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Simple LED fader

ryan

Feb 6, 2011
2
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Feb 6, 2011
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2
Hi there! I'm looking to build a little project, but I don't really know where to start.

I want to have 3 fader-type potentiometers that will control the R, G and B parts of an RGB LED. I have a bit of experience with micro controllers and it's something that would be easy with that, but I want to build it as an analog circuit.

The things I know are this:
I'll need to use a transistor to control the voltage to the LED from the battery, as a pot would die a quick death otherwise.

Any one have any tips regarding how to start? Should I just breadboard something up and try things until it works?

Thanks!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
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25,510
You really want the pots to control the current of a variable constant current source.

Basically, you have a sense resistor in series with the LED and you use an op-amp to compare the voltage across the sense resistor with the voltage on the pot.

An alternative is to use the pot to control the duty cycle of a PWM signal used to power the LED(s).
 

ryan

Feb 6, 2011
2
Joined
Feb 6, 2011
Messages
2
right! I want to eventually make this something that I could produce cheaply (by hand) and sell. Also I want it to be quite compact, so I'm shooting for non-chip construction. Is it possible to create a PWM signal w/o a microcontroller? Maybe just an IC w/ osc?

By op amp you mean a transistor right?

Thanks for your help. Very useful.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
By op-amp I mean op-amp. They're "chips" (or ICs -- I don't get your distinction). Basically a device containing a stack of transistors that provides you with a high gain amplifier that can be used for all manner of purposes.

Perhaps the simplest oscillator you can make that can have its duty cycle easily varied is one involving a 555 oscillator. Again, "a chip", but saves you heaps of effort and can probably drive most LEDs directly.
 
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