I'm trying to make an LED light up from the AC output of a power amplifier when the amplifier is producing almost full power at approximately 54v. The LED should be much dimmer or off
completely below this voltage. I was thinking about using a diode in reverse parallel with the LED and in series with a large resistor. Is there a better way to do this, possibly with some type of fine adjustment through a trim pot?
I was thinking about using a diode in reverse parallel with the LED and in series with a large resistor.
Sounds like what I'd do. Just be careful. AC is no joke.
Yes a series diode and a properly sized dropping resistor will power the LED safely. However you will not get a smooth on/off transistions at anywhere near your 54v value. Rather you will get a rather gradual dropping off of brightness, some LEDs still show light at 1 ma when set up for normal 20ma current draw.
It would take extra circuitry (comparitor op-amp, maybe a 555 timer used as a threshold detector, etc) to give you a sharp transition in brightness.
Lefty
Any change you could point me in the direction of some similar circuits that might use such a configuration of 555 and comparator? I have those parts but never used a 555...
Any change you could point me in the direction of some similar circuits
What and ruin the fun for you to research and learn? 
Just kidding, sorry, but I don't have a circuit at hand and I'm up to my ### in my own projects at this time. Working on a 5X5X5 LED cube display 
Arduinos sure are fun and empowering. 
Lefty
The problem you have is that most comparators won't work up to that voltage and you have to power them with voltage rails that are at least as big as the biggest signal.
The solution is to put your waveform through a potential divider before you apply it to a comparator.
Two 39V zener diodes in series the opposite way round and a series resistor as well. Don't put them in parallel as they work as ordinary forward-biased diodes too!
They might need to be fairly beefy though at that voltage, at least 1.3W
That solution rules out the requirement:-
possibly with some type of fine adjustment through a trim pot?
Maybe but I read "possibly with" as a wish, not a requirement.
It's clear you are not an engineer then.
Haha I suppose that "possibly with" makes me sound kind of like some high maintenance designer or something! ;D
Really I'm not, I promise! I'm working on the comparator route now, but not really getting what I'm looking for. I'm looking at some LED Vu meters now.