LED Color Organ Mod problem

Hi Gang
I built this LED Color Organ - http://blog.makezine.com/2010/10/18/circuit-skills-led-color-organ-spon/ - and it works great.
Wanting more, I modified it to drive three high-power LEDs - 400ma each

Problem is that I don't have the circuit around the TIP120 Darlingtons right exactly. the Red channel works really well, but the Blue & Green channels are difficult to balance, and after a while operating, the Blue channel just stays on - even if I disconnect the input to the base.

Also, when I turn on the power, all three LEDs glow a bit. Drawing about 11ma total, even with the base input disconnected, so it seems that the TIP120s aren't totally turned off.

Here's the circuit with my modification boxed in on the right side.

Can someone here tell me what I'm missing? I'm not a total electronic noob, but know enough to be dangerous. (as may be obvious)

Thanks!

- a -

The drawing shows the LEDs backwards, but let's assume that's just the drawing.

If you imagine no signal coming from the left, your transistors and LEDs will be biased to on, but the base current is limited to what comes through the 100k resistors from Vcc, i.e. not much. That's why the LEDs are dim.

You need a better biasing scheme.

LEDs are shown backwards.
R 9, R 24, & R 29 seem a little low. I would think they should be adjusted so that LEDs are just extinguished without signal.

wayneh said:

The drawing shows the LEDs backwards, but let's assume that's just the drawing.

If you imagine no signal coming from the left, your transistors and LEDs will be biased to on, but the base current is limited to what comes through the 100k resistors from Vcc, i.e. not much. That's why the LEDs are dim.

You need a better biasing scheme.

Click to expand...

Thanks, yes, the backwards LEDs are just the drawing - threw it together in a hurry to make this post.

I thought the bias was the problem, but I don't really understand how to do it and how it works.

I'll try making some adjustments - trial-&-error style - per Bernard and report back what I find.

Thanks guys!

Bernard said:

LEDs are shown backwards.
R 9, R 24, & R 29 seem a little low. I would think they should be adjusted so that LEDs are just extinguished without signal.

Click to expand...

So, trial & error found that 180K set the LEDs to just barely glowing without input signal, so I settled for 220K (next up in my parts drawer).

When I turned the music back on, the LEDs barely flashed. So, I changed the base resistors to 1K. Then, the Red & Blue worked pretty good, but Green was barely working unless I turned P3 all the way up.
Next, I changed R28 to 820ohm (next lower in the drawer) and that seemed to help.

Here's the current circuit (with LED polarity fixed to match the actual circuit)

Another question; i seem to recall someone saying that removing caps C12, 13 & 14 would make the LEDs react more quickly. That so? (kinda seems like it to me).

THX!

Without Cs , LEDs will try to follow each beat, & a little dimmer?

Bernard said:

Without Cs , LEDs will try to follow each beat, & a little dimmer?

Click to expand...

Yes, Thanks. I tried it and that was the result. not what I wanted.

- a -