Pork Barrel: LED / LFO mod question

[schweltzer]

Hello all,

I've just tested my pork barrel before boxing: good news the effect works! but the bad news: The LED/LFO mod does not appear to work - that is the LEDs do not light up.
I followed the instructions, used a 22k resistor and used two 3mm ultrabright LEDs in series, and connected the negative terminal of LED-2 to the ground of my small test rig. The effect works fine, but the LEDs do not light up. Im using a 9v power supply (test rig is powered by a new 9v battery).

Does anyone have a suggestion as to why the LEDs do not light up?


Thanks

Dave

[RobA]

Measure the voltage at the pad on the PCB where you connected the mod. If it's not high enough to cover the forward voltage drops of the two diodes combined, then they aren't going to light up.

[blearyeyes]

I just built this one, why two LEDs?

[schweltzer]

blearyeyes: I bought two pots that can be fitted with LEDs to light up from below. I thought it would be cool to have both light up.
Will check with a multi meter tonight.

[blearyeyes]

Funny I want to do that as well.
Check out the forward voltage requirements of different color LEDs as they vary. Might be your voltage isn't enough and you need to lower the resistors value... Good luck.

[wilsona]

Ooooh! Where do you get lighty-up pots?

[RobA]

Small Bear has them, http://www.smallbearelec.com/servlet/Detail?no=1186. They have clear shafts and you use clear knobs to go on them. The illumination comes from an LED you put in the back of the pot.

If the combined forward voltage of the two LED's is too high, then lowering the resistor value isn't going to help. If the voltage drop is less than the available minimum voltage there, you might be able to get it to work by going to a small enough resistor. But, that might change the way the depth mod works.

I think you could still rig up a way to do this with the two pots, but you'd have to build a little buffered daughter board to drive the LED's. You could tap off the LFO at the junction of R37 and C21 (also the base of Q5) and put in a buffer to drive the LED's with there. Maybe you could drive one LED with the depth mod the way it is normally done and then drive the second LED by replicating the Q5 R38, R39 and depth mod section for the second LED (tapping the LFO signal at the base of Q5 for this replicated section).

[blearyeyes]

I have a basic question that came to mind..

When combining LEDs how do they react when wired in Series Vrs. wired in Parallel?

[RobA]

I have a basic question that came to mind..

When combining LEDs how do they react when wired in Series Vrs. wired in Parallel?

Good point. In parallel, they'll only have the voltage drop of a single LED, but the currents will be different. How this interacts with the signal to the MN3101 could get weird. It might be fine though because of the buffer that Q5 gives you. You'd want to have a resistor for each LED you did in parallel. You'd want to replace R39 with two LED's with series resistors and then recombine these at the switch. You might have to play with the values you use for the CLR's, try starting with a couple of 47kΩ and see how that works. If it's not great, work your way down from there until you get an optimal balance between LED brightness and the depth control.