Looking for a little guidance on a project I am working on. The requirement is to drive 8 RGB LED's in parallel. There will be three analog potentiometers (10K Ohms) to control the red, green and blue's intensity so any color can be displayed. I am using the concept from Matthew Beckler's Home Page (RGB LED :: mbeckler.org) to use the input and output circuits for the Arduino.
My questions are about the power and limiting resistor needed for the PWM leads into the Arduino. For the input, was planning on using the native 5V on the Arduino. For driving the RGB LED's (Radio Shack T-1 3/4 5mm Full Color LED Model: 276-028), was going to use a separate 12V power supply. Is that enough voltage for the supply to drive all 8 LED's to white (all on)?? What would the limiting resistor value be for each of the color branches? Cannot seem to do the math correctly to figure this out. Below is the planned circuit diagram. Any advice you would be greatly appreciated
Do not want to overdrive the PWM leads in the Arduino and want to be sure there is enough voltage & current to have all the LED's at full. Would this be better to run the LED color's in series?
The "right way" is with a separate current-limiting resistor for each LED.[/b] You can't run them in series, because RGB LEDs are either common-anode or common cathode. You'd need six wires coming-out in order to wire the 3 internal LEDs in series with other LEDs.
If you do it the way you've shown, you are assuming that the current through the limiting-resistor will divide equally among the LEDs. You might get acceptable results and you might not.
Also, you need a MOSFET or transistor on each output/PWM channel for two reasons - The LEDs are going to require around 20mA each, and you can only draw 40mA from an Arduino output. The 12V supply through the LED to the Arduino can also damage it.
What would the limiting resistor value be for each of the color branches? Cannot seem to do the math correctly to figure this out.
You basically need to know [u]Ohm's Law[/u] (R = V/I) and [u]Kirckchoff's Laws[/u] (Voltage is divided among series components, and the same current flows through series components).
So with a 12 V supply, and say 2V across the LED, that leaves 10V across the resistor. You can then apply Ohm's Law to the resistor. i.e. 10V/20mA = 0.5k Ohms = 500 Ohms. The different colors have different forward voltage, so check LED the specs. (You have to use Ohm's Law on the resistor, not the LED, because the "resistance" of the LED changes when the voltage & current change.)
Or, with 8 LEDs in parallel you'd still have 10V across the resistor, but 160mA through it. 10V/160mA = 62.5 Ohms. And, with that much current, it's a good idea to calculate the power dissipation in the resistor (Voltage squared/R or Current squared X R = 1.6W). That means you need a 2W resistor... Using 5V instead of 12V would reduce the power & heat wasted in the resistor.
Yes that is right although your resistors are too high for 20mA,
To find it subtract the Vf of the led from 5V then divide by 20mA to get the resistance.
Grumpy_Mike:
Yes that is right although your resistors are too high for 20mA,
To find it subtract the Vf of the led from 5V then divide by 20mA to get the resistance.
But OP supply voltage is 12V and not 5V
the resistors value are good for 12V
