Erica Synths EDU DIY EG Eurorack Module Kit - Manual (Page 18)

THE STATUS LED
Thankfully, there’s a quick and easy way to achieve this: by implementing a simple status
LED, which we’ll drive using our envelope’s output. This way, the LED will tell us what
voltage level the envelope is currently pushing out out by shining more or less bright. So
far, so simple. But there are two slight problems with this. First, LEDs are (as I said before)
pretty power hungry. And though our op amp bu
ff er is capable of providing enough
current to light one up, this would pretty much completely occupy it. Second, LEDs are
super quick to burn out if we push too much current through them.
So we need to make
sure we provide enough, but not too much current for our status LED . Doing this is
much easier than it might sound, though. We only need three components in total: our
LED, an NPN transistor and a small resistor.
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Here’s how this works. Whenever our bu ff er sends out a
non-zero voltage, a tiny current will flow into the
transistor’s base. This will cause that transistor to open
up, resulting in a much bigger current flowing from the
positive rail through the current limiting resistor and then
our LED, lighting it up.
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The nifty thing about this setup is that the higher the
voltage coming from our bu
ff er, the more the
transistor will open up – and the brighter the LED
will get
. All while the 470 Ω resistor prevents the current
from increasing too much. Note that it’s not really the op
amp that’s providing the power to light up the LED here
– instead, we’re getting it pretty much straight from the
power supply.
Read more about transistors in the components & concepts appendix (page 35). 13
You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right 14
here: https://tinyurl.com/y7gr9hnj – you can change all values by double clicking on components.
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