I want to design an audio mixer. The audio signal comes from an audio jack and is then buffered by an op amp. I want to have an LED that indicates whether the signal is close to saturation for this op amp.
I have a voltage divider that gives me the comp reference at 75% between V- and V+.
Will the following circuit work for my use case? Do I need to add a diode to act as a peak detector?
There are so many approaches to this. It seems that what you are looking for is a red LED which will warn you when the level is close to (presumably audible) clipping. This is a common requirement, the usual approach to this is to have the LED common at either 3dB or 6dB below the maximum output level. The usual approach is to use a comparitor which charges an RC circuit (this effectively works as a monostable as suggested by others but is a bit simpler). Ideally you would want to detect either negative or positive peaks, but many designs economise a bit by only looking at one peak.
LM393 is indeed a comparator (probably how you got to it) but as audio opamps like NE5532 and TL074 are typically plentiful in a mixer, you can just as well use one of those as a comparator, and keep your parts count down a bit.
simulate this circuit – Schematic created using CircuitLab
It's usual for such a circuit to have a fairly fast attack time (here I set a time constant of 0.5ms) but (as very short periods of clipping are inaudible) not too fast, and a longer release time to make clipping on short peaks visible. I didn't work the values through thoroughly, and in any case you usually end up tweaking the values manually as the LED you choose will itself affect discharge time. It's not super critical, you just want to make sure you see the peaks.
The series diode and RC network should do this well enough for most applications. Note that you might prefer a 5532 over an 072 for this, as it have better current drive.
The LM339 has an open collector output so can’t drive its output high – you’d need to add a pullup resistor for that to work.
It would be better to take advantage of the LM339’s strong pulldown strength and have that turn on the LED. Connecting input to ‘-‘ input then cause the LED to come on when threshold is exceeded.
To keep the LED on for some persistence a capacitor can be added to the ouput. This will discharge immediately and only slowly charge through the LED resistor. This cap may have to be fairly large to get an acceptable persistence.
Note this circuit will only work if the audio input is swinging between 0 and VCC.
