Hi guys! well I wanted to as the topic says, use LED to show how are the signal I want to be shown but if I directly connect the output of, for exemple a LFO, its voltage would drop. So I thought of using some sort of buffering circuit. Then two schematics came in my mind:
The transistor
The voltage follower Op-Amp
So what do you think I should use?
Op amps are always my first choice
I like the op amp better
EDIT: Ok, ignore that, you absolutely can make a changing brightness LED both ways. The Transistor method is fine so long as your input signal doesn’t exceed the base-emmiter breakdown voltage of the transistor, which for a 3904 is about +/-6v. Whereas the opamp would function between the voltage supplied to its power in and out, its just a more expensive (in terms of price of parts) solution to the same problem.
Really glad I picked up a copy of The Art of Electronics, lol
For all my trigger/gate/LFO … i use the Transistor schem with a 100K resistor add between Signal and pin 2 of transistor.
Inductance is another old time option.
Magnetic man! It was the future!!
Okay thank you everyone for your advices. I think I will go for the transistor method since I have lots of transistor 
Later if I got more tl07x I’ll do it with opamps because it sounds safer to use (I won’t kill the opamps)
(Thanks to reminded me that those transistors needed a 100k resistors at the gate!!)
To avoid having the LED not turn on until the forward voltage is exceeded, and more generally to have it respond more or less proportionally to the voltage, you want to drive the LED with a current proportional to the voltage. One way is to put the LED in the feedback of an op amp; for instance, this is what I used to drive the LED of a vactrol in the Megamodule:
Honestly I’m not sure how necessary the capacitor and feedback resistor are.
Oooh, so you add a cap between the neg input and the input of the opamp as well as a pull down resistor?
Using this circuit makes the brightness of the LED proportional to the input signal which make a linear response curve?
The cap and R7 form a high pass filter, I guess? which might improve the stability, but I’m not sure how necessary they are.
There is no current into the op amp input, so none in the feedback loop, so the voltage at the top of R8 equals the voltage at the - input, which equals the input voltage at the + input. Then by Ohm’s law the current through R8 is proportional to the input voltage. But that’s the same as the current through the LED. So this supplies an LED current proportional to the input voltage.
I think the cap just provides a steady voltage (DC?) To the inverting input so the output is less jumpy waiting? Nope. Not got this. Thinking amplifier stuff. As you were.
Wouldnt this be a passive lowpass RC style filter?
Yeah it’s probably a passive low pass filter to avoid the led from blinking at high frequencies. Thanks for the explanation
I think the keyword for voltage to current conversion is “transconductance amplifier.”
A few weeks ago I made one to make an LED shine with the output volume of my guitar. Just an LM324 from my scrap op-amp box, a resistor and an LED, if I remember.
Oh yeah you made a VU meter but the meter is the led if I understood?
Yes, that’s it ! And what is nice is that when you play two notes slightly out of tune, you see the “beat” (i hope i use the right word, in french we say “battement”). The led blinks, and it gets steady when you are in tune. It’s just a blinking led, but i found it nice to view something that you hear.
here is more or less the schematic I used : https://imgur.com/a/8nUrHqC
(found here)
And after, i have learnt that the transimpedance amplifier is a current to voltage converter… such barbaric words 
Okay nice !! And yes “je comprend tkt” ^^
t’es d’où en france ?
Je suis de Mantes la Jolie dans le 78 et toi ?
à côté de Saint-Brieuc, en Bretagne
