Adding LEDs Question

[DanCarlson]

Hey all,

I am in the process of planning a build of musicpcb.com's Echo Base pedal. I love pedals that have a ton of features, so I plan on building it with all of the suggested mods. These mods include adding 4 footswitches (bypass, dry signal kill, wet signal kill, "dub-madness"). I would love to have an LED for each of these, but I am worried that that many LEDs will drain too much power from the circuit. I am also fairly sure I know how to wire them, but if anyone has a diagram I would appreciate it!

Thanks!

[Mike]

Use superbright LEDs at low current.

For example, I have a boatload of superbright yellow LEDs. They are brighter at 2mA than standard yellow LEDs are at 20mA. Four of them would equate to 8mA if they are all on at once. The PT2399 uses something like 30mA.

Wiring them is easy. V+ -> resistor -> led+, led- -> Gnd --OR-- V+ -> led+, led- ->resistor -> Gnd. It doesn't matter. To switch them off, disconnect the V+ or Gnd to the LED.

Calculate the resistor with Ohm's law. You'll need to know your V+, your LED's forward voltage drop (Vf), and your desired current.

Here's an example:

V+ = 9V
Vf = 2.2v
Desired current = 2mA

9V - 2.2V = 6.8V.
R = 6.8(v) / 2(mA)
R= 3.4K

So, each LED w/ a 2.2V Vf on a 9V supply will each use the closest standard value to 3.4k.



Mike

[eatyourguitar]

I would like to add that you need to do some actual testing the first time to get that 2ma number. even if it is written on the datasheet, it may be too bright for what you like or not bright enough. the next time you build something, you don't need to test it. you can just do the math like mike said. if all your pedals are 9v DC then you only need to remember the resistor value, not the current consumption. when you buy LED's from a different supplier or a different color, you would need to test them again to get the brightness right. I buy my LED's from the cheapest place so there is usually no datasheet at all and the parts vary a lot.

[soldersqueeze]

By "wet signal kill" do you mean the LFO bypass switch or something different? I've just built one of these with most of the mods but don't remember that one.