 10K.

I guess it looks like I may have been into some other type of "pot" drawer on this one! :eek::eek::p

So today I am once again "One of them" too! :p
 
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Sceadwian

Banned
Was it a RED LED?
 

tcmtech

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Was it a RED LED?
:confused:

Um.. About 644 nm? Give or take. ;)

My electro senses tell me this may be a loaded question! :eek::D
 
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