I would like to change a LED in a circuit, in order to change its color. The circuit is this one (LED on the right) : enter link description here

First, can someone confirm that the "LED part" of the circuit "has a purpose" (I don't know how to say this), like explained in part 3 of this comment.

Then, is it possible to change the LED with one of a different color, without messing up the behavior of the circuit ? I would like to put a purple one instead of the red one which is connected right now.

My intent was to remove the LED and measure the voltage "at its place", and use the value as the supply voltage to calculate a resistor value. That would lead me to add a resistor+LED in place of the current LED (or maybe just the new LED if the calculous gives me a resistor value of zero ?).

Or is it a bad idea and there is no other solution than to let the current LED in place ?

Thanks a lot to whoever can help me !


the LED is serving dual purpose, it's showing an indication when the pedal is on and passing voltage into the 4.7M which eventually turns on the JFET 2N4320

using a higher voltage LED will reduce the peak voltage on the 10k resulting in less drive to the circuit before the FET

You're going to get a different effect, but still mostly within the range of performance you get from a 9v battery, it probably won't be very noticeable.

  • \$\begingroup\$ Thank you. I have an other question, maybe I'm asking too much, let me know. I've found a blog explaining that it is possible to remove the JFET and replace it by a wire. It seems that it allows me to remove the 10k resistor, the 4,7M resistor, the diode and the cap, and then I can put any LED I want. This is supposed to have other benefits (remove a plop when activating the effect) when it is done as well as a rewire of the 3pdt switch. What do you think ? Does it look realistic to you ? Thanks \$\endgroup\$ – jdiogon Jul 13 '18 at 11:50
  • \$\begingroup\$ sites.google.com/site/cctsim/home/chorus \$\endgroup\$ – jdiogon Jul 13 '18 at 11:53
  • \$\begingroup\$ I went ahead and did what it says on this page : it's working perfectly. Thanks a lot everybody for the answers. \$\endgroup\$ – jdiogon Jul 13 '18 at 19:03
  • \$\begingroup\$ if you make that other change I think the pedal won't be able to control the effect anymore. \$\endgroup\$ – Jasen Jul 13 '18 at 21:22
  • \$\begingroup\$ @jdiogon I think you will end up with a circuit where pressing the footswitch will turn the LED on, but do nothing else at all. \$\endgroup\$ – Simon B Jul 13 '18 at 21:31

You need to calculate a new value for the 10k ohm resistor depending on the new LED. But I suspect that the value 10k isn't changing that much since LED have from 1,x to 3,x V forward voltage.

And to have a 10k in series with a LED indicates that you have 220V as Vcc.

Resistor calculator: https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor

  • \$\begingroup\$ It's weird : it's a guitar effect pedal, the Vcc is 9V. \$\endgroup\$ – jdiogon Jul 13 '18 at 9:20

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