Help with signal presence indicator circuit

Hi guys,

The modified schematic doesn't light up the LED.
I'm sure I have to change the values of R1 to R6 because the input circuit is different, so please help me understand how it works and get the values right.

Thank you in advance

Did you omit the diode for some reason?

Bri

Hi, you should put the diode (remember it still will be cliping indicator just for one side, if you have assymetrical signal may clip and not light if you use it as clipping detector, for signal presence would be OK unless you have a big negative DC Offset when led will be always on)

If you just want it to work only put the diode in its place and should be working and stop reading, if you want to go deeper on the circuit keep reading.

Remember INA163 is a complete mic pre, I assume you change the mic input for a line one.

Without the diode 10k 1µ will act as low pass filter at 15Hz so no audio signal will reach neither the 2nd opamp nor the LED. With the diode this will act as 10ms attack, so not realy a peak detector (5ms standard)

If you want an standard peak detector use 5K for R1 and 650K for R2. If you want clip detector at 600Ω using LM833 put 4K7 for R4 and 10K for R3 (about 19 dBu) 7K5 (2 15K in parallel) seems a little low for driving the led, but if the bright is OK they are working fine, if they turn on but not so bright as you want you could lower this value. Just a recomendation, don't use the same IC for both opamps, you could hear a click when the diode turns on and off because crosstalk and the current draw of the led, for even less noise you should use another ground path and decoupling caps for the supply rails.

Cheers.

JS

@Bri, I omitted the diode because I thought it was only for signal polarity. I should have looked at its datasheet :-[

@joaquins, thank you for the in-depth explanation and I understand your recommendation. On my board, I use 2x NE5534 for two channel line input, 1x TL072 for two channel signal presence indicator. Both have their supply pins coupled with 0.1uF ceramic cap.

Thank you for pointing out my error. Cheers!

It still doesn't work. I wired a 1N4148 diode like in this schematic, and the LED didn't light up. When I reversed the diode, the LED always stayed on even tho there's no signal coming in, also the line input op-amp got really hot and died. I had to put in a new one into the socket.

You have the diode in the wrong way round. The idea is it acts like a little peak detector and produces a negative voltage for the comparator.

Cheers

Ian

Right. And I also found out why the LED stayed on infinitely, a faulty TL072 on my board. I replaced it, and everything is working now  ;D

Thank you guys! Cheers!

** Updated schematic. Hoping it will be useful to anyone who's looking for this circuit **

Glad it worked out.  FWIW, you can diddle with the values of R3 and R4 to adjust the triggering threshold.

Bri

Sorry for the silly questions:
-is U1 there as a buffer/summing not to load whatever is before that stage?
-are the resistors around U1 just normal 10K?

thank you,
Mattia.

Q1 dont understand very well but, it's an input stage, it puts 10K load (or something similar) to the stage before it (from another device usually)

Q2, I don't know what you call normal, but it should be 1% (or matched) for better CMRR.

JS