If I were looking to tack on a line level indicator LED into a circuit, would this work in general principal?
I think that the led should not be on the same line as the signal, but in addition with a transistor for example or another opamp
For a simple on off indicator, use a resistor + diode from the op amp output to ground, in parallel with the resistor + jack, not in series.
For something that responds kind of linearly to the voltage, use an op amp driver, see Vactrols are magic! - #10 by sebastian and further explanation in the North Coast Synthesis article linked from there.
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Vactrols! Of course!
Uh, no, vactrols have nothing to do with it. I mean, you can use that kind of circuit to drive a vactrol, that’s why it came up, but you don’t need a vactrol to drive an LED (which would be using an LED to drive an LED, and then it’s turtles all the way down).
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I agree that you want to put it on a separate circuit and have the LED’s cathode going to ground.
Is there going to be a bipolar signal driving it?
LEDs don’t like being reverse biased too much.
You can either put a protection diode the opposite direction in parallel with the LED (like How to Connect a Protection Diode in a Circuit ) or put a diode in the feedback of your op-amp follower (like Precision rectifier - Wikipedia).
Also, if it’s line level, as in a few hundred mV rms, you may need some gain in your buffer to overcome the LEDs forward bias voltage. Typical red LEDs need around 1.7V before they will turn on.
Cheers
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Bit late to this @zorch but in the guitar synth world the 80’s had a lot of LED oddness. Below is a schematic from the ZD3 hexaphonic driver (the guitar bit on the Z3 synth.
LED circuit in the top right. Wondered if that might help. It’s belt and braces but works on very low levels and high.
I mention the ZD3 only because I’ve been asked to make a clone again and this time I’ll use easyeda.
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Right, for a bipolar signal you need a protection diode — which can be another LED, antiparallel, which will light up when the signal is negative. Or a 2-legged bicolor LED, which is really the same thing in a single package. Though if it’s an audio signal it’ll change sign too fast to see both colors, it’ll just look yellow instead of red/green.
Overcoming the forward voltage is what the op amp LED driver is about. It supplies a current proportional to the input voltage, which corresponds to LED brightness proportional to input voltage.
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