Color Mixing w LEDs

[Wade4Green]

I built the circuit for Color Mixing w LEDs, exactly as described & shown, 3 times, EXCEPT i have to use battery packs instead of powered breadboard. A parent also tried. 1st used Radio Shack parts, then ordered Jameco parts.
The 3 color LEDs are either OFF OR ON! No variation of brightness when turning knobs on pots, so there are limited colors when diffused (tried several translucent to milky materials)
IS THE DIFFERENT POWER SOURCE THE PROBLEM?

I AM getting variation without op-amp, using resistors higher than 330, but do not know how this will affect life of bulbs.

(this is proj #2 because of capacitor problems, regional fair is in 1 week)

[rmarz]

Wade4Green - What voltage is your battery pack? What is the resistance of your potentiometer? You should be able to control the current with a properly selected pot value alone, but with a bit more precision if you use it in conjunction with a series fixed resistor. There should be visual differences in light output between current values of 5 to 30 mA. If you have a multimeter it might help you select the right resistor values. If you send more information it might be helpful. The 10K pot will not give you a good result and is only suitable if used with the op-amp circuit. If your battery pack is something like 6V, only a small portion of the 10 K ohm pot's resistance (a few hundred ohms) will allow sufficient current to produce light from the LED. That's why you may be seeing only 'on or off' indication near the end of the pot's rotation. Life of the LED's decreases if driven over 30 mA. Forward voltage drops differ with color, red may be 1.8V, green 3.0V and blue up to 4.0V. Therefore your pot and resistor values will differ a bit depending on which LED they are hooked up to. You can use ohms law to help with the resistance and current calculations.

Rick Marz

[Wade4Green]

THANKS for your reply. Your information helped. I don’t know anyone who can help troubleshoot. My dad is an electronic/computer technician but doesn't know much about LED circuits. My 1st project (FM transmitter) didn’t work using parts bought locally. Since this is an engineering project, I need a working product to test.
I looked at LED mood lamp projects but they had programmable chips. I decided to try “Color Mixing with Red, Green and Blue LEDs" from this site then build a 2nd one to see how I could change it into a lamp. I bought local parts but finally ordered exact ones from project list. Electrical hook-ups aren’t allowed at the fair so I tried to come close to voltage of the powered breadboard.

Here is PROJECT INFO:
I built the 1st circuit using instructions from "Color Mixing with Red, Green and Blue LEDs". Already had wire, DMMs, etc. I used these Jameco electronic components from Materials List:
•Potentiometer, Linear Taper, 1/2W, 10K ohm
•Resistors 330 ohm, 1/4 WATT, 5%
•Integrated Circuit, LF411CN
•LED, High-Efficiency Red, 642NM
•LED, Blue, Water Clear, T1-3/4
•LED, Green, 505NM
•Knobs 1/4"shaft, 14.6MM (red, green, blue)
(EXCEPT solderless breadboard with regulated power supply (+/-5 V, +/-15 V))

What voltage is your battery pack? What is the resistance of your potentiometer?
Rick Marx

Using Pots listed online Linear Taper, 1/2W, 10K ohm
(first pots bought are wider & supposed to be same except 10W instead of 1/2W - I think the same problem occurred w them. Also have same RadioShack pot with 2” shaft)

Battery wiring to Radio Shack IC Breadboard:
6v pack to pot; 6v & 9v packs: 6v +pos to Pins 7 & -neg to 9v +pos, 9v -neg to Pins 4 & 330 resistors.
I checked color variations before and after mounting wired pots on a plastic mini tote and put circuit inside, then tried different translucent materials on top. (online project used tuna can, cardboard box & wax)

2nd CIRCUIT DIFFERENCES: I built another identical one then started trying different things. I used Ohm's Law to choose last resistors used. It works well now, but I dont have time to wait for possible problems, such as shortening bulb life.
Potentiometer (same) MY battery packs & wiring: a 6v pack to pot; a 6v - to rail that goes to resistors then LEDs, + pos to rail connected to pot.
NO op-amp
RESISTORS: 270 to Red & 270 Green; 470 to Blue (parallel)
I hope this is enough info. I am trying to finish Sat & Sun so i can start on display.

WWB:nwb (typed by mom, a slow typer too but uses more than 2 fingers)

[rmarz]

Wade4Green - You are on the right track. Your resistor values are in the right range to safely limit the current from a 6 volt battery pack. I'm confused by your comment on the resistors - "RESISTORS: 270 to Red & 270 Green; 470 to Blue (parallel)". Assuming you have abandoned the op amp version and are just using current limiting resistors, they should be in series with the battery and LED. It sounds like you may have wired the 10K pot across the 6 volt battery and picking the output voltage for the LED from the wiper connection. This will certainly divide the 6 volts, but there is only 0.6 mA flowing through the 10K pot. If that is the case, when the wiper contact gets near the 6 volt connection you risk putting 6 V directly to the LED and burning it out. The attached schematic should look like what you should be using. The Pot should have a value of perhaps 1K ohms, the fixed resistors in the range of hundreds of ohms. If you find the LED typical forward voltage specs to be similar to what I mentioned previously, these values may be a starting point.

LED Fwd Voltage R1 value

Red 1.8v 160 ohms
Green 3.0v 120 ohms
Blue 4.0v 82 ohms

These values will limit the current from a 6 v battery pack to about 25 mA assuming the forward voltage specs are close. Check your LED data sheet if possible, or use higher value fixed resistors until you conclude you have safe current limiting.

Rick Marz

led.pdf

(11.34 KiB) Downloaded 294 times

[Wade4Green]

I was wrong about the resistors in last post - still trying different resistors on my
circuit. they vary but not as much as i hoped.

i couldn't get the circuit w op-amp to vary, so i started trying different things.
still cant get the op-amp version to vary but at least i can test colors w pot all the way up. i can compare 2 circuits for data, but am not sure how to graph them. i want a project goal different than mixing colors & plant to make a lamp.

my transmitter didnt work as well at my 1st fair yr, i won an nnovative engineering award fr NSPE!